Affirmed in part, vacated in part, and remanded by unpublished PER CURIAM opinion. Judge SHEDD wrote a separate opinion concurring in part and dissenting in part.
Unpublished opinions are not binding precedent in this circuit.
PER CURIAM:
On consolidated appeal, Appellants Anthony Sellers, Alcindo Rochelle Matthews, and Sigmund Diaola James challenge their convictions and subsequent sentences stemming from a drug conspiracy. Appellants dispute the admissibility of evidence gained from searches of their vehicles. Appellants Sellers and James also dispute the district court’s calculation and proportionality of their life sentences. For the following reasons, we affirm Appellants’ convictions. We conclude, however, that the district court erred in sentencing Appellant James by applying the murder cross-reference provision in United States Sentencing Guidelines Manual (“Guidelines”) § 2Dl.l(d) and in treating as relevant conduct, an unrelated and uncharged murder. Accordingly, we vacate James’s sentence and remand for resentencing.
I.
The events of this case stem from the surveillance of Sellers, Matthews, James, and others by Drug Enforcement Agency (“DEA”) agents and Orangeburg County officers regarding possible drug-related activities in Orangeburg, South Carolina. On May 21, 2007, South Carolina state highway patrol officers stopped James for speeding. Upon running James’s license and registration, Officer James LaChance was informed James had an outstanding warrant in Orangeburg County. After James was detained, he alerted Officer LaChance to money inside the vehicle. Officer LaChance then returned to the vehicle and located the money inside the console along with a rear-facing video camera. Other officers, including a K-9 search team, subsequently continued to search James’s vehicle.
Following James’s arrest and the search of James’s vehicle, DEA agents continued to surveil James’s whereabouts. On January 16, 2008, the DEA secured a wiretap of James’s telephone. On January 19, 2008, the DEA, acting without a warrant, placed an electronic Global Positioning System [323]*323(“GPS”) device on the exterior of James’s vehicle. Over the course of the next several days, DEA agents used the GPS device to track James’s whereabouts. On February 2, 2008, the GPS device ceased transmitting, apparently because the device’s batteries had been exhausted. On March 6, 2008, agents removed the device from James’s vehicle. Relying partially on information gained from the use of the GPS device, the DEA then secured another wiretap of James’s phone on February 13, 2008, and again on March 16, 2008. A total of seven wiretaps were issued from January 2008 to July 2008 to secure evidence of the scope of the drug conspiracy at issue in this case.
On August 14, 2008, Sellers was stopped by police for an improper lane change while driving. Upon approaching the vehicle, Officers Phillip Furtick and Terry Logan noticed a strong odor of marijuana. Officer Furtick asked Sellers and his passenger to step out of the vehicle, at which point Sellers admitted to there being marijuana inside the vehicle. Officer Logan also observed a partially hidden bag of what appeared to be cocaine under the passenger seat as the passenger exited the vehicle. The police officers then placed Sellers and his passenger under arrest and searched the vehicle. The search uncovered marijuana, cocaine, a pistol, and roughly $3,000.
On September 17, 2008, as a result of the evidence gained from the search of James’s and Sellers’s vehicles, the GPS surveillance, the wiretaps, and other information, James, Sellers, and Matthews were indicted on drug conspiracy charges.
Following a series of superseding indictments, in the third superseding indictment James was ultimately indicted on the following seven charges: (1) conspiracy to possess with intent to distribute five kilograms or more of cocaine and 50 grams or more of cocaine base in violation of 21 U.S.C. §§ 841(a)(1), 841(b)(1)(A), and 846; (2) use of a communication facility to facilitate said conspiracy in violation of 21 U.S.C. § 843(b) and 18 U.S.C. § 2; (3) possession with intent to distribute 500 grams or more of cocaine in violation of 21 U.S.C. §§ 841(a)(1), 841(b)(1)(B), and 18 U.S.C. § 2; (4) possession with intent to distribute a quantity of cocaine in violation of 21 U.S.C. §§ 841(a)(1), 841(b)(1)(C), and 18 U.S.C. § 2; (5) money laundering in violation of 18 U.S.C. §§ 2,1956(a)(l)(A)(i), 1956(a)(l)(B)(i), and 1956(h); (6) possession with intent to distribute a quantity of cocaine within 1,000 feet of a school in violation of 21 U.S.C. §§ 841(a)(1), 841(b)(1)(C), 860(a), and 18 U.S.C. § 2; and (7) possession with intent to distribute 500 grams or more of cocaine within 1,000 feet of a school in violation of 21 U.S.C. §§ 841(a)(1), 841(b)(1)(B), 860(a), and 18 U.S.C. § 2.
