TARNOW, Senior District Judge.
Plaintiff-Appellant Michael Gardner filed suit under the Federal Tort Claims Act (hereinafter “FTCA”), 28 U.S.C. § 1346, 2671, et seq., against Defendant-Appellee United States, alleging that due to the negligence of the government, through its employees, in failing to treat his condition, he suffered from an impaired mental state that caused him to be charged with aggravated assault and sentenced to a term of incarceration. The United States filed a motion to dismiss, which the district court granted. Gardner now appeals. Gardner also filed a motion asking us to certify two questions of state law to the Tennessee Supreme Court. For the reasons that follow, we AFFIRM the district court’s dismissal of Gardner’s complaint and DENY the motion for certification.
I. BACKGROUND
On October 7, 2003, Gardner entered a Veterans Affairs hospital in Murfreesboro, Tennessee with a shotgun and other firearms and opened fire on employees. Gardner was charged on November 10, 2003 in a nine-count criminal complaint with three counts of attempting to kill an employee of the United States, four counts of forcibly assaulting and intimidating an employee of the United States, one count of using, carrying, and discharging a firearm during and in relation to a violent crime, and one count of knowingly possessing a firearm in a federal facility, with intent to use it in the commission of a crime.
On December 10, 2004, Gardner pled guilty to aggravated assault under 18 U.S.C. § 111. He was sentenced in February 2005 to eight of years imprisonment, three years of supervised release, and a $100 special assessment.
On December 21, 2006, Gardner filed this suit under the FTCA in federal district court in Tennessee against the United States, alleging that government employees were negligent in treating him between July 2002 and October 2003. He states that due to this negligent treatment:
Gardner became suicidal, dangerous and, left untreated, acted violently resulting in conviction and imprisonment. Specifically, Defendant’s employees/agents were negligent in failing to timely diagnose, manage and treat Michael Gardner for his depression and mental instability, which resulted in pain and suffering, emotional distress, permanent impairment, suicidal thoughts, impaired memory, a major depressive disorder, acute stress disorder and disassociative amnesia. As a direct and proximate result of the above negligent acts, negligent omissions, negligent medical treatment and/or medical malpractice by the Defendant’s, agents, employees and/or servants, Plaintiff Michael Gardner suffered a complete mental breakdown causing him to be charged with aggravated assault and sentenced [72]*72to federal incarceration for eight (8) years.
Compl. at 2. The complaint sought “damages resulting from the negligent treatment ..., including pain, suffering, mental anguish, loss of enjoyment of life, loss of earning capacity, and loss of liberty” and requested a two-million-dollar judgment. Id. at 3.
Gardner subsequently moved to file an amended complaint, which the district court permitted over the government’s opposition. The amended complaint added one paragraph to the original complaint, specifically including an allegation that Gardner was insane when he committed the aggravated assault for which he had pled guilty. The added paragraph states:
Plaintiff alleges that he was not of sound mind, had suffered a complete mental breakdown, was insane and not capable of appreciating the differences between right and wrong at the time he committed the acts as alleged in the criminal proceedings for which he was charged and pleaded guilty resulting in his sentencing to a federal prison. Plaintiff Michael Gardner’s impaired mental condition was the proximate result of the negligence of the United States, through its employee(s) in failing to treat him for his emotional and mental disorders in accordance with the recognized standards of professional practice.
Am. Compl. at 2.
The government moved to dismiss the amended complaint on August 15, 2007, arguing that Gardner’s claims were barred by collateral estoppel and, citing to various cases including Moss v. Mid-South Hospital, 1989 WL 134666 (Tenn.Ct.App.1989) (unpublished), Tennessee public policy.
Gardner argued in response that collateral estoppel did not apply. Noting that Moss was unpublished, Gardner also filed a motion to certify to the Tennessee Supreme Court the question: “Whether under Tennessee law a Plaintiff may bring a medical malpractice case based upon psychiatric malpractice where it is alleged that as a result of a mental healthcare provider’s psychiatric malpractice the Plaintiff committed, while of unsound mind and insane, criminal acts which resulted in incarceration.” Mot. for Certification of Questions of State Law to Tenn. Sup.Ct. at 1. Gardner’s certification motion maintained that an Illinois Court of Appeals case, Boruschewitz v. Kirts, 197 Ill.App.3d 619, 144 Ill.Dec. 73, 554 N.E.2d 1112 (Ct.App.1990), cited Moss but did not follow its holding and that the Tennessee Supreme Court had not ever reached the issue addressed in Moss.
