James Lee Worthington was charged in a one count indictment with possessing approximately 843 pounds of marijuana with intent to distribute, in violation of Title 21, U.S.C., Section 841(a)(1). He pled not guilty, and was found guilty as charged by the district court in a bench trial, after waiving a jury. The hearing on Worthington’s motion to suppress the evidence as to his warrantless arrest and the search of his aircraft was conducted simultaneously with the trial on the merits. The basis for appellant’s conviction was the court’s finding that probable cause existed for the arrest and search. This appeal is from the judgment of conviction and Worthington’s sentence to 3 years confinement to be followed by a special parole term of 3 years.
The appellant’s primary contention is that the district court erroneously denied his motion to suppress the seized marijuana as the fruit of an illegal arrest and search. He asserts further that the court should have conducted an in camera interview with the confidential informer so as to determine whether to permit disclosure of the informer’s identity and access to him as a witness. We find these contentions to be lacking in merit and affirm the conviction.
The following recitation of uncontested facts is distilled from the transcript of the hearing, in combination with the fact findings of the trial judge. The two versions differ slightly in immaterial particulars. On April 27, 1974, federal Drug Enforcement Administration (DEA) Agent Gary Morrison, received a telephone call from a Mr. Anderwald, the owner of Air Central1 at the Harlingen Airport located near Harlingen, Texas in the lower Rio Grande Valley.2 Mr. Anderwald told Agent Morrison *1278that a Cessna aircraft with its rear seats removed had just arrived, piloted by a bearded young man. Agent Morrison testified at trial that the aircraft described by Anderwald is the type commonly used to transport contraband. Shortly thereafter, Agent Morrison received a call from a person who had previously furnished him with reliable information. This informer told Agent Morrison that a bearded young man had just arrived in Harlingen and would leave in a small aircraft with a large amount of marijuana. He also reported that this young man was meeting with other persons at a green mobile home behind the Casa Blanca Motel in Harlingen. No further information was obtained from the informer. Agent Morrison verified the location of the green trailer. He also learned that the Cessna aircraft had been recently leased to a James Worthington.
A Customs officer, who had observed the aircraft land, noted that it contained boxes marked “Cessna Aircraft Parts” in place of the rear seats. He and Agent Morrison kept the airplane under surveillance and were soon joined by two Customs agents. In the early morning hours of April 28, 1974, an electronic tracking device (“beeper”) was placed on Worthington’s aircraft by the Customs agents to assist in the surveillance. Later that morning the agents saw appellant Worthington arrive at the airport in a Mercury Capri with another man. The Customs agents then took off in their aircraft. From the air, they observed appellant take off. Visual contact with his plane was soon lost. A few minutes later the “beeper” stopped transmitting information and did not function thereafter. But by monitoring radio communications, the Customs agents were able to determine that appellant’s aircraft was headed for Hooks Airport in Houston. When they arrived there, they saw appellant unload the boxes and replace the rear seats. The agents later examined the boxes and found them empty. A DEA agent based in Houston told one of the Customs agents at Hooks Airport that appellant was suspected of drug dealing in the Houston area.
Later that same day, Worthington returned to the airport, again removed the rear seats, and took off in the aircraft. The Customs agents, maintaining visual contact, followed appellant to Harlingen Municipal Airport; from there he went to the green trailer behind the Casa Blanca Motel. The Customs agents followed Worthington when he took off again from Harlingen about 8:30 that night. He landed and refueled at Brownsville, Texas, and then flew back to Harlingen. The aircraft remained on the ground for approximately fifteen minutes, then took off and flew to Mid-Valley Airport, Weslaco, Texas. It remained there for another fifteen minutes before leaving. Due to bad weather conditions, both planes landed at Victoria, Texas, around midnight. The Customs plane taxied in front of appellant’s aircraft which had already stopped. One of the agents got out of the Customs plane with a flashlight and his gun in hand. He could see burlap sacks, containing brick-like objects, “piled about” appellant. The agent testified that, in light of his experience, the sacks appeared to contain marijuana. He then told appellant to raise his hands and remain seated in his aircraft. The other Customs agent searched appellant for weapons. Appellant was advised of his Miranda rights at this time. He and his aircraft were flown to Corpus Christi and placed in the custody of a DEA agent.