Sellers was indicted on the following five charges: (1) conspiracy to possess with intent to distribute five kilograms or more of cocaine and 50 grams or more of cocaine base in violation of 21 U.S.C. §§ 841(a)(1), 841(b)(1)(A), and 846; (2) possession with intent to distribute a quantity of cocaine in violation of 21 U.S.C. §§ 841(a)(1), 841(b)(1)(C), and 18 U.S.C. § 2; (3) knowingly possessing and using a firearm during, in relation to and in furtherance of a drug trafficking crime in violation of 18 U.S.C. § 924(c)(1); (4) felon in possession of a firearm in violation of 18 U.S.C. §§ 922(g)(1), 924(a)(2), and 924(e); and (5) aiding and abetting in possession with intent to distribute a quantity of cocaine within 1,000 feet of a school in violation of 21 U.S.C. §§ 841(a)(1), 841(b)(1)(C), 860(a), and 18 U.S.C. § 2.
Matthews was indicted on the following charge: money laundering in violation of [324]*32418 U.S.C. §§ 2, 1956(a)(l)(A)(i), 1956(a)(l)(B)(i), and 1956(h).
Appellants pled not guilty and a jury trial commenced. At trial, Appellants moved to suppress evidence gained from the search of James’s vehicle, information gathered from the GPS device including the wiretaps that relied in part on the GPS data, and the search of Sellers’s vehicle. The district court denied Appellants’ motions to suppress. The jury found James, Sellers, and Matthews guilty on all counts.
During sentencing, the district court found by a preponderance of the evidence that in 2004 James had also committed the murder of Vance Davis. Relying on testimony from witnesses confirming James murdered Davis and at the time James was involved in the drug trade, the district court attributed Davis’s murder to James as conduct relevant to the drug conspiracy under Guidelines § lB1.3(a). Over James’s objection, the district court adopted the pre-sentence investigation report and applied the cross-reference found in Guidelines § 2Dl.l(d)(l) for first degree murder, Guidelines § 2A1.1, to increase James’s base offense level from 38 to 43.
James and Sellers were sentenced to life in prison on August 19, 2010 and June 17, 2010, respectively. Matthews was sentenced to 24 months in prison on June 17, 2010. Appellants each filed timely appeals to this court. James, Sellers, and Matthews challenge the district court’s denial of their motions to suppress evidence gained from the searches of James’s and Sellers’s vehicles and the information gained from the' GPS and resulting wiretaps. James and Sellers also appeal the district court’s imposition of their life sentences.
Thus, this court possesses jurisdiction pursuant to 28 U.S.C. § 1291 and 18 U.S.C. § 3742.
II.
On appeal, Appellants challenge the district court’s denial of their motions to suppress various categories of evidence. The following categories warrant our discussion here, and each will be addressed in turn: (A) evidence gained from the search of James’s vehicle; (B) evidence gained from the search of Sellers’s vehicle; and (C) the installation and use of a GPS device on James’s vehicle and the subsequent wiretaps that relied on the GPS information.
When reviewing a district court’s denial of a motion to suppress, this court reviews legal conclusions de novo, and any factual determinations for clear error. United States v. Kelly, 592 F.3d 586, 589 (4th Cir.2010). When the district court denies a defendant’s motion to suppress, as the reviewing court, we construe the evidence in the light most favorable to the government. Id. In the event we find a constitutional error on direct review, “the government has the burden of proving that a constitutional error was ‘harmless beyond a reasonable doubt.’ ” Bauberger v. Haynes, 632 F.3d 100, 104 (4th Cir.2011) (quoting Chapman v. California, 386 U.S. 18, 24, 87 S.Ct. 824, 17 L.Ed.2d 705 (1967)).
