ROBERT MADDEN HILL, Circuit Judge:
Appellant Martin Cardenas Alvarado (Cardenas) was convicted on various drug related charges. On appeal, Cardenas claims that there was insufficient evidence to support his convictions and that the prosecutor violated his Fifth Amendment privilege against self-incrimination by commenting on his silence in violation of
Doyle v. Ohio,
426 U.S. 610, 96 S.Ct. 2240, 49 L.Ed.2d 91 (1976). Finding no merit to these contentions, we affirm.
I.
On Thursday, January 9,1986, at approximately 8:30 p.m., two United States Border Patrol agents, Ed Barrett and Fred Bau-man, were on surveillance on the Mexico border approximately five and one-half miles east of the Ysleta port-of-entry near El Paso, Texas. They parked their marked van on the crossover of an irrigation ditch six to eight hundred yards from the Rio Grande boundary with Mexico. They knew that the particular area was commonly used by undocumented Mexican field workers to cross illegally into the United States to work, usually in groups of two to four people.
The agents noticed unusual movement along a drainage canal, and using binoculars they could see a string of people carrying bundles approaching them. The people were walking single file, evenly spaced, but close together. As the people walked closer to the van, they suddenly disappeared from the agents’ view.
The agents immediately turned on the van’s headlights and drove along the drainage ditch to cut them off before they could run back across the border. Upon arriving at the south end of the ditch, approximately 300 yards away, Cardenas ran in front of the van. The agents captured him but were unable to catch two other people they saw running back into Mexico. The agents found tracks from Mexico into the United States that looked like other people had crossed the canal. They also noticed tracks corresponding to Cardenas’ attempted flight and those of the group that escaped. The agents questioned Cardenas as to his nationality and papers. They asked Cardenas if he or anyone in his group was armed, and he said no. They put Cardenas in the Border Patrol van and continued along the ditch to the area where they had seen the others disappear.
Back at the north end of the ditch, Barrett and Bauman found nine other people lying in the drainage ditch, and interspersed among them were bundles of mari-' juana — four packages to the bundle— rigged to be carried on a person’s back. The agents told them not to move and asked if they or anyone else in the area had weapons. They answered in the negative. The agents then moved each person out of the ditch and searched him. When the situation seemed under control, the agents radioed for assistance and read each person his
Miranda
rights.
Border Patrolmen Lopez and Andrew arrived about twenty minutes later. All who were apprehended were loaded into the two vehicles. The agents began gathering the bundles of marijuana that had been dropped along the drainage ditch. They found 119 packages weighing approximately 25 pounds each for a total of 3200 pounds of marijuana. The marijuana had a value of $1.6 million. The packages were bundled and tied with rope to be carried over the shoulders so that one person could carry four packages containing a total of 100 pounds of marijuana. Each of the rope halters was unique. A white dust or powder from the packages of marijuana covered the clothes of those taken into custody, including Cardenas.
Later that night, after being given additional
Miranda
warnings at the nearby
Ysleta Border Patrol’'station, Barrett asked Cardenas where he was taking the marijuana, whose it was, and how much he had been paid. After Cardenas responded that he did not know, agent Segarra of the Drug Enforcement Administration (DEA) asked what happened, to which Cardenas responded, “Well, I really don’t know.” When asked about their possession of the marijuana, the others also said that they knew nothing; they did not know who owned the marijuana or where they were to take it. One indicated that he had been paid eight dollars to bring the load across the border.
Each of the persons apprehended, including Cardenas, lived in the same town in Mexico; several were related. Each admitted entering the United States illegally on many previous occasions, and each had been arrested several times by the Border Patrol and returned to Mexico. A federal grand jury returned a four count indictment charging Cardenas and eight others with (1) one count of having conspired to import in excess of 50 kilograms of marijuana into the United States from Mexico, in violation of 21 U.S.C. §§ 952(a), 960(a)(1), and 963; (2) one count of having imported over 50 kilograms of marijuana into the United States from Mexico in violation of 21 U.S.C. §§ 952(a) and 960(a)(1); (3) one count of having conspired to possess over 50 kilograms of marijuana with intent to distribute in violation of 21 U.S.C. §§ 841(a)(1) and 846; and (4) one count of possession of over 50 kilograms of marijuana with intent to distribute in violation of 21 U.S.C. § 841(a)(1).