Appellant’s chief contention is that his warrantless arrest was illegal due to an absence of probable cause, and thus any evidence derived from the arrest and the ensuing search should have been suppressed as seized in violation of his Fourth Amendment rights. He places the time of arrest at the moment the Customs plane was taxied in front of appellant’s aircraft and Customs Agent Williams disembarked with a flashlight and a gun. The district court found that the government had sufficient probable cause to arrest based on the informer’s tip, corroborated by the agents’ observations and appellant’s erratic behavior. The court concluded in the alternative that even if the facts then known were insufficient to provide probable cause for *1279arrest, sufficient facts were present to justify brief detention of appellant for investigation. Cf. Terry v. Ohio, 1968, 392 U.S. 1, 16-19, 88 S.Ct. 1868, 1877-79, 20 L.Ed.2d 889, 903-05. Appellant’s contention regarding the time of his arrest was specifically rejected by the district court, which fixed the time of arrest as after the brief detention and the sighting of the burlap sacks in plain view: “The obvious presence of the marijuana in the plane conferred then on them sufficient probable cause for arrest.” The court elaborated: “At some point after Williams had gotten out of the plane, he told Worthington to put up his hands, that he was under arrest. The Court is satisfied this took place after Williams saw the sacks in the plane.”
Time of arrest is a question of fact which depends upon an evaluation of the testimony of those who were present at the time. Rios v. United States, 1960, 364 U.S. 253, 262, 80 S.Ct. 1431, 1436-37, 4 L.Ed.2d 1688, 1694. The conclusion of the district court is supported by the record. We therefore hold that appellant was “seized” in a reasonable manner in light of the surrounding exigent circumstances. Terry v. Ohio, supra.
The facts in this case, as they were known to the Customs agents at the time, provided ample justification for appellant’s stop and subsequent arrest. It is true that an informer’s tip is not adequate grounds for an arrest or search absent verified, consistent reliability of the informer, an indication of how the informer came by his information, or details of the alleged criminal activities. Spinelli v. United States, 1969, 393 U.S. 410, 416, 89 S.Ct. 584, 589, 21 L.Ed.2d 637, 643-44; United States v. Freund, 5 Cir. 1976, 525 F.2d 873, 875-76, cert. denied 426 U.S. 923, 96 S.Ct. 2631, 49 L.Ed.2d 377; Bailey v. United States, 5 Cir. 1967, 386 F.2d 1, 3, cert. denied 1968, 392 U.S. 946, 88 S.Ct. 2300, 20 L.Ed.2d 1408. However, if independent investigation by government agents yields information consistent with and corroborative of the informer’s tip, the warrantless arrest is legal. Draper v. United States, 1959, 358 U.S. 307, 79 S.Ct. 329, 3 L.Ed.2d 327; United States v. Freund, supra, 525 F.2d at 875-76; United States v. Anderson, 5 Cir. 1974, 500 F.2d 1311, 1316; United States v. Summerville, 5 Cir. 1973, 477 F.2d 393, 395; United States v. Squella-Avendano, 5 Cir. 1971, 447 F.2d 575, 581, cert. denied 404 U.S. 985, 92 S.Ct. 450, 30 L.Ed.2d 369. Here, one of the government agents had received a tip from an informer. While the tip was lacking in detail, its accuracy and reliability were corroborated by the subsequent surveillance of appellant Worthington’s activities.
Moreover, we view the rapid airport hopping and the transporting of empty boxes, when added to the tip as grounds for reasonable suspicion by the agent that appellant was involved in criminal activity. See United States v. Robinson, 5 Cir. 1976, 535 F.2d 881; United States v. Maslanka, 5 Cir. 1974, 501 F.2d 208, cert. denied sub nom. Knight v. United States, 1975, 421 U.S. 912, 95 S.Ct. 1567, 43 L.Ed.2d 777; United States v. McCann, 5 Cir. 1972, 465 F.2d 147, cert. denied sub nom. Kelly v. United States, 1973, 412 U.S. 927, 93 S.Ct. 2747, 37 L.Ed.2d 154. At a minimum the Customs and DEA agents were justified in stopping appellant to investigate further. “[T]he courts have recognized the right of police officers to stop and detain an individual ... with less than probable cause.” (Citations omitted). United States v. Robinson, supra, 5 Cir. 1976, 535 F.2d 881, 883. It is the legitimate function of law enforcement agents to detect and prevent crime. See United States v. McCann, supra, 465 F.2d at 157-58. It is their duty to be alert for suspicious activities and to follow up with appropriate investigation within constitutional limits. United States v. Allen, 5 Cir. 1973, 472 F.2d 145, 147. The agent’s observations must lead him “reasonably to conclude in light of his experience that criminal activity may be afoot. ...” Terry v. Ohio, 1968, 392 U.S. 1, 30, 88 S.Ct. 1868, 1884, 20 L.Ed.2d 889, 911.