When reviewing a district court’s determination of necessity for the authorization of the interception of communications under 18 U.S.C. § 2518, we review for an abuse of discretion. United States v. Wilson, 484 F.3d 267, 280 (4th Cir.2007).
A.
Search of James’s Vehicle
James argues the district court erred when it denied his motion to suppress and found evidence gained from the search of his vehicle admissible as the product of a valid inventory search.
[325]*325As a starting point, to comply with the Fourth Amendment’s prohibition against unreasonable searches and seizures, generally police must obtain a warrant before conducting a search. Kelly, 592 F.3d at 589 (4th Cir.2010). A warrantless search may be valid however, and the resulting evidence admissible, if the search is conducted “ ‘within one of the narrow and well-delineated exceptions’ to the Fourth Amendment’s warrant requirement.” United States v. Currence, 446 F.3d 554, 556 (4th Cir.2006) (quoting Flippo v. West Virginia, 528 U.S. 11, 13, 120 S.Ct. 7, 145 L.Ed.2d 16 (1999)). One such exception includes evidence gained from a valid inventory search. United States v. Matthews, 591 F.3d 230, 234 (4th Cir.2009) (citing United States v. Banks, 482 F.3d 733, 738-39 (4th Cir.2007)).
The inventory search exception is applied when police officers impound vehicles or detain suspects. Matthews, 591 F.3d at 235. Its justifications are two-fold; an inventory search serves to protect the detainee’s property, and also protects the police from accusations of theft and from potentially dangerous items. Id. Once a suspect is detained, the inventory search exception does not give arresting officers carte blanche to rummage through the detainee’s property looking for possible evidence of criminal activity. In order for the inventory search exception to apply, the search must have been performed pursuant to standardized criteria — such as a uniform policy — and such criteria must have been administered in good faith. United States v. Banks, 482 F.3d 733, 739 (4th Cir.2007). Importantly, “nothing prohibits the discretion of police officers in making inventory searches so long as that discretion is based on standard criteria and on the basis of something other than the suspicion of criminal activity.” United States v. Ford, 986 F.2d 57, 60 (4th Cir.1993) (citing Colorado v. Bertine, 479 U.S. 367, 375, 107 S.Ct. 738, 93 L.Ed.2d 739 (1987)).
In this case, after police stopped and arrested James, James indicated to police that he had a significant amount of cash inside the vehicle. Prior to discovery of the money and rear-facing camera, Officer LaChance conducted the inventory search in compliance with state highway patrol procedures and in good faith in response to James’s notification that he had money in the vehicle. At the suppression hearing, Officer LaChance testified that the South Carolina state highway patrol inventory search procedures require officers to start searching on one side of a vehicle and progressively search to the other side. In the police video submitted to the district court, Officer LaChance clearly begins his search with the driver’s side door, before moving to the driver’s seat, and finally the inner console. Once Officer LaChance located the money in the inner console, he stopped his search and sought confirmation from his fellow officers that the money had been found. Also during this search, Officer LaChance noted the rear-facing camera installed on the dashboard.
Most of James’s objections center around the events that followed. After initially locating the money and rear-facing camera, a K-9 crew was called in and other officers repeatedly searched portions of James’s vehicle. James argues these repeated searches were not conducted pursuant to police inventory policy and were administered merely in an attempt to locate incriminating evidence.
James’s reliance on these arguments is misplaced. This court need not consider whether the subsequent searches were valid inventory searches simply because they produced no evidence that was ever admitted at trial. The money and rear-facing camera were discovered while Officer La-[326]*326Chance first initiated his inventory search pursuant to official policy. If, in fact, the subsequent searches by other officers and the K-9 search were not performed according to official policy or were performed in bad faith, it is of no moment because there is no indication in the record that any testimony related to the subsequent searches or physical evidence discovered during the subsequent searches was ever admitted at trial. Therefore, the district court did not err in denying James’s motion to suppress and allowing the money and photographs to be admitted as evidence that was discovered pursuant to a valid inventory search.
B.