Each of the defendants except Cardenas testified at trial. They admitted knowing that their actions were illegal but claimed that they were forced at gunpoint to lead a group of thirty in carrying the marijuana into the United States. They testified that they had heard there was work in the United States for a man known as El Bancero. They had taken a bus together from the El Million area in Mexico where they all lived to a town near the Rio Grande. They walked from there to the crossing point at the river where they were waiting to cross that night to work the next morning. A stranger whom they could not describe approached and offered them work, but he did not say what kind or where. Suddenly, after a white stake-bed truck loaded with marijuana and twenty more people arrived, four men with what appeared to be machine guns jumped out and ordered them to take the bundled marijuana across the river.
After a three and one-half day trial, the jury returned a verdict of guilty as to all four counts against all defendants, including Cardenas. Cardenas was sentenced to a term of five years imprisonment on each of counts one and three, and five years imprisonment followed by a five-year special parole term as to each of counts two and four, all four terms to run concurrently. The eight codefendants received identical sentences. Cardenas then perfected this timely appeal.
II.
Cardenas first attacks the sufficiency of the evidence to support his convictions. In drug conspiracy cases, the government must prove beyond a reasonable doubt that a conspiracy existed, that the accused knew of the conspiracy, and that he knowingly and voluntarily joined it.
United States v. Jackson,
700 F.2d 181 (5th Cir.),
cert. denied sub nom. Hicks v. United States,
464 U.S. 842, 104 S.Ct. 139, 78 L.Ed.2d 132 (1983). It is unnecessary in drug conspiracy cases to prove an overt act in furtherance of the conspiracy.
United States v. Kupper,
693 F.2d 1129, 1134 (5th Cir.1982). We will not, however, “lightly infer a defendant’s knowledge and acquiesence in a conspiracy.”
Jackson,
700 F.2d at 185. A showing that the defendant merely associated with those participating in a conspiracy is insufficient.
Id.
Similarly, the government cannot prove a conspiracy by presenting evidence that only places the defendant in “a climate of activity that
reeks of something foul.”
United States v. Galvan,
693 F.2d 417, 419 (5th Cir.1982).
The government need not prove the existence of a formal agreement to establish a conspiracy, but it must do more than “pile inference upon inference upon which to base a conspiracy charge.”
United States v. Sheikh,
654 F.2d 1057, 1063 (5th Cir.1981) (citation omitted),
cert. denied,
455 U.S. 991, 102 S.Ct. 1617, 71 L.Ed.2d 852 (1982). Circumstantial evidence, however, can be used to show the existence of a conspiracy.
United States v. Aguirre Aguirre,
716 F.2d 293 (5th Cir.1983);
see also United States v. Acosta,
763 F.2d 671 (5th Cir.),
cert. denied sub nom. Weempe v. United States,
— U.S. -, 106 S.Ct. 179, 88 L.Ed.2d 148 (1985).
In order to sustain a conviction for the crime of possession of marijuana with intent to distribute, the government must prove three elements: (1) knowing (2) possession of marijuana (3) with intent to distribute it.
United States v. Vergara,
687 F.2d 57, 61 (5th Cir.1982) (quoting
United States v. Richards,
638 F.2d 765, 768 (5th Cir.1981),
cert. denied,
454 U.S. 1097, 102 S.Ct. 669, 70 L.Ed.2d 638 (1981)). A conviction on the importation offense requires proof of similar elements, the principal difference being that the government must show that the defendant played a role in bringing the marijuana from a foreign country into the United States.
See United States v. Jonas,
639 F.2d 200, 205 (5th Cir.1981).
Possession of contraband may be actual or constructive, may be joint among several defendants, and may be proved by circumstantial as well as direct evidence.
United States v. Wilson,
657 F.2d 755, 760 (5th Cir.1981),
cert. denied,
455 U.S. 951, 102 S.Ct. 1456, 71 L.Ed.2d 667 (1982);
see also United States v. Stanley,
765 F.2d 1224, 1240 (5th Cir.1985). Constructive possession is the “knowing exercise of, or the knowing power or right to exercise dominion and control over the proscribed substance.”
United States v. Marx,
635 F.2d 436, 440 (5th Cir.1981). In addition, intent to distribute may be inferred from the possession of a large quantity of an illegal substance.
Vergara,
687 F.2d at 62.
Before examining the record to determine if there is sufficient evidence to support the convictions, we must be mindful of our role in reviewing the sufficiency of the evidence. “The verdict must be sustained if there is substantial evidence, taking the view most favorable to the Government, to support it.”