There was no legitimate, logical explanation for appellant’s behavior. The specific, articulable facts within the agents’ *1280knowledge, from the informer’s tip to appellant’s erratic flight schedule, together with the rational inferences drawn therefrom were sufficient to create a reasonable suspicion that criminal activity was in progress. See Terry v. Ohio, supra, 392 U.S. at 21, 88 S.Ct. at 1880, 20 L.Ed.2d at 906; United States v. Rias, 5 Cir. 1975, 524 F.2d 118; United States v. McCann, supra, 465 F.2d 147. There was much more here than merely an “inchoate and unparticularized suspicion or ‘hunch’ . . . Terry v. Ohio, supra, 392 U.S. at 27, 88 S.Ct. at 1883, 20 L.Ed.2d at 909. We therefore find that the investigatory stop at the Victoria, Texas airport was warranted for the purpose of investigation.3
As soon as the investigatory stop was made, and the flashlight beam was directed into the plane, the agents had probable cause from visual observation to believe that the plane contained contraband. Gunnysacks containing brick-like objects were revealed. Williams testified that, from his experience he was familiar with such packages, and thus knew they often contained kilo bricks of marijuana. We find no error in the court’s finding that it was at this time that he advised appellant of his rights and arrested him. Probable cause is present at the instant the facts and circumstances known to the arresting officer warrant belief by a prudent person that a crime has been or is being committed. Adams v. Williams, 1972, 407 U.S. 143, 92 S.Ct. 1921, 32 L.Ed.2d 612; Dyke v. Taylor Implement Mfg. Co., Inc., 1968, 391 U.S. 216, 88 S.Ct. 1472, 20 L.Ed.2d 538; Beck v. Ohio, 1964, 379 U.S. 89, 85 S.Ct. 223, 13 L.Ed.2d 142; Draper v. United States, 1959, 358 U.S. 307, 79 S.Ct. 329, 3 L.Ed.2d 327; Brinegar v. United States, 1949, 338 U.S. 160, 69 S.Ct. 1302, 93 L.Ed. 1879; Williams v. United States, 5 Cir. 1968, 404 F.2d 493. Moreover, the exigent circumstances necessary to justify a warrantless arrest were present here, in view of the “mobility of the [vehicle], the possibility of flight, and subsequent destruction of evidence . . . .” United States v. Troise, 5 Cir. 1973, 483 F.2d 615, 617, cert. denied 414 U.S. 1066, 94 S.Ct. 574, 38 L.Ed.2d 471. See Carroll v. United States, 1925, 267 U.S. 132, 45 S.Ct. 280, 69 L.Ed. 543; United States v. Church, 9 Cir. 1973, 490 F.2d 353, 355, cert. denied 1974, 416 U.S. 983, 94 S.Ct. 2385, 40 L.Ed.2d 760.
Observation of the burlap sacks containing brick-shaped objects in plain view justified the arrest and subsequent search. “It has long been settled that objects falling in the plain view of an officer who has a right to be in the position to have that view are subject to seizure and may be introduced in evidence.” (Citations omitted). Harris v. United States, 1968, 390 U.S. 234, 236, 88 S.Ct. 992, 993, 19 L.Ed.2d 1067, 1069.4 Discovery of such evidence was not the result of a search. Marshall v. United States, 5 Cir. 1970, 422 F.2d 185, 188-89. The stop and search were legitimately conducted and accordingly we reject appellant’s claim that the search was tainted by an illegal arrest.
