ALVIN B. RUBIN, Circuit Judge:
Under the border search doctrine, federal agents may without a warrant inspect mail arriving in this nation from abroad. We here consider whether the foreign mail may be followed into the hands of its addressee and, immediately after delivery, be seized and searched without a warrant. Concluding that such a search is permitted by the border search rule when there has been continuous surveillance of the mail and reasonable suspicion that it contains contraband, we deny a motion to suppress the results of the search and affirm the conviction of the recipient for possession of heroin with intent to distribute it.
I. The Facts
On December 5,1977, Raymond Richards, an airline steward stationed at the Miami International Airport, applied for a post office box at the Miami Springs, Florida post office, located near the airport. He designated the boxholder as Mehling Arts & Crafts, furnished identification showing that he was Christopher Thompson and signed the application in that name. The application was approved and a box assigned to Mehling was opened for use on December 15.
On March 2, 1978, a sealed parcel addressed to Mehling Arts & Crafts at the Miami Springs box number arrived at the Foreign Mail Center in New York. The customs declaration stated that it contained Thai silk and uncut stones. The parcel was opened and inspected by a customs agent who found 10 cigarette packages. He opened one of these, found cigarettes and sent the parcel to another officer for further inspection because it appeared to contain a tobacco product. The second officer opened two more cigarette packages; each contained vials of material that on field test was found to be heroin. Neither the amount of heroin in the parcel nor its strength was established in New York.
The parcel was reassembled, resealed and sent to the postal inspector in Miami in a locked pouch for controlled delivery. Government agents delivered it to the Miami Springs post office on March 13 with instructions to put a notice of arrival in the Mehling box. A postal employee testified that she had never seen mail in the Mehling box until this parcel arrived. Drug Enforcement Administration agents set up surveillance of the box and waited for someone to claim the parcel.
[768]*768Richards arrived in a half hour or less, received the notice and asked for the package. There was a slight delay because he had no identification showing a connection with Mehling, but the package was given to him. Government agents watched Richards take the package. They continued to observe him as he walked out of the building, down a side street for a short distance and into a parking lot behind the post office. The agents exercised care to assure that he had no opportunity to leave the scene with the parcel. They arrested him just as, or just after, he entered his car. The agents addressed Richards as Christopher Thompson and he responded by giving his correct name. The agents gave him Miranda warnings and asked him why he had picked up the package. He replied by asking the agents what was in the parcel. When they told him that it contained narcotics, Richards said he knew nothing about it. He explained that someone had telephoned him, asked him to pick up the package and promised to pay him for doing so.
The government agents took Richards to a police facility a few blocks away. He was fingerprinted, photographed, booked and placed in a holding cell. While he was there, the sealed parcel was opened without a warrant outside of his presence. Later its contents were analyzed by a chemist, who found 27.4 grams of 93% heroin hydrochloride. He estimated that, when “cut,” it would produce 12,000 street level doses.
Richards was indicted for possession of heroin with the intent to distribute it, in violation of 21 U.S.C. § 841(a)(1). At his trial, he testified that he operated a mail order business importing and selling uncut gems and maintained a postal box at the Miami post office in connection with that business. According to his testimony, Richards had recently started Mehling Arts & Crafts as a mail order export-import business and needed a separate post office box for it, but none was available at the Miami post office. He testified that he used a false name to apply for the Miami Springs box because he feared that operating two outside enterprises might affect his job as a steward. A personnel administrator for the airline testified that it was permissible for the company’s employees to have outside jobs and that many held other jobs, but he acknowledged that in some circumstances outside employment might have an adverse affect on an employee’s job.
II. Sufficiency of the Evidence
To prove commission of the crime of possession of heroin with intent to distribute it, the government must establish three essential elements: (1) knowing (2) possession of heroin (3) with intent to distribute it. United States v. Johnson, 469 F.2d 973, 976 (5th Cir. 1972); United States v. James, 555 F.2d 992, 999 (D.C.Cir.1977). Richards contends the evidence was insufficient to prove either that he knew the parcel contained a controlled substance or that he had the intention of distributing it.