On April 7, 2008, the district court denied the certification motion without any explanation, stating only that the motion was reviewed and was being denied.
On September 30, 2009, the district court granted the government’s motion to dismiss. Although the district court cited Moss in its order, it did not rely on Tennessee public policy as a ground for dismissing the amended complaint. Rather, it concluded that collateral estoppel barred Gardner’s claims. The court observed that Gardner had pled guilty to aggravated assault under 18 U.S.C. § 111 and found that he “had a full and fair opportunity to litigate the issue of his mental state”; therefore, he could not now claim insanity at the time of his criminal acts and “prove that the medical malpractice was the proximate cause of his harm alleged: his incarceration for aggravated assault.” Order and Mem. Op. Granting Summ. J. at 5.
Gardner now appeals. Additionally, he moves this Court to certify two questions of state law to the Tennessee Supreme Court, including the question he had previously asked the district court to certify, as [73]*73well as the question of whether collateral estoppel bars his claims under Tennessee law.
II. DISCUSSION
A. Standard of Review
This court “review[s] de novo the district court’s ruling on a motion to dismiss a claim.” Jones v. City of Cincinnati, 521 F.3d 555, 559 (6th Cir.2008). In order to defeat a motion to dismiss, a party’s:
complaint must contain either direct or inferential allegations respecting all material elements to sustain a recovery under some viable legal theory. We need not accept as true legal conclusions or unwarranted factual inferences, and con-clusory allegations or legal conclusions masquerading as factual allegations will not suffice. [T]he complaint’s [f]actual allegations must be enough to raise a right to relief above the speculative level, and state a claim to relief that is plausible on its face.
Terry v. Tyson Farms, Inc., 604 F.3d 272, 275-76 (6th Cir.2010) (citations and internal quotation marks omitted). Moreover,:
In determining whether to grant a Rule 12(b)(6) motion, the court primarily considers the allegations in the complaint, although matters of public record, orders, items appearing in the record of the case, and exhibits attached to the complaint, also may be taken into account. This circuit has further held that documents that a defendant attaches to a motion to dismiss are considered part of the pleadings if they are referred to in the plaintiffs complaint and are central to her claim.
Amini v. Oberlin College, 259 F.3d 493, 502 (6th Cir.2001) (citations, internal quotation marks, and emphasis omitted).
Additionally, “[t]he availability of collateral estoppel is a mixed question of law and fact which this court reviews de novo.” Hammer v. INS, 195 F.3d 836, 840 (6th Cir.1999).
Finally, “[i]t is an established principle of law that a reviewing court can sustain the judgment of a lower court on any ground that finds support in the record[,]” United States v. Anderson County, 761 F.2d 1169, 1174-1175 (6th Cir.1985), “including grounds not relied upon by the district court.” Hensley Mfg. v. ProPride, Inc., 579 F.3d 603, 609 (6th Cir.2009).
B. Discussion
1. FTCA
Gardner filed this suit under the FTCA, which grants federal district courts jurisdiction over civil actions seeking money damages against the United States for personal injury caused by negligence of government employees acting within the scope of their employment “if a private person[ ] would be liable to the claimant in accordance with the law of the place where the act or omission occurred.” 28 U.S.C. § 1346(b)(1); see also Massachusetts Bonding & Ins. Co. v. United States, 352 U.S. 128, 77 S.Ct. 186, 1 L.Ed.2d 189 (1956). Here, the underlying acts occurred in Tennessee; thus, its substantive law controls. Under Tennessee law, “a plaintiff bringing a negligence action must prove: (1) a duty of care owed by the defendant to the plaintiff; (2) conduct falling below the applicable standard of care amounting to a breach of that duty; (3) an injury or loss; (4) causation in fact; and (5) proximate, or legal cause.” Kellner v. Budget Car & Truck Rental, Inc., 359 F.3d 399, 403 (6th Cir.2004) (citations and internal quotation marks omitted).
While state law guides the underlying negligence claim, the parties do not dispute that federal law controls the question of whether collateral estoppel bars this [74]*74action.1 See Resp. to Certification Mot. at 4; Reply to Certification Mot. at 2.
2. Collateral Estoppel
Collateral estoppel applies to a federal court’s decision if:
(1) the precise issue raised in the present case must have been raised and actually litigated in the prior proceeding; (2) determination of the issue must have been necessary to the outcome of the prior proceeding; (3) the prior proceeding must have resulted in a final judgment on the merits; and (4) the party against whom estoppel is sought must have had a full and fair opportunity to litigate the issue in the prior proceeding.
Smith v. SEC, 129 F.3d 356, 362 (6th Cir.1997).