Search of Sellers’s Vehicle
Sellers argues the district court erred when it denied his motion to suppress and found evidence gained from the search of his vehicle admissible. Sellers contends the search of his vehicle violated his Fourth Amendment right to be free from unreasonable searches and seizures, because the search did not fall within the parameters of the exception to the warrant requirement for the search of a vehicle incident to a recent occupant’s arrest, as articulated by the Supreme Court in Arizona v. Gant, 556 U.S. 332, 129 S.Ct. 1710, 173 L.Ed.2d 485 (2009).
Sellers mistakenly relies only on select portions of the Gant decision, and ignores other Supreme Court precedent. In Gant, the Supreme Court clarified the rules governing a search of an automobile incident to arrest set forth in New York v. Belton, 453 U.S. 454, 101 S.Ct. 2860, 69 L.Ed.2d 768 (1981), and held that two circumstances could authorize a warrantless search of a vehicle incident to an arrest: (1) the possibility of access to the vehicle by a recent occupant; or (2) the likelihood of discovering evidence related to the offense of arrest. 556 U.S. at 343, 129 S.Ct. 1710 (“Accordingly, we ... hold that ... [police may] search a vehicle incident to a recent occupant’s arrest only when the arrestee is unsecured and within reaching distance of the passenger compartment at the time of the search.... [W]e also conclude that circumstances unique to the vehicle context justify a search incident to a lawful arrest when it is reasonable to believe evidence relevant to the crime of arrest might be found' in the vehicle.” (footnote, citations, and internal quotation marks omitted)).
Here, police officers made a valid stop of Sellers for a traffic violation. See United States v. Hassan El, 5 F.3d 726, 730 (4th Cir.1993). Upon making the stop, the officers were permitted to direct Sellers and his passenger out of the vehicle. See Maryland v. Wilson, 519 U.S. 408, 415, 117 S.Ct. 882, 137 L.Ed.2d 41 (1997). If these were the only facts of this case, then the search would not have been permissible under Gant because Sellers and his passenger did not have access to the interior of the vehicle, and there would have been no basis for officers to believe there was a likelihood of discovering evidence related to a traffic violation inside the vehicle.
Importantly, however, the police officers in this case possessed more information. Officers Furtick and Logan noticed a strong odor of marijuana emanating from the vehicle, Sellers admitted to there being marijuana inside the vehicle, and Officer Logan observed a partially hidden bag of what appeared to be cocaine under the passenger seat as the passenger exited the vehicle.
In Gant, the Supreme Court noted that the search ineident-to-arrest exception is not the only exception that may justify the search of a vehicle. 556 U.S. at 346, 129 S.Ct. 1710. Indeed, “[o]ther established [327]*327exceptions to the warrant requirement authorize a vehicle search under additional circumstances when safety or evidentiary concerns demand.” Id. Most germane to this case, “[i]f there is probable cause to believe a vehicle contains evidence of criminal activity, United States v. Ross, authorizes a search of any area of the vehicle in which the evidence might be found.” Id. at 347, 129 S.Ct. 1710 (citation omitted).
In this case, the drug-related odor, admission, and visual identification clearly gave police officers probable cause to believe the vehicle contained evidence of criminal activity and justified the search under United States v. Ross, 456 U.S. 798, 102 S.Ct. 2157, 72 L.Ed.2d 572 (1982), even though the criminal drug activity was not the original offense that justified the arrest. The district court, therefore, did not err in denying Sellers’s motion to suppress the drugs, gun, and money gathered during the search of Sellers’s vehicle.
C.
GPS Device
James argues the district court erred by denying his motion to suppress evidence gained from the installation of a GPS device on his vehicle and its subsequent use. James contends the GPS tracking data should have been suppressed, as well as the resulting wiretaps that relied in part on the GPS data, as fruit of the poisonous tree.1
While James’s appeal was pending before this court, the Supreme Court in United States v. Jones, — U.S.-, 132 S.Ct. 945, 181 L.Ed.2d 911 (2012) addressed the precise issue implicated by this case: whether the installation of a GPS tracking device to a target’s vehicle, and its subsequent use, without a valid warrant, constituted an unlawful search. In Jones, the Supreme Court relied on the common-law trespassory test for a search — noting that the Katz v. United States, 389 U.S. 347, 88 S.Ct. 507, 19 L.Ed.2d 576 (1967) reasonable-expectation-of-privacy test merely supplemented but did not replace the trespassory test — and found that such installation and use violated the Fourth Amendment. 132 S.Ct. at 952. The Supreme Court also noted, “[tjrespass alone does not qualify [as a search], but there must be conjoined with that what was present here: an attempt to find something or to obtain information.” Id. at 951 n. 5. Just as in Jones, the DEA agents in this case attached a GPS device to a target’s vehicle and used the device to gain information about the target’s whereabouts, all absent a valid warrant.2 The search in this case, therefore, violated James’s Fourth Amendment rights.
