Jose Angel Diaz-Carreon (“Diaz-Car-reon”) appeals his convictions on two counts of controlled substances violations: importation of marijuana into the United States from Mexico in violation of 21 U.S.C. §§ 952(a), 960(a)(1) (1982) and possession of marijuana with intent to distribute in violation of 21 U.S.C. § 841(a)(1) (1982). He argues that the evidence was insufficient to prove a “knowing” violation of the law and that the prosecutor made improper comments that compromised Diaz-Car-reon’s right to a fair trial. Unable to find that the district court committed reversible error, this Court affirms Diaz-Carreon’s convictions.
I. FACTS AND PROCEDURAL HISTORY
Approximately 5:40 a.m. on June 20, 1989, Jose Angel Diaz-Carreon attempted to drive a stake bed pickup truck with California license plates across the international border into El Paso, Texas. He presented an amnesty card to the customs inspector at the Paso Del Norte Port of Entry and declared that he was bringing nothing into the United States. The inspector, finding it unusual that a person arriving early from Mexico in a vehicle with California plates declared neither lunch nor luggage, conducted a cursory inspection of the truck. She walked around the truck and tapped the sides of the unusual convex sideboards. Her taps produced different sounds — some solid, some hollow.
The customs inspector returned to the driver’s side of the vehicle and asked Diaz-Carreon several questions. Diaz-Carreon responded that he was driving the truck to his residence in Canutillo, Texas. He explained that he was not the owner of the truck and admitted that he had no driver’s license. As the customs inspector continued her questioning, Diaz-Carreon’s previously friendly demeanor deteriorated into extreme and noticeable nervousness. He became increasingly agitated and unable to communicate. Suspecting that some of the truck’s stakes might contain controlled sub *953 stances, the customs inspector directed Diaz-Carreon to secondary inspection.
Customs officials at secondary inspection further questioned Diaz-Carreon. He stated that he did not own the truck and was driving it to its owner in Anthony, New Mexico (approximately six miles from Ca-nutillo, Texas). He again maintained that he was bringing nothing into the United States. Meanwhile, federal agents conducted a successful canine sniff test on the pickup truck. The agents dismantled the sideboards on the truck, discovering approximately 161 pounds of marijuana. Customs officials subsequently escorted Diaz-Carreon to a detention cell and searched him for contraband and weapons. Toward the end of the search, before being informed that agents had found marijuana in the truck, Diaz-Carreon nervously volunteered in Spanish, “If the truck is loaded, I didn’t know about it.”
Diaz-Carreon waived his legal rights and consented to an interview with a special agent of the customs service. In the interview, Diaz-Carreon stated that he lived in Puerta de Anapra, Mexico, but was traveling to Anthony, New Mexico, to search for work on a ranch. He revealed that a man known simply as Ruben had loaned him the truck so that he could find employment. However, Diaz-Carreon commented that he had met Ruben only a few days earlier and did not know where Ruben could be found. Moreover, Diaz-Carreon could not explain how Ruben would recover the pickup truck. 1
On June 20, 1989, Diaz-Carreon was charged with importation of marijuana into the United States from Mexico 2 and possession of marijuana with intent to distribute. 3 After a short trial that began on October 18,1989, the jury returned a guilty verdict on both counts. The district court sentenced Diaz-Carreon to forty-one months of imprisonment on each count, the sentences to be served concurrently.
II. DISCUSSION
On appeal, Diaz-Carreon raises two arguments. First, he complains that the evidence was insufficient to prove a “knowing” violation of the law. Second, he complains that the prosecutor made improper statements that compromised Diaz-Car-reon’s right to a fair trial. This Court will examine each of these arguments in turn.
A. Sufficiency of the Evidence
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. Williams-Hendricks,
In considering Diaz-Carreon’s allegations, this Court “must view the evidence and all reasonable inferences that may be drawn from the evidence in a light most favorable to the government.”
United States v. Prieto-Tejas,
In the instant case, the Government had the difficult task to prove that DiazCarreon knowingly possessed and imported marijuana. The Government could have, and did in fact, offer evidence that DiazCarreon was the driver of a vehicle which contained contraband. Knowledge of the presence of a controlled substance often may be inferred from the exercise of control over a vehicle in which the illegal substance is concealed.