Our review of the jury verdict is limited to determining whether the trier of fact could reasonably conclude that the evidence excludes every reasonable hypothesis, except that of guilt. United States v. Hawkins, 614 F.2d 85, 87 (5th Cir.), cert. denied, 446 U.S. 955, 100 S.Ct. 2926, 64 L.Ed.2d 814 (1980), United States v. Squella-Avendano, 478 F.2d 433, 436 (5th Cir. 1973). The issue of guilt remains a question for the jury unless we conclude that the jury must necessarily have had a reasonable doubt. United States v. Shaw, 555 F.2d 1295, 1300 (5th Cir. 1977); United States v. Warner, 441 F.2d 821, 825 (5th Cir.), cert. denied, 404 U.S. 829, 92 S.Ct. 65, 30 L.Ed.2d 58 (1971). The supporting evidence may be direct or circumstantial and we do not discriminate against sufficiently probative evidence because it is indirect. Id. at 825.
The evidence, examined in the light most favorable to the government, Glasser v. United States, 315 U.S. 60, 80, 62 S.Ct. 457, 469, 86 L.Ed. 680, 704 (1942), establishes that: (1) Richards opened the post office box in a false name and had no convincing explanation for doing so; (2) no mail except the parcel containing heroin had come to the post office box in the three months during which it was rented; and (3) the excuse Richards advanced after he was ar[769]*769rested, that someone else had asked him to pick up the package, was in truth inculpatory and indicated guilty knowledge because the package was addressed to his company and he had rented the box.
These facts are circumstantial evidence on the issue of knowledge. Because no one has a window to a man’s mind, knowledge must often be proved by indirect evidence. Johnson v. Wright, 509 F.2d 828, 831 (5th Cir.), cert. denied, 423 U.S. 1014, 96 S.Ct. 445, 46 L.Ed.2d 384 (1975); Jackson v. United States, 330 F.2d 679, 681 (8th Cir.), cert. denied, 379 U.S. 855, 85 S.Ct. 105, 13 L.Ed.2d 58 (1964). The evidence was sufficient to permit the jury to infer that Richards knew from what he said and did that the package contained narcotics. See also United States v. Squella-Avendano, 478 F.2d 433 (5th Cir. 1973) (similar amount of evidence held sufficient).
Richards having indisputably possessed heroin when he was arrested, we are left only with the question whether the evidence was sufficient to support the jury’s conclusion that he intended to distribute it. That intention may not be inferred from possession alone, for contraband may be destined for personal use, not distribution. However, the possession of a quantity of narcotics so large that it could not be used by the possessor alone justifies the conclusion that he had an inventory for distribution rather than personal consumption. See United States v. Grayson, 625 F.2d 66 (5th Cir. 1980); United States v. Soto, 591 F.2d 1091, 1103 (5th Cir.), cert. denied, 442 U.S. 930, 99 S.Ct. 2862, 61 L.Ed.2d 298 (1979); United States v. Ratio, 587 F.2d 199, 201 (5th Cir. 1979); United States v. Johnson, 469 F.2d 973, 977 (5th Cir. 1972). The jury was warranted in concluding that one who possessed 12,000 doses of heroin did not intend to use the supply merely for personal euphoria. If, therefore, the warrantless post-delivery search was constitutionally proper and the evidence derived from it was properly admitted, the essential elements of the crime were proved.
III. Search of the Package
Richards does not challenge the admissibility of evidence obtained from the initial search of the package in New York and the field test that first disclosed its contents. He stipulated that the package contained heroin when searched in New York and that the chain of custody between New York and Miami Springs was complete. Because the initial test did not establish the quantity of heroin in the package, however, the second search was crucial to proving his intent to distribute.
A. Standing to Assert Fourth Amendment Rights
The question of Richards’ standing to contest the search was not raised in the court below. On appeal the government argued proleptically that the Supreme Court decision in Rakas v. Illinois, 439 U.S. 128, 99 S.Ct. 421, 58 L.Ed.2d 387 (1978), foretold the coming demise of the concept of “automatic standing” for possessory crimes. In United States v. Salvucci, -U.S.-, 100 S.Ct. 2547, 65 L.Ed.2d 619 (1980), the Court followed the intimations of Rakas v. Illinois, 439 U.S. at 135 n.4, 99 S.Ct. at 426 n.4, 58 L.Ed.2d at 396 n.4, and Brown v. United States, 411 U.S. 223, 229, 93 S.Ct. 1565, 1568-69, 36 L.Ed.2d 208, 214 (1973), and held that automatic standing does not justify the assertion of fourth amendment protection.