Glosser v. United States,
315 U.S. 60, 80, 62 S.Ct. 457, 469, 86 L.Ed. 680 (1942). We have stated that
[i]t is not necessary that the evidence exclude every reasonable hypothesis of innocence or be wholly inconsistent with every conclusion except that of guilt, provided a reasonable trier of fact could find that the evidence establishes guilt beyond a reasonable doubt. A jury is free to choose among reasonable constructions of the evidence.
United States v. Bell,
678 F.2d 547, 549 (5th Cir.1982) (en banc) (footnote omitted),
aff’d on other grounds,
462 U.S. 356, 103 S.Ct. 2398, 76 L.Ed.2d 638 (1983). Under this standard, our review of the record convinces us that there is sufficient evidence to support Cardenas’ convictions.
According to the testimony of the eight codefendants and the observations of the Border Patrol agents, Cardenas was one of a group of thirty people who traveled from El Million by bus and then walked at nightfall to an area on the Mexican border. The defendants loaded the bundles of marijuana on their backs and carried it into the United States. When Cardenas spotted the Border Patrol van, he started running back toward Mexico when the agents caught him. His tracks corresponded with those of the group and approximately thirty bundles of marijuana were found in the area.
Cardenas places great emphasis on the fact that he was in a group of three or four others separate from the other eight who were arrested with the contraband and that he was spotted and arrested several hundred yards away from the other defendants. Cardenas alleges that the area where he was arrested is known to be an area where illegal aliens who want to work in the United States seek entry. Thus, Cardenas contends that his conviction is improper because there was insufficient evidence to connect him to the other defendants.
We disagree with Cardenas’ contentions. The fact that he was found in a different area than the other defendants does not preclude a jury from concluding that he was guilty of both the substantive crimes and the conspiracy when there is sufficient evidence that creates a nexus between Cardenas and the other defendants. We believe that there is sufficient evidence to create this nexus. The Border Patrol agents testified that field workers normally cross the border in the morning and usually cross in small groups. In contrast, the defendants in this case met at 8:30 p.m. Furthermore, the other defendants testified that Cardenas was one of a group of at least ten people who had traveled to the border together. The fact that he was captured as part of a small group does not negate the ability of the jury to conclude that Cardenas was part of a conspiracy to import and distribute the marijuana into the United States. Also, as noted earlier, Cardenas ran back toward Mexico when he was spotted by the Border Patrol agents. The jury was entitled to infer that Cardenas’ flight was indicative of guilt.
See United States v. Meneses-Davila,
580 F.2d 888, 896 & n. 16 (5th Cir.1978) (citing cases). Further, the packages of marijuana were covered in a white powder or dust that was also on the clothes of Cardenas and the other defendants. This evidence would allow the jury to infer that Cardenas had possession of the marijuana. Moreover, the large amount of marijuana discovered—3200 pounds — is sufficient to allow the jury to infer intent to distribute.
See, e.g., Vergara,
687 F.2d at 62;
Meneses-Davila,
580 F.2d at 896 (250 pounds of marijuana). The destination of the marijuana was the United States; this factor is sufficient additional proof for the importation offense.
Jonas,
639 F.2d at 205. In essence, the jury verdict was dependent on the credibility assessments of the jury, and we will not second-guess those judgments. The defense of coercion was adequately presented to the jury which rejected it.
After examining the record, we hold that there is sufficient evidence to support Cardenas’ convictions.
III.
A.
Cardenas also contends that the prosecutor improperly commented on his post-arrest silence in violation of
Doyle v. Ohio,
426 U.S. 610, 96 S.Ct. 2240, 49 L.Ed.2d 91 (1976). In
Doyle
the Supreme Court held that impeachment of a defendant with respect to his post-arrest silence, offered on the theory that silence is inconsistent with an exculpatory story told at trial, violates a defendant’s due process rights. The Court’s conclusion was based on the ambiguity inherent in a defendant’s silence after he has been arrested and informed of his
Miranda
rights. A defendant’s silence may indicate that he is exercising the rights of which he has just been advised; it does not necessarily mean that a defendant does not have an exculpatory story to tell.
Subsequent Supreme Court cases, however, have narrowed the applicability of
Doyle.
In
Fletcher v. Weir,
455 U.S. 603, 102 S.Ct. 1309, 71 L.Ed.2d 490 (1982) (per curiam), the Court held that it does not violate due process for a state to permit cross-examination as to post-arrest silence when a defendant chooses to take the stand.