Furthermore, “[n]o rule of law precludes a prior criminal conviction from having preclusive effect in a subsequent civil proceeding between the government and the defendant.” Beaty, 245 F.3d at 624. “A guilty plea is as much a conviction as a conviction following [a] jury trial.” Gray v. Comm’r, 708 F.2d 243, 246 (6th Cir.1983) (affirming lower court’s ruling that collateral estoppel barred plaintiffs complaint and observing that “[n]umerous federal courts have held that a conviction for federal income tax evasion, either upon a plea of guilty, or upon a jury verdict of guilt, conclusively establishes fraud in a subsequent civil tax fraud proceeding through application of the doctrine of collateral estoppel.”)
a. Gardner’s failed arguments
Gardner argues on appeal that the district court erred in granting the motion to dismiss based on collateral estoppel. He maintains that the district court’s “implied reasoning process appears to have been that pleading guilty to a crime of intent somehow foreclosed proof of causation with respect to the VA’s prior negligent treatment.” Br. of Appellant at 11. According to Gardner, Tennessee law does not foreclose a psychiatric malpractice action simply because a person pleads guilty to a crime or commits an intentional act. In support of this argument, Gardner cites a line of Tennessee state cases involving situations where individuals committed suicide and suits were then brought against care providers.2
Gardner’s “suicide” argument (and the cases offered in support) was never raised below and is therefore forfeited. See United States v. Ninety-Three Firearms, 330 F.3d 414, 424 (6th Cir.2003) (“This court has repeatedly held that it will not consider arguments raised for the first time on appeal unless our failure to consider the issue will result in a plain misear-[75]*75riage of justice”) (citation and internal quotation marks omitted). However, irrespective of forfeiture, Gardner’s argument lacks merit, as the cited cases specifically involve the intentional act of suicide (which is not illegal under Tennessee law, despite Gardner’s claims — without citation — to the contrary3) and were suits resulting from failure to provide proper medical care. None of those cases involve an individual who waived an insanity defense, plead guilty to a general intent crime in federal court, and then brought suit against their medical provider claiming the provider’s negligence made them insane and caused them to commit the crime.4 Those cases do not even discuss the issue of collateral estoppel.
Gardner also contends that, when he pled guilty, he never conceded that improper medical care was not a factor in his criminal acts and thus this issue, which is the gravamen of this suit, was never actually litigated. His state of mind, in his view, “is irrelevant” to his civil action. However, this argument misses the mark, as Gardner’s entire complaint is premised on a claim of negligent treatment that resulted in his becoming insane and then committing the underlying offenses. Indeed, Gardner amended his complaint to include the allegation that he was insane when he committed aggravated assault. In such a suit, the fact finder would thus have to evaluate his state of mind.
Gardner further directs the court to an Illinois state-court decision, Talarico v. Dunlap, 177 Ill.2d 185, 226 Ill.Dec. 222, 685 N.E.2d 325 (1997), and a Kentucky state-court decision, Gossage v. Roberts, 904 S.W.2d 246 (Ky.1995), maintaining that those cases support a holding that collateral estoppel does not bar the claims at issue here. Gardner’s reliance on these cases is misplaced, as both are state-law cases that do not determine what federal law is regarding collateral estoppel.
b. Gardner’s differing standards argument
Gardner additionally argues that since the test for establishing insanity in civil cases in Tennessee differs from the test for proving insanity in federal criminal proceedings, the issue of insanity being litigated in this matter is unlike what was addressed in the prior proceedings and collateral estoppel would therefore not apply. The government argued before the district court that the test for insanity is the same in both the criminal and civil contexts.
The district court found that Gardner pled guilty in his criminal case to aggravated assault under 18 U.S.C. § 111, which is a general-intent crime that “requires only that a defendant intend to do the act that the law proscribes.” United States v. Kimes, 246 F.3d 800, 806 (6th Cir.2001) (citation and internal quotation marks omitted). Gardner acknowledged that he was pleading guilty because he was guilty and that he was “entering into this plea after being advised of and understanding the potential for raising defenses and seeking sentencing departures based upon mental health issues[.]” Tr. of Plea Hr’g [76]*76Ex. 2, Dec. 10, 2004, at 14 (emphasis added). The district court found that “[because Plaintiff cannot relitigate the question of his intent, he cannot now assert that medical malpractice resulted in a complete mental breakdown — rendering him insane and therefore devoid of intention— at the time of the criminal acts.” Order and Mem. Granting Summ. J. at 5 (emphasis added).