Fourth amendment protection is accorded only to a person who has a privacy interest in the area searched, Rawlings v. Kentucky,-U.S.-, 100 S.Ct. 2556, 65 L.Ed.2d 633 (1980), Rakas v. Illinois, 439 U.S. 128, 99 S.Ct. 421, 58 L.Ed.2d 387 (1978), a rubric adopted from a phrase used in Katz v. United States, 389 U.S. 347, 353, 88 S.Ct. 507, 512, 19 L.Ed.2d 576, 583 (1967). Instead of conducting a separate inquiry into standing, we now focus directly on whether the defendant possesses a legitimate expectation of privacy in the area searched. Rawlings v. Kentucky,-U.S.-, 100 S.Ct. 2556, 65 L.Ed.2d 633 (1980); Rakas v. Illinois, 439 U.S. 128, 99 S.Ct. 421, 58 L.Ed.2d 387 (1978).
[770]*770The package was sealed and addressed to Mehling, which, in effect, was Richards. These facts alone indicate “an expectation that the contents would remain free from public examination.” United States v. Chadwick, 433 U.S. 1, 11, 97 S.Ct. 2476, 2483, 53 L.Ed.2d 538, 548 (1977). See generally Katz v. United States, 389 U.S. at 352, 88 S.Ct. at 511-12, 19 L.Ed.2d at 582. Moreover, sealed mail historically has been considered to have a high degree of privacy, and government intrusion into mailed parcels is limited by the fourth amendment. See United States v. Van Leeuwen, 397 U.S. 249, 90 S.Ct. 1029, 25 L.Ed.2d 282 (1970); Ex parte Jackson, 96 U.S. 727, 24 L.Ed. 877 (1878).
Although the Supreme Court has rejected the importation into search and seizure law of arcane distinctions evolved in property law, Rakas v. Illinois, 439 U.S. at 149-50 n.17, 99 S.Ct. at 434, n.17, 58 L.Ed.2d at 405 n.17, we note that Richards was in lawful possession of the package when it was seized. Cf. Rakas v. Illinois, 439 U.S. at 153, 99 S.Ct. at 435, 58 L.Ed.2d at 407 (Powell, J., concurring) (“property rights reflect society’s explicit recognition of a person’s authority to act as he wishes in certain areas, and therefore should be considered in determining whether an individual’s expectations of privacy are reasonable”). It is, therefore, not dispositive that Richards denied ownership of the package. Considering all the circumstances, we conclude that he had a legitimate expectation that the contents of the package were private,1 and has standing to assert fourth amendment protection.
B. Border Search
Under the fourth amendment, all warrantless searches and seizures are unreasonable except those conducted in a few narrowly defined situations where the circumstances justifying the search outweigh privacy rights. Coolidge v. New Hampshire, 403 U.S. 443, 91 S.Ct. 2022, 29 L.Ed.2d 564 (1971); United States v. Sink, 586 F.2d 1041 (5th Cir. 1978), cert. denied, 443 U.S. 912, 99 S.Ct. 3102, 61 L.Ed.2d 876 (1979). Because the fourth amendment expressly prohibits only unreasonable warrantless searches, it patently incorporates a balancing test, weighing in one measure the level of intrusion into individual privacy and in the other the public interest to be served. United States v. Martinez-Fuerte, 428 U.S. 543, 555, 96 S.Ct. 3074, 3081, 49 L.Ed.2d 1116, 1126-1127 (1976); United States v. Himmelwright, 551 F.2d 991, 994 (5th Cir.) cert. denied, 434 U.S. 902, 98 S.Ct. 298, 54 L.Ed.2d 189 (1977). Thus, a warrantless search is justified when it is incident to a lawful arrest, Chimel v. California, 395 U.S. 752, 89 S.Ct. 2034, 23 L.Ed.2d 685 (1969); when it is conducted with probable cause under exigent circumstances, Schmerber v. California, 384 U.S. 757, 86 S.Ct. 1826, 16 L.Ed.2d 908 (1966); when it involves a vehicle, Chambers v. Maroney, 399 U.S. 42, 90 S.Ct. 1975, 26 L.Ed.2d 419 (1970); or when it is made for administrative purposes to satisfy a special governmental need and necessity outweighs the invasion entailed, Camara v. Municipal Court, 387 U.S. 523, 87 S.Ct. 1727, 18 L.Ed.2d 930 (1967).