Id.
at 607, 102 S.Ct. at 1312. In
Jenkins v. Anderson,
447 U.S. 231, 100 S.Ct. 2124, 65 L.Ed.2d 86 (1980), the Court held that impeachment of a testifying defendant by reference to his pre-arrest silence does not violate a defendant’s constitutional rights. The Court noted that allowing impeachment through pre-arrest silence does not offend due process because “no governmental action [has] induced [the defendant] to remain silent.”
Id.
at 240, 100 S.Ct. at 2130. Consequently, if testimony relates to pre-arrest silence, the constitutional claim has no merit; if it relates to post-arrest silence, there has been a constitutional violation.
Even if we find that there has been a constitutional violation, however, our inquiry does not end. We must then consider whether or not the
Doyle
violation is harmless error under the standards set out in
Chapman v. United States,
547 F.2d 1240, 1248-49 (5th Cir.),
cert. denied,
431 U.S. 908, 97 S.Ct. 1705, 52 L.Ed.2d 393 (1977). In
Chapman
we described three general types of
Doyle
violations:
[1] When the prosecution uses defendant’s post arrest silence to impeach an exculpatory story offered by defendant at trial and the prosecution directly links the implausibility of the exculpatory story to the defendant’s ostensibly inconsistent act of remaining silent, reversible error results even if the story is transparently frivolous.
[2] When the prosecutor does not directly tie the fact of defendant’s silence to his exculpatory story, i.e., when the prosecutor elicits that fact on direct examination and refrains from commenting on it or adverting to it again, and the jury is never told that such silence can be used for impeachment purposes, reversible error results if the exculpatory story is not totally implausible or the indicia of guilt not overwhelming.
[3] When there is but a single reference at trial to the fact of defendant’s silence, the reference is neither repeated nor linked with defendant’s exculpatory story, and the exculpatory story is transparently frivolous and evidence of guilt is otherwise overwhelming, the reference to defendant’s silence constitutes harmless error.
Id.
at 1249-50 (citations and footnote omitted). The second and third
Chapman
categories are not rigid rules but instead are guidelines to help a court examine a
Doyle
violation.
Alderman v. Austin,
695 F.2d 124, 126 n. 7 (5th Cir.1983) (en banc). In addition, we have held in cases following
Chapman
that a prosecutor’s remarks must be evaluated in context.
E.g., United States v. Shaw,
701 F.2d 367, 381 (5th Cir.1983),
cert. denied,
465 U.S. 1067, 104 S.Ct. 1419, 79 L.Ed.2d 744 (1984). In
Shaw
we stated that:.
The alternative tests for determining whether a prosecutor’s or witness’s remarks constitute comment on a defendant’s silence are whether the “manifest intent” was to comment on the defendant’s silence or, alternatively, whether the character of the remark was such that the jury would “naturally and necessarily” construe it as a comment on the defendant’s silence. Both the intent of the prosecutor and the character of the remarks are determined by reviewing the context in which they occur, and the burden of proving such intent is on the defendant.
Id.
at 381 (citations and footnote omitted).
Finally, we note that Cardenas did not object at trial to the testimony that he asserts violates
Doyle.
In such circumstances, our review is even more limited. Consequently, if the prosecutor’s statements violate
Doyle
and are not harmless error under
Chapman,
we must then determine whether plain error has occurred.
United States v. Allston,
613 F.2d 609, 611-12 (5th Cir.1980).
See also Shaw,
701 F.2d at 382 n. 9. An error that fails to meet the harmless error standard under
Chapman
may nevertheless not be egregious enough to constitute plain error.
See All-ston,
613 F.2d at 611-12. In the context of jury instructions, we have stated that plain error occurs when “the error is so great as to result in the likelihood of a grave miscarriage of justice.”
United States v. Franklin,
586 F.2d 560, 569 (5th Cir.1978) (quoting
United States v. Smith,
502 F.2d 1250, 1256 (5th Cir.1974)),
cert. denied sub nom., Bonamo v. United States,
440 U.S. 972, 99 S.Ct. 1536, 59 L.Ed.2d 789 (1979). We believe the same standard is applicable here. With these principles in mind we examine Cardenas’ claims.
B.
Cardenas asserts that the testimony elicited by the government on its direct examination of Border Patrol Agents Barrett and Segarra violated
Doyle.
We disagree that the testimony violates
Doyle. Doyle
is inapplicable to these statements. During these conversations, there is no indication that Cardenas invoked his
Miranda
rights. The burden rested on Cardenas to invoke his right to remain silent.