We hold that the district court was correct that the issue of intent, for purposes of the instant action, was already determined by Gardner’s guilty plea in his criminal case. Gardner, by his plea, admitted he intended to do the act of aggravated assault. He cannot now deny that in this action.5
Under federal law, insanity is an affirmative defense that may be raised in a criminal case. 18 U.S.C. § 17 states: “It is an affirmative defense to a prosecution under any Federal statute that, at the time of the commission of the acts constituting the offense, the defendant, as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts. Mental disease or defect does not otherwise constitute a defense.” The defendant must prove this defense by “clear and convincing evidence.” Id. Gardner waived this insanity defense in his criminal case and pled guilty, thus conceding that he was not insane under the federal standard at the time he committed his crime. Therefore, Gardner cannot now claim in his civil suit that, under the federal insanity standard, he actually was insane at the time he committed the act.
Accordingly, if the Tennessee standard for establishing insanity in civil cases contained the same elements and required the same standard of proof as the federal standard, then collateral estoppel, as the district court found, would bar Gardner’s suit.
On the other hand, if Gardner were correct that the elements and standard of proof for establishing insanity differ in the civil negligence context from the criminal context, his guilty plea would not foreclose his ability to raise issues in this action regarding his mental state under Tennessee law. A finding that he was insane under a state standard for insanity that differed from the federal standard (in that the elements differ and the standard of proof differ) would not conflict with a conclusion in the negligence action that he intended to commit the act of aggravated assault, which was necessary element. Such a finding would not negate his intent. See United States v. Allen, 449 F.3d 1121, 1125 (10th Cir.2006) (“insanity is, as the statute expressly says, an affirmative defense. The fundamental concept of an affirmative defense is that it does not negate an element of the adversary’s case.”); see also United States v. Vela, 624 F.3d 1148, 1156 n. 11 (9th Cir.2010) (“We are satisfied that a jury could reasonably find that an insane person had a specific intent to commit a crime and yet was unable to appreciate the nature and quality or the wrongfulness of his acts”) (citation and internal quotation marks omitted); United States v. Gonyea, 140 F.3d 649, 651 (6th Cir.1998) (“insanity is a defense to all crimes, regardless of whether they require general or specific intent”); United States v. Willis, 1999 WL 591440 at *6, 1999 U.S.App. LEXIS 18298 at *18 (6th Cir.1999) (unpublished) (“We think it is important to distinguish between the use of psychological testimony to negate an element of the crime [77]*77and the use of such testimony as an affirmative defense to the crime.”)
It is ultimately unnecessary for this Court to resolve the parties’ dispute as to what the proper elements and standard of proof are under Tennessee law for establishing insanity in a Tennessee civil negligence action, as the district court’s ruling may be affirmed on grounds other than collateral estoppel.
3. Public Policy
Although the government argued before the district court that Tennessee public policy barred Gardner’s claims, that court did not reach a decision on the issue. However, this ground also supports the dismissal of the suit.
In Moss, 1989 WL 134666, an individual convicted of murder filed a negligence action against medical providers alleging that, as a result of their negligent treatment, he suffered from mental illness and committed murder. The Tennessee Court of Appeals observed: “According to our research, this is the first time a Tennessee appellate court has had an opportunity to address the issue of whether a mental health care provider is subject to liability for injury to a former patient caused by the former patient’s conviction of a crime. Very few courts throughout this country have addressed this issue.” Id. at *2. The court went on to hold that Tennessee recognizes the general rule:
that a person cannot maintain an action if, in order to establish his cause of action, he must rely, in whole or in part, on an illegal or immoral act or transaction to which he is a party, or to maintain a claim for damages based on his own wrong or caused by his own neglect, ... or where he must base his cause of action, in whole or in part, on a violation by himself of the criminal or penal laws....
Id. at *2 (citation omitted). Since the plaintiff was seeking recovery for an injury caused by his own criminal acts, the court concluded “that it would be, plain and simply, wrong as a matter of public policy to allow recovery.” Id. at *2 (citation and internal quotation marks omitted).
The government argued before the district court that Moss barred this suit. In response, Plaintiff relied on a decision from the Illinois Court of Appeals, Boruschewitz v. Kirts, 197 Ill.App.3d 619, 144 Ill.Dec. 73, 554 N.E.2d 1112 (Ct.App.1990), where a plaintiff who plead guilty but mentally ill to murder brought suit against medical providers for negligent treatment that he alleged led him to commit his illegal act. The court found that the suit did not violate Illinois public policy and allowed it to proceed.