*1281Appellant also argues that the search is illegal because of the “beeper” placed on his aircraft by a Customs agent. We find this contention to be without merit. Attempted use of the “beeper” is without significance in this case since it malfunctioned shortly after its installation and its use produced no evidence whatsoever. We are not called upon to decide the admissibility of evidence discovered by a “beeper”, as in United States v. Holmes, 5 Cir. 1975, 521 F.2d 859, aff’d en banc 1976, 537 F.2d 227. Since the marijuana was not “come at by [the] exploitation of . . . ” the electronic device, we need not be concerned with the legality of the installation of the “beeper”. See Wong Sun v. United States, 1963, 371 U.S. 471, 487-88, 83 S.Ct. 407, 417, 9 L.Ed.2d 441, 455.
Appellant finally urges that the district court erred when it failed to conduct an in camera interview with the informer. We find that no such error was made, under the balancing test established in Roviaro v. United States, 1957, 353 U.S. 53, 77 S.Ct. 623, 1 L.Ed.2d 639. Disclosure of an informer’s identity is required only if necessary to a fair determination of an accused’s guilt or innocence. “We believe that no fixed rule with respect to disclosure is justifiable. The problem is one that calls for balancing the public interest in protecting the flow of information against the individual’s right to prepare his defense.” Id. at 62, 77 S.Ct. at 628-29, 1 L.Ed.2d at 646. The circumstances in each case are determinative of the proper balance. Id. “The inquiry centers on the likelihood that the informant possesses facts which are relevant and helpful to the accused in preparing his defense on the merits.” United States v. Freund, supra, 525 F.2d 873, 876.
The evidence in the present case indicates that the informer played no part in the offense charged, and was not present at the time of arrest.5 The Customs agents, through their own follow-up investigation and surveillance, came upon the marijuana which was plainly visible in appellant’s possession in the aircraft. There is no basis here for even a bare supposition that the informer possessed facts either relevant or helpful to Worthington’s defense.
We conclude that the facts found by the district court are amply supported by the evidence, that the legal conclusions drawn therefrom are sound, and that the judgment appealed from should be
AFFIRMED.
. Not further identified in the record.
. Harlingen is approximately ten miles from the Mexican border.
. We agree with the trial court and reject appellant’s contention that a full-blown arrest occurred at the precise moment that the Customs plane was positioned in front of appellant’s plane, and Customs agent Williams emerged with gun and flashlight in hand. We hold that the stop was made in an acceptable manner under the circumstances present. Blocking the plane was a reasonable manner in which to effect the stop and maintain the status quo. ■f An investigatory stop is not automatically an I arrest simply because an officer draws his gun. See United States v. Maslanka, supra, 501 F.2d at 213, n. 10. From following appellant during his inexplicable flights, and based upon their experience with drug traffickers, the agents reasonably feared death or serious injury when they stopped and approached appellant’s plane on the dark, deserted airstrip. Carrying a drawn gun was reasonable under the circumstances.
. Under the plain view doctrine the officers must discover the evidence by inadvertence and while they have a legitimate reason for being present. Coolidge v. New Hampshire, 1971, 403 U.S. 443, 91 S.Ct. 2022, 29 L.Ed.2d 564. However, the fact that the agents expected to find marijuana does not destroy the necessary inadvertence. See United States v. Cushnie, 5 Cir. 1973, 488 F.2d 81, 82, cert. denied 1974, 419 U.S. 968, 95 S.Ct. 233, 42 L.Ed.2d 184.
. Although we found an in camera interview to be appropriate in United States v. Freund, supra, the facts in the case at bar differ significantly from those in Freund. In that case, appellant Freund claimed that the stop was merely for a routine check and therefore the officer lacked probable cause to believe that his truck contained contraband. Freund relied on the DEA’s written case summary, describing the stop as a routine check, to support his theory. Moreover, at the trial, it developed that the informer, who had not supplied any information in the case, was present at the search and arrest. Id. at 874-75. Because of these unusual circumstances, an in camera hearing was appropriate to determine if disclosure would be necessary to help Freund prepare his defense. Id. at 877-78. In the present case, there is no controversy regarding the agents’ observations. Nor is it even claimed that the informer took part in the offense or was present at the time of arrest.