In its briefing, the government does not challenge the applicability of Jones to the present case. Rather, the government argues that because Jones was decided after the events of this case, the DEA’s actions should be viewed through the good-faith standard as stated in Davis v. United States, — U.S.-, 131 S.Ct. 2419, 180 L.Ed.2d 285 (2011), and therefore any evidence gained from the GPS installation and its use should not be subjected to the exclusionary rule. This court need not venture along this line of inquiry for the result is the same even if Davis does not apply.
[328]*3281.
GPS Tracking Data
Even assuming Davis does not apply to this case, and the district court erred in denying James’s motion to suppress evidence gained from the GPS device, there is no indication that any data gained from the GPS device was ever introduced at trial. The government has stated that no such evidence was introduced, and Appellants do not offer anything in the record to the contrary. It is self-evident that there can simply be no error in the admission of evidence when evidence is not admitted. Appellants simply do not identify any direct tracking information gained from the use of the GPS device that was ever introduced either in testimony or by exhibit.
2.
Wiretaps
The main thrust of James’s argument on appeal, however, is not that the district court should have explicitly suppressed direct GPS data, but rather that the district court should have suppressed any evidence that was gained as a proximate result of the GPS data — specifically the February through July 2008 wiretaps. In essence, James argues that because the wiretap applications in part contained information derived from an illegal GPS search, any evidence gained from the wiretaps should be suppressed as “fruit of the poisonous tree.” James argues this result is required because without the GPS data, the wiretap applications lacked the required showing of probable cause and necessity. We disagree.
Congress has provided a statutory framework to guide the regulation of wiretapping and electronic surveillance. United States v. Apple, 915 F.2d 899, 904 (4th Cir.1990). In order to issue a wiretap, a judge must determine, on the basis of the application for the wiretap, that probable cause exists to believe that (1) an individual is committing, has committed, or is about to commit an offense enumerated in 18 U.S.C. § 2516; (2) particular communications concerning that offense will be obtained by the wiretap; and (3) the target facilities will be used in connection with the offense. 18 U.S.C. § 2518(3); see United States v. Webster, 639 F.2d 174, 177 (4th Cir.1981). The trial judge must also determine (4) the necessity for the wiretap — that is, normal investigative procedures have been tried and have failed or reasonably appear to be unlikely to succeed if tried or appear to be too dangerous. 18 U.S.C. § 2518(3)(c).
The wiretap statute also provides grounds for the suppression of wiretap communications. Id. §§ 2515; 2518(10)(a). For example, a defendant may seek suppression of wiretap communications when “the order of authorization or approval under which [the communication] was intercepted is insufficient on its face.... ” Id. § 2518(10)(a)(ii). Although at issue here is the availability of suppression for a statutory violation, as a opposed to a constitutional violation, Fourth Amendment principles may also inform the court’s analysis. See United States v. Clerkley, 556 F.2d 709, 719 (4th Cir.1977) (“In other words, the violation must substantially impinge upon Fourth Amendment values sought to be protected by Congress in restricting and rendering uniform the use of wiretaps.”); see also United States v. Baranek, 903 F.2d 1068, 1072 (6th Cir.1990) (noting that although the wiretap statute may provide greater protection than the Fourth Amendment, Fourth Amendment evidence suppression doctrines are still relevant).