Warrantless searches may also be made at the border. These searches, undertaken pursuant to the historical right of the sovereign to protect itself by stopping and examining persons and property crossing into this country, are deemed reasonable simply by virtue of the fact that they occur at the border. United States v. Ramsey, 431 U.S. 606, 97 S.Ct. 1972, 52 L.Ed.2d 617 (1977); Carroll v. United States, 267 U.S. [771]*771132, 45 S.Ct. 280, 69 L.Ed. 543 (1925). This sovereign right was recognized by the First Congress, Act of July 31, 1789, ch. 5,1 Stat. 29 (1789), and statutes exempting border searches from warrant and probable cause requirements have been in force in this country ever since.2
A border search need not take place at the actual border. It may be conducted at a place considered “the functional equivalent of the border,” such as the port where a ship docks in this country after entering our territorial waters from abroad, United States v. Prince, 491 F.2d 655 (5th Cir. 1974), or the airport where an international flight lands, United States v. Klein, 592 F.2d 909 (5th Cir. 1979).
While the mere fact that a person or thing has once crossed the border does not sanction a search of it forever after, we have also recognized that the need to protect personality and property against warrantless invasion must be balanced against the myriad difficulties facing customs and immigration officials who are charged with the enforcement of smuggling and immigration laws. We have, therefore, recognized in the doctrine of “extended border search,” the government’s power, under certain circumstances, to search without a warrant persons and things after they have entered the country.3
Various panels of this court have described the requisites for a warrantless extended border search in terms that are not reconcilable. At its most permissive, the standard has been phrased as requiring only a “reasonable suspicion of a customs agent” if the search occurs in the “border area.” See, e. g., United States v. Hill, 430 F.2d 129, 430-31 (5th Cir. 1970). We have also phrased the standard as requiring either “direct contact” or “nexus” with the border area coupled with a reasonable suspicion of secreted contraband. See, e. g., United States v. Salinas, 439 F.2d 376, 379 (5th Cir. 1971); United States v. Bowman, 502 F.2d 1215, 1219 (5th Cir. 1974).
On several occasions, however, this court has exacted stricter criteria and has required the government to show both a likelihood that the person or thing has crossed the border and reasonable certainty that any contraband discovered by the search was present when the border was crossed. See, e. g., United States v. Fogelman, 586 F.2d 337 (5th Cir. 1978); United States v. Martinez, 481 F.2d 214 (5th Cir. 1973), cert. denied, 415 U.S. 931, 94 S.Ct. 1444, 39 L.Ed.2d 489 (1974). Because the primary justification for the relaxation of fourth amendment standards in these situations is the crossing of a border, these constraints are warranted and the less exacting standards of Hill, Salinas, and Bowman have been criticized. See, e. g., Note, From Bags to Body Cavities: The Law of Border Search, 74 Colum.L.Rev. 53, 60-61 (1974); 3 W. LaFave, Search and Seizure: A Treatise on the Fourth Amendment 298 (1978).
In United States v. Johnson, 588 F.2d 147 (5th Cir. 1979), this court rejected [772]*772the bare requirement of “border nexus” as being inconsistent with the decision of the Supreme Court in Almeida-Sanchez v. United States, 413 U.S. 266, 93 S.Ct. 2535, 37 L.Ed.2d 596 (1973), and held that “[a] search is not a valid border search unless it appears by a preponderance of the evidence, direct or circumstantial, that a border crossing has occurred.” United States v. Johnson, 588 F.2d at 154 (footnote omitted).