United States v. Richardson,
While Diaz-Carreon’s control of the pickup truck does not in itself constitute sufficient evidence that he knowingly possessed and imported marijuana, the Government in the instant case produced significant additional evidence that tends to establish Diaz-Carreon’s guilty knowledge. Specifically, the Government asserts that the following circumstances adequately evidence the defendant’s consciousness of guilt: (1) Diaz-Carreon’s nervousness; (2) Diaz-Car-reon’s conflicting statements to customs officials; and (3) Diaz-Carreon’s implausible story.
Nervousness.
Nervous behavior at an inspection station frequently constitutes persuasive evidence of guilty knowledge.
See Richardson,
Inconsistent statements to customs officials.
Perhaps the strongest evidence of a
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criminal defendant’s guilty knowledge is inconsistent statements to federal officials.
See, e.g., Richardson,
Implausible story.
This Court has acknowledged that a “less-than-credible explanation” for a defendant’s actions is “part of the overall circumstantial evidence from which possession and knowledge may be inferred.”
United States v. Phillips,
Considered together, these circumstances presented sufficient evidence of Diaz-Car-reon’s consciousness of guilt. The jury certainly was not
obligated
on these facts to find that Diaz-Carreon possessed guilty knowledge; “it could well have rejected inferences which are reasonable to draw from these facts.”
Del Aguila-Reyes,
B. Prosecutorial Comments
The prosecutor occupies a distinctive position in the criminal justice system: he is the hammer that sparks fire on the anvil of justice.
6
He can strike a devastating blow to the career of a recidivist; he can release the shackles on an innocent victim of the system. But with great power comes great responsibility — responsibility that
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easily can be abused. “Justice is served only when convictions are sought and secured in a manner consistent with the rules that have been crafted with great care over the centuries.”
United States v. Murrah,
This Court recognizes, however, that a criminal defendant bears a substantial burden when attempting to demon.strate that improper prosecutorial comments constitute reversible error. “A criminal conviction is not to be lightly overturned on the basis of a prosecutor’s comments standing alone.”
United States v. Lowenberg,
First, Diaz-Carreon argues that the Government prosecutor improperly attempted to shift the burden of proof. 7 During argument, the prosecutor stated:
Ladies and gentlemen, what this case comes down to is one thing. The credibility that you are going to give the witnesses that testified. And it’s like a balance. You’re either going to believe the Defendant and you’re going to disbelieve everyone else, or you’re going to believe other people and you’re going to disbelieve the Defendant. And that’s all it comes down to.
Record at 253. Divorced from context, this statement might appear to suggest that in order to acquit Diaz-Carreon, the jury must disbelieve all of the Government’s witnesses, and believe all of Diaz-Car-reon’s witnesses. Of course, such an implication would be incorrect: a jury could believe all of the Government witnesses and disbelieve all of the defense witnesses, but still find that the Government failed to meet its burden of proof. 8 When the pros *957 ecutor’s argument is considered in context, however, it is apparent that the prejudicial effect of the questionable comments is limited. The prosecutor’s remarks followed a heated attack by Diaz-Carreon’s counsel on the credibility of the customs agents who testified for the Government. The prosecutor correctly (if perhaps inartfully) noted that witness credibility was a critical issue in the case against Diaz-Carreon. Then, following the questionable comments, the prosecutor attempted to demonstrate that the Government witnesses were more credible than the defense witnesses. At no point did the prosecutor inform the jury the facts it must find in order to reach a certain verdict — indeed, the prosecutor expressly disavowed that he had a prerogative to do so. 9 Rather, the prosecutor’s apparent intent was (1) to inform the jury simply that it was likely to believe the witnesses of one side at the expense of the other side’s witnesses, and (2) to suggest that the Government witnesses were the witnesses the jury could find most believable. In this light, such a suggestion was not impermissible. 10
In another complaint of prosecutorial burden-shifting, Diaz-Carreon contends that the prosecutor, in response to a defense argument that the Government had bungled an opportunity to obtain important physical evidence by failing to search for Diaz-Carreon’s fingerprints on the concealed marijuana, improperly suggested that the defendant had an obligation to introduce fingerprint evidence. The prosecutor commented:
As far as fingerprints, ladies and gentlemen, as an Assistant U.S. Attorney, I should insist that Officer Shreve and the Department of Customs, hire fingerprint experts just to throw away. Maybe we ought to hire an expert who can dust the packages so we can take fingerprints off of each one of these. Maybe we’ll get 100 fingerprints that are comparable off of 171, 182 packages that we have, and then we compare them against known fingerprints, and then that fingerprint expert or those fingerprint experts bring in a bill for $10,000 for the work that they’ve done and I charge it to the U.S. Government, because ladies and gentlemen, we don’t have a national debt. And I have money in my budget to just throw away, because I should present that kind of evidence to you.