In this case, setting aside the allegedly impermissible GPS information contained in the wiretap application, the surviving [329]*329information contained in the application remained sufficient to support a finding of probable cause and necessity required to issue the wiretap. See United States v. Gillenwaters, 890 F.2d 679, 681-82 (4th Cir.1989) (recognizing that courts have set aside suspect material in an affidavit for a search warrant and then evaluated probable cause, even when the suspect information was obtained through an illegal search). Like the probable cause standard for a search warrant, the probable cause standard necessary to comply with 18 U.S.C. § 2518(8) requires not absolute certainty, but rather a “fair probability” that evidence of the subject offense will be uncovered given the totality of the circumstances. United States v. DePew, 932 F.2d 324, 327 (4th Cir.1991) (citing United States v. Alfano, 838 F.2d 158, 161-62 (6th Cir.1988)). Similarly, the showing of “necessity” for the wiretap is not prodigious and “the Government ‘need only present specific factual information sufficient to establish that it has encountered difficulties in penetrating the criminal enterprise or in gathering evidence [such that] wiretapping becomes reasonable.’ ” United States v. Wilson, 484 F.3d 267, 281 (4th Cir.2007) (quoting United States v. Smith, 31 F.3d 1294,1298 (4th Cir.1994)).
Excising the allegedly impermissible GPS information from the wiretap applications in this case, it is clear the wiretap applications satisfy the probable cause and necessity standards. For example, the affidavits set forth the array of investigatory techniques DEA officers employed to shed light on the drug conspiracy. The techniques employed by DEA officers included extensive stationary and mobile physical surveillance, interviews, telephone pen registers, telephone toll records, and the analysis of public records. The affidavits also describe the difficulties faced by officers in this case, including: performing discreet mobile and stationary surveillance and collecting garbage in the sparsely populated rural environment of the targets; electronic counter-surveillance measures and “look-outs” employed by the targets; evasive measures taken by targets in response to perceived surveillance; and the inability to insert undercover agents in, or recruit confidential sources from, the target’s organization. This showing was sufficient. Although the district court abused its discretion in relying on the additional GPS information in issuing the wiretaps, inclusion of this information was harmless beyond a reasonable doubt and a good faith inquiry under Davis is not required. See Bauberger v. Haynes, 632 F.3d 100, 104 (4th Cir.2011).
Therefore, we affirm the convictions of Appellants James, Sellers, and Matthews.
III.
Appellants James and Sellers also challenge the district court’s imposition of a life sentence. James argues the district court erred when it found by a preponderance of the evidence that he murdered Vance Davis and attributed the murder at sentencing, as “relevant conduct” to the drug conspiracy in this case. James and Sellers also both argue their life sentences are not proportional to their crimes and therefore violate the Eighth Amendment’s prohibition on cruel and unusual punishment.
In reviewing a sentence imposed by the district court, we review for both procedural and substantive reasonableness. United States v. Lynn, 592 F.3d 572, 575 (4th Cir.2010). We review unpreserved challenges of procedural sentencing error for plain error. Id. at 576-77. We review preserved challenges of procedural sentencing error for an abuse of discretion and reverse unless we conclude that the error was harmless. Id. at 576. If the [330]*330sentence is proeedurally reasonable, we then review the underlying substantive reasonableness of the sentence under an abuse of discretion standard. Id. at 575. If we find the sentence to be proeedurally unreasonable, we are foreclosed from reviewing the underlying substantive reasonableness of the sentence. United States v. Horton, 698 F.3d 463, 472 (4th Cir.2012).
In reviewing whether a sentence is within the constitutional limits of the Eighth Amendment’s prohibition against “cruel and unusual punishments” an appellate court asks whether the imposed sentence is proportionate to the crime committed. Solem v. Helm, 463 U.S. 277, 290, 103 S.Ct. 3001, 77 L.Ed.2d 637 (1983). To determine proportionality, we consider “(1) the gravity of the offense and the harshness of the penalty; (2) the sentences imposed on other criminals in the same jurisdiction; and (3) the sentences imposed for commission of the same crime in other jurisdictions.” United States v. Wellman, 663 F.3d 224, 231 (4th Cir.2011) (citing Solem, 463 U.S. at 292, 103 S.Ct. 3001).