United States v. Mattoni,
698 F.2d 691, 695 (5th Cir.1983). Instead of remaining silent, Cardenas responded to the agents’ questions. Consequently,
Doyle
does not apply.
See Lofton v. Wainwright,
620 F.2d 74, 77 (5th Cir.1980)
(Doyle
not applicable when defendant speaks).
Cardenas also complains that portions of the prosecutor’s opening statement and closing argument improperly commented on his silence.
In deciding whether or not the prosecutor’s comments improperly commented on Cardenas’ silence, we first note that all but one of the comments in the prosecutor’s opening remarks that Cardenas objects to refer to his codefendants and not to him. Cardenas does not have standing to raise possible violations of the constitutional rights of third parties. The general rule is that one may not claim standing in the courts to vindicate the constitutional rights of a third party.
Singleton v. Wulff,
428 U.S. 106, 113-14, 96 S.Ct. 2868, 2873-74, 49 L.Ed.2d 826 (1976). In
Rakas v. Illinois,
439 U.S. 128, 99 S.Ct. 421, 58 L.Ed.2d 387 (1978), the Court held that, due to the personal nature of a person’s fourth amendment rights, a person can only complain about violations of his own fourth amendment rights and not the fourth amendment rights of others.
Id.
at 139-40, 99 S.Ct. at 428. The Court has also recognized that the fifth amendment privilege against self-incrimination is a purely personal right.
Bellis v. United States,
417 U.S. 85, 89-90, 94 S.Ct. 2179, 2183-84, 40 L.Ed.2d 678 (1974). Thus, the
Beilis
court held that a former partner of a law firm could not invoke the privilege against self-incrimination to refuse to produce records of the law firm as directed by a grand jury subpoena. The Court held that the law partnership had an institutional identity independent of its individual partners and that, since the records were held by the partner in a representative capacity, the partner could not show a violation of
his
fifth amendment rights. Similarly, a defendant’s
Miranda
warnings are based on the fifth amendment privilege not to be compelled to incriminate himself.
Miranda v. Arizona,
384 U.S. 436, 86 S.Ct. 1602, 16 L.Ed.2d 694 (1966). We believe the teachings of
Beilis
are equally applicable in cases where
Doyle
violations have allegedly occurred: a defendant can only complain about violations of his own rights. Thus, we examine the prosecutor’s comments only to the extent that they may have violated Cardenas’ rights under
Doyle.
We agree with Cardenas that the first quoted remark from the prosecutor’s opening statement and the quoted remarks from the prosecutor’s closing argument violate
Doyle.
All of the prosecutor’s comments concern a time period after the Border Patrol agents had apprehended Cardenas and his
Miranda
protection against interrogation had commenced. It seems clear that not only did the prosecutor intend to comment on Cardenas’ silence, but the jury would naturally construe it in that way. The prosecutor’s comment would naturally be seen by the jury as a comment on Cardenas’ failure to offer an exculpatory story immediately as well as a general attack on his credibility. This violates
Doyle.
As we have already stated, we must now determine whether the
Doyle
violation is harmless error.
Shaw,
701 F.2d at 382. We believe that the prosecutor’s comments in his opening statement and closing argument constitute harmless error. This case does not seem to fall squarely into any of the categories of cases discussed in
Chapman.
None of the statements which violate
Doyle
came out in the course of the testimony of witnesses but instead occurred only in the opening statement and closing argument. The defense attorney explicitly told the jury in his opening statement that comments by the attorneys were not to be considered evidence by them in deciding the defendants’ guilt or innocence. Furthermore, in its charge to the jury, the district court reiterated the fact that statements by attorneys are not evidence in the case. We also find it significant that the comments by the prosecutor did not rebut Cardenas’ defense of noninvolvement but instead rebutted his codefendants’ coercion story.
The government also presented strong evidence of Cardenas’ guilt. For example, Cardenas had white powder from the packages of marijuana on his clothes. In addition, the evidence showed that a close connection existed between Cardenas and the other codefendants. Finally, Cardenas was crossing the border at an unusual time if he was a field worker as he asserted; he also fled when he was spotted by the Border Patrol agents. Under these circumstances, we believe the
Doyle
violations in this case constitute harmless error and
a fortiori
are not plain error.
IV.
We find sufficient evidence in the record to support Cardenas’ convictions. Furthermore, we find the
Doyle
violations by the prosecutor to be harmless error. Accordingly, Cardenas’ convictions are AFFIRMED.