Gardner again cites Boruschewitz on appeal in support of his argument that Tennessee public policy permits this suit. However, the Illinois Supreme Court’s decision that Illinois public policy did not bar the suit before it has little relevance to what Tennessee public policy permits.6
Gardner also argues on appeal that the line of cases cited above regarding suicide demonstrates that public policy permits this suit. However, that argument was not raised below and is therefore forfeited. Regardless of forfeiture, Gardner’s argument fails, as those cases do not even discuss Tennessee public policy pertaining to recovering damages for one’s own criminal acts. As addressed above, they involve [78]*78suits by parties against medical providers following a suicide, whereas this case involves a party who waived a federal insanity defense, pled guilty to committing a felony, and is now attempting to recover damages based on claims that the negligence of medical providers caused him to go insane and commit the acts.
Although Moss is an unpublished case and is therefore not controlling, it still may serve as persuasive authority. See Hood v. Keller, 229 Fed.Appx. 393, 398 n. 5 (6th Cir.2007) (unpublished) (unpublished decisions of this Court “may constitute persuasive authority especially where there are no published decisions which will serve as well”) (citation and internal quotation marks omitted); see also Eady v. Morgan, 515 F.3d 587, 601 (6th Cir.2008). Its facts are analogous to those in the instant suit. Gardner has not offered a sufficient reason why this Court should disregard Tennessee public policy as enunciated in Moss and instead rely on out-of-state cases and a line of Tennessee cases that does not address the public policy concerns raised in this matter.
Tennessee public policy therefore bars Gardner’s complaint.
4. Motion for Certification
Finally, Gardner moves this Court to certify two questions to the Tennessee Supreme Court:
1) Whether under Tennessee law a Plaintiff is collaterally estopped (by virtue of a guilty plea to criminal charges) from pursuing a medical malpractice case based upon psychiatric malpractice where it is alleged that as a result of a mental healthcare provider’s psychiatric malpractice the Plaintiff committed, while of unsound mind and insane, criminal acts which resulted in a voluntary guilty plea and incarceration.
2) Whether under Tennessee law a Plaintiff may bring a medical malpractice case based upon psychiatric malpractice where it is alleged that as a result of a mental healthcare provider’s psychiatric malpractice the Plaintiff committed, while of unsound mind and insane, criminal acts which resulted in incarceration. Compare Moss v. Mid-South Hospital, 1989 WL 134666 (Tenn. Ct.App.1989) with Boruschewitz v. Kiris, 197 Ill.App.3d 619, 144 Ill.Dec. 73, 554 N.E.2d 1112 (Ct.App.1990).
Br. of Appellant at 1.
“As the certification of an issue of state law to a state supreme court is a matter of the district court’s discretion, we review certification decisions for an abuse of discretion.” Murphy v. Owens-Corning Fiberglas Corp., 1997 WL 705185, at *3, 1997 U.S.App. LEXIS 30530 at *10-11 (6th Cir.1997) (international citation omitted) (unpublished). This Court has found that “Certification is most appropriate when the question is new and state law is unsettled. As the Tenth Circuit has noted, however, the federal courts generally will not trouble our sister state courts every time an arguably unsettled question of state law comes across our desks. When we see a reasonably clear and principled course, we will seek to follow it ourselves.” Pennington v. State Farm Mut. Auto. Ins. Co., 553 F.3d 447, 450 (6th Cir.2009) (citations and internal quotation marks omitted).
Gardner’s request for certification of the first question, which was not raised before the district court, lacks merit since both parties agree that the issue of whether collateral estoppel bars the claims at issue is a question of federal law. Appellant’s Mot. Reply at 2.
The district court did not abuse its discretion in denying Gardner’s certification motion as to the second question, where the only basis for certification Gardner [79]*79offered was that the Tennessee Court of Appeals’ decision in Moss was unpublished and the Illinois Court of Appeals in Borus-chewitz concluded that plaintiffs suit was not barred by Illinois public policy. How the Illinois state courts interpret Illinois law has no bearing on how a Tennessee state court interprets Tennessee law.
Gardner also makes additional arguments regarding certification not raised below. Regardless of forfeiture, the arguments lack merit, as Gardner again relies on the line of “suicide cases.” As already discussed, said cases are not relevant to the issue of whether this suit may proceed. Gardner further maintains that Tenn. Sup. Ct. R. 28 permits certification when there is no controlling precedent on an issue but, as discussed above, there is persuasive authority supporting the conclusion that public policy bars this suit.
III. CONCLUSION
For the foregoing reasons, we AFFIRM the district court’s dismissal and DENY Gardner’s certification motion.