Relevant Conduct Enhancement
During sentencing, the district court found by a preponderance of the evidence that James had committed the murder of Vance Davis and that this qualified as relevant conduct under the sentencing guidelines. Specifically, the district court relied on two witnesses who confirmed that James murdered Davis. Over James’s objection, the district court adopted the pre-sentence investigation report and applied the cross-reference found in Guidelines § 2Dl.l(d)(l) for first degree murder, Guidelines § 2Al.l. This application of the cross-reference increased James’s offense level under § 2D1.1 from 38 to 43.
In reviewing an enhancement for relevant conduct for procedural error, we conduct two separate analyses. First, we review for clear error the district court’s factual finding that the defendant committed the relevant conduct offense by a preponderance of the evidence. Horton, 693 F.3d at 474-75. Second, we review de novo the district court’s legal determination that the relevant conduct offense qualifies for the applicable cross-reference provision in the Guidelines. Id.
As this court noted in Horton, the clear error standard of review is “very deferential” toward the factual findings made by the district court. 693 F.3d at 474. At sentencing, the district court heard testimony by two witnesses, Robert Jones and Avery Haigler, each indicating James had murdered Vance Davis. Although James pointed out that there were certain discrepancies between other evidence and the testimony of Jones and Hai-gler, that fact alone, does not overcome the fact that there was a sufficient amount of evidence for the court to conclude, by a preponderance of the evidence, that James murdered Davis.
Finding the district court did not clearly err in its factual determination that James murdered Davis, we now turn to the legal question of whether the relevant conduct offense can be applied under the cross-reference provision. In reviewing the district court’s determination of relevant conduct for sentencing purposes, the interrelation of two provisions in the Guidelines requires discussion: (1) § 2Dl.l(d) (“the Cross-Reference Provision”); and (2) § 1B1.3 (“the Relevant Conduct Guideline”).
The offense level for James’s offenses of conviction — the drug conspiracy and possession convictions — is established under Guideline § 2D1.1. Subsection (d) of § 2D1.1 provides the Cross-Reference [331]*331Provision, which states in relevant part: “If a victim was killed under circumstances that would constitute murder under 18 U.S.C. § 1111 ... apply § 2A1.1 (First Degree Murder) or § 2A1.2 (Second Degree Murder), as appropriate, if the resulting offense level is greater than that determined under this guideline.” USSG § 2Dl.l(d)(l).
Unless otherwise specified in the Guidelines, the application of the Cross-Reference Provision depends on whether the cross-referenced offense — in this case, first degree murder — constitutes relevant conduct under the Relevant Conduct Guideline found in § lB1.3(a). Horton, 693 F.3d at 476. Neither the parties, the district court, nor the Pre-Sentence Investigation Report identifies whether the cross-referenced offense should be considered relevant conduct under § 1B1.3 (a)(1) or § 1B1.3 (a)(2), or any other provision for that matter. Under any provision, however, we find the cross-referenced murder offense not to be relevant conduct attributable to the drug conspiracy in this case.
The Relevant Conduct Guideline, § IB 1.3(a), states in relevant part, that the application of a cross-reference shall be determined on the basis of:
(1)(A) all acts and omissions committed, aided, abetted, counseled, commanded, induced, procured, or willfully caused by the defendant; and
that occurred during the commission of the offense of conviction, in preparation for that offense, or in the course of attempting to avoid detection or responsibility for that offense;
(2) solely with respect to offenses of a character for which § 3D1.2(d) would require grouping of multiple counts, all acts and omissions described in subdivisions (1)(A) and (1)(B) above that were part of the same course of conduct or common scheme or plan as the offense of conviction;....
USSG § lB1.3(a).
Here, the Davis murder cannot be considered relevant conduct under either provision. First, with respect to § lB1.3(a)(l)(A), the evidence is insufficient to conclude that the murder occurred during, in preparation for, or in the course of attempting the drug conspiracy in this case. While all the witnesses at sentencing agreed James had murdered Davis, their testimony was inconsistent, at best, as to whether the murder involved drug-related activities, or was an act of vengeance for Davis’s alleged attempted robbery of James’s mother’s residence.