The government must also be able to show, with reasonable certainty, that conditions remained unchanged from the time of the border crossing until the subsequent warrantless search. United States v. Fogelman, 586 F.2d 337, 343 (5th Cir. 1978); Cf. United States v. Walters, 591 F.2d 1195, 1198 (5th Cir. 1979) (extended border search doctrine, requiring unchanged conditions, applied to strip search). In other words, it must be established with reasonable certainty that, when searched, the person or thing was in the same condition as when the border was crossed. This requirement is usually met by proving either that the object searched was subject to constant surveillance from the time it crossed the border or that, under the circumstances, the contraband was not likely to have been introduced during any breaks in the surveillance. See, e. g., United States v. Martinez, 481 F.2d 214, 218-19 (5th Cir. 1973), cert. denied, 415 U.S. 931, 94 S.Ct. 1444, 39 L.Ed.2d 489 (1974) (35 minute break in surveillance held insufficient to establish changed circumstances in view of the fact that it took the authorities two hours to unload the 628 pounds of marijuana hidden in a secret compartment of a truck). Finally, before conducting a warrantless extended border search, the government agents must possess a reasonable suspicion, supported by articulable facts, that the person or thing searched is involved in illegal activity, such as smuggling contraband.4 United States v. Kenney, 601 F.2d 211, 213 (5th Cir. 1979); United States v. Martinez, 481 F.2d 214, 219 (5th Cir. 1973), cert. denied, 415 U.S. 931, 94 S.Ct. 1444, 39 L.Ed.2d 489 (1974).
There appears to be no sound reason to distinguish between incoming mail and other property that crosses our border. Affixing a postage stamp to a parcel should not grant it immunity that would not be accorded a package carried by a traveller.5 In United States v. Ramsey, 431 U.S. 606, 97 S.Ct. 1972, 52 L.Ed.2d 617 (1977), the Supreme Court held that a search by custom officials of incoming international letter-class mail without a warrant was constitutionally permissible. While the Court reserved the question of the validity of such a search if conducted at a point distant from the border, 431 U.S. at 615 n.11, 97 S.Ct. at 1978, n.11, 52 L.Ed.2d at 626 n.11, it rejected the argument that mail should be treated differently from and receive more constitutional protection than incoming persons or vehicles: “It is clear that there is nothing in the rationale behind the border-search exception which suggests that the mode of entry will be critical.” 431 U.S. at [773]*773620, 97 S.Ct. at 1980, 52 L.Ed.2d at 629. “The critical fact is that the envelopes cross the border and enter this country, not that they are brought in by some mode of transportation rather than another. It is their entry into this country from without it that makes a resulting search ‘reasonable.’ ” 431 U.S. at 620, 97 S.Ct. at 1980-81, 52 L.Ed.2d at 629. The Court found no historical or constitutional reason for treating mail differently from persons or vehicles entering the country. See also United States v. King, 517 F.2d 350 (5th Cir. 1975), cert. denied sub nom. Pearson v. United States, 446 U.S. 966, 100 S.Ct. 2943, 64 L.Ed.2d 825 (1980) (approving a search of mail that was made prior to delivery to the addressee at an inland post office).
Extending the doctrine of Ramsey and King, we see no reason to prohibit an extended border search of international mail under the same conditions that would properly permit one to be made of persons or other property that has crossed the border. See also United States v. Pringle, 576 F.2d 1114, 1117 (5th Cir. 1978). There is no more reason to draw a bright line at the moment mail is delivered than there is to draw it after the incoming parcel crosses the border.
Thus, we apply this circuit’s extended border search doctrine to searches of mail and conclude that such a warrantless search is reasonable (a) when it is established by a preponderance of the evidence that the mail has crossed the border; (b) when it appears with reasonable certainty that the mail’s contents have not been altered since it entered the United States and, (c) if the search occurs after delivery of the mail to the addressee, when the government agents who made the search had a reasonable suspicion of smuggling activity. While these requisites have not been exacted in every Fifth Circuit extended-border-search decision, and while, even in those of our decisions requiring them, each has not been separately set forth with this specificity, each of them has been implicitly or explicitly demanded in many of our prior decisions. Thus summarized, they are distillate of a variety of decisions. Each of them has been satisfied here.6
The search was conducted with respect to a package that had undeniably crossed our international border and remained unchanged subsequent to that crossing. In addition, although reasonable suspicion is ordinarily sufficient to warrant such inland searches, even of mail, the government officials were certain that contraband was being smuggled in the package. Therefore, all the criteria that sanction warrantless border searches are met and we hold that the search was not unreasonable.7
Accordingly, the judgment of the district court is AFFIRMED.