Although James’s pre-sentence report indicates that Lenell Walker stated to police officers that Davis stole drugs from James, prompting James’s retribution, the testimony of Robert Jones and Avery Hai-gler at sentencing provides no such connection. At most, Jones and Haigler indicated that at the time of Davis’s murder, James was involved in the drug trade. Neither witness, however, indicated Davis successfully stole drugs from James or that they believed Davis was motivated by James’s involvement in the drug trade. In fact, both Jones and Haigler testified that James was infuriated not because of any connection the attempted break-in may have had to drugs, but rather because the attempted break-in occurred at his mother’s residence. In short, their testimony, and the overall weight of the evidence, does not connect the attempted robbery and the Davis murder with drug-related activity.
Furthermore, there was no evidence Davis was ever a part of the drug conspiracy in this ease, or that James ever sold [332]*332drugs to, or purchased them from, Davis.3 Even if the murder happened to be “drug-related” in some sense, nothing connects the Davis murder to the particular drug conspiracy in this case.
James was never charged with Davis’s murder nor had he otherwise ever been implicated in the murder up until the present case. We cannot simply assume every act committed by a convicted criminal, no matter how heinous, is connected and relevant to the offense of conviction. To do so would turn the relevant conduct analysis into an impermissible conduit for punishing uncharged and unproven conduct and would circumvent the criminal process.
Second, § lB1.3(a)(2) is also inapplicable to the present case. As we recently held in Horton, in order for § 1B1.8(a)(2) to apply, both the offense of conviction and the relevant conduct offense must be capable of grouping. Horton, 693 F.3d at 479. Because homicide offenses are explicitly excluded from grouping, § 1B1.3(a)(2) cannot apply. See id. at 477 (noting that § 1B1.3(a)(2) relies on § 3D1.2(d), which in turn excludes Chapter Two, Part A offenses (except § 2A3.5), which include the guideline for murder).
We therefore conclude that the district court committed procedural error in finding the Davis murder to be relevant conduct to the underlying conspiracy in this case, and thus increasing James’s base offense level under the cross-reference provision.
Proportionality of Life Sentences
With respect to Appellant James, because we find significant procedural error in the calculation of his sentence, we do not reach the underlying substantive reasonableness of his sentence. See Horton, 693 F.3d at 472. Finding no error with respect to Appellant Sellers, however, nothing prevents us from considering the proportionality of Sellers’s life sentence.
As noted, in reviewing the proportionality of a life sentence, this circuit applies the three-part test found in Solem, 463 U.S. at 292, 103 S.Ct. 3001. We examine “(1) the gravity of the offense and the harshness of the penalty, (2) the sentences imposed on other criminals in the same jurisdiction, and (3) the sentences imposed for commission of the same crime in other jurisdictions.” United States v. Kratsas, 45 F.3d 63, 66 (4th Cir.1995) (citing Solem, 463 U.S. at 292, 103 S.Ct. 3001).
In this case, the district court correctly applied the Solem test and determined Sellers’s statutorily required life sentence was not constitutionally disproportionate in violation of the Eighth Amendment. Under the first prong of the Solem test, Sellers’s offenses were considerably serious. Sellers was determined by the district court to be part of a conspiracy to distribute five kilograms or more of cocaine. This circuit has recognized that smaller amounts of drugs in a conspiracy can be equally serious. United States v. Kellam, 403 Fed.Appx. 815, 817 (4th Cir. [333]*3332010) (finding the gravity of 500 grams to 1.5 kilograms of drugs included in a conspiracy to be sufficiently serious to satisfy the first Solem prong). Sellers also possessed a significant criminal history related to drug convictions. This circuit has repeatedly found that life sentences for similar major drug conspiracies for defendants with a history of drug convictions are not disproportionate to similar sentences under the Sentencing Guidelines and sentences imposed by states within this circuit. See Id. at 817 (citing United States v. D’Anjou, 16 F.3d 604, 613-14 (4th Cir.1994)).
Accordingly, Sellers’s life sentence is not constitutionally disproportionate to constitute an Eighth Amendment violation.
IV.
For the aforementioned reasons, we affirm the convictions of James, Sellers, and Matthews. We also affirm Sellers’s sentence. We vacate James’s sentence and remand to the district court for resentenc-ing.4
AFFIRMED IN PART, VACATED IN PART, AND REMANDED.