Record at 257. According to Diaz-Car-reon, this argument implied that if the defendant desired fingerprint evidence, he should have introduced it himself. Diaz-Carreon concedes, however, that he failed to object to the prosecutor’s allegedly questionable comments on fingerprint evidence. Thus, this Court reviews the comments under the “plain error” standard
11
— whether the comments “seriously affected the fairness, integrity, or public reputation of judicial proceeding^] and resulted in a miscar
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riage of justice.”
United States v. Goff,
Finally, Diaz-Carreon argues that the prosecutor impugned the character of defense counsel. 13 During argument, the prosecutor commented:
Ladies and gentlemen, if there’s a zealot in this particular courtroom, it’s the defense attorney. Thank God he’s a defense attorney and not part of the Government.
Record at 256. Such a comment was not only improper, it was disgraceful. The comment was a personal attack on the demeanor, as well as the status, of the defense counsel; it was clearly irrelevent to the guilt or innocence of the criminal defendant and apparently intended solely to prompt the jury to question the believability of defense counsel’s arguments. 14
Nonetheless, in response to the prosecutor’s improper comment, the defense counsel objected and received exactly the remedy he requested. The district court sustained the defendant’s objection and, in strong language, instructed the jury to disregard the prosecutor’s comment. Further, the court admonished the prosecutor to discontinue his personal attacks on defense counsel. 15 After receiving this favorable instruction from the court, the defense counsel elected not to pursue his objection; he neither requested nor received a mistrial.
The prosecutor’s attack on the demean- or — and perhaps even the character — of defense counsel was undoubtedly inappropriate. It is not sufficient merely to conclude, however, that the prosecutor’s comments were improper. Inappropriate or improper prosecutorial remarks are not necessarily reversible error. To support a reversal of the defendant’s conviction, the comments
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must “affect substantially the defendant’s right to a fair trial.”
United States v. Murrah,
Persistent questionable prosecutorial comments demand careful scrutiny to ensure that a defendant’s rights are not compromised. Here, the combination of certain circumstances, principally the defense counsel’s failure adequately to preserve error and the district court’s able use of firm curative instructions, persuades this Court that Diaz-Carreon’s right to a fair trial was not reversibly compromised. However, the prosecutor’s repeated reliance throughout the trial on sarcasm, innuendo and misstatement comes dangerously close to reversible error. Prosecutors are reminded that the bounds of this Court’s tolerance are narrow. Continued careless prosecutorial comments can — and will— cause the reversal of otherwise carefully wrought convictions.
III. CONCLUSION
This Court is unpersuaded that the evidence is insufficient to support the district court’s finding of a “knowing” violation of the law or that the prosecutor’s improper remarks compromised Diaz-Carreon’s right to a fair trial. Accordingly, the judgment of the district court is affirmed.
AFFIRMED.
Notes
. Diaz-Carreon testified at trial that he had given Ruben precise directions to the ranch where Diaz-Carreon sought employment.
. See 21 U.S.C. § 952(a) (1982), which provides in pertinent part:
(a) It shall be unlawful to import into the customs territory of the United States from any place outside thereof (but within the United States), or to import into the United States from any place outside thereof, any controlled substance in schedule I or II of subchapter I of this chapter, or any narcotic drug in schedule III, IV, or V of subchapter I of this chapter....
.See 21 U.S.C. § 841(a) (1982), which provides in pertinent part:
(a) Except as authorized by this subchapter, it shall be unlawful for any person knowingly or intentionally—
(1) to manufacture, distribute, or dispense, or possess with intent to manufacture, distribute, or dispense, a controlled substance....
. The Government argues that Diaz-Carreon was hardly an unwitting carrier. It notes that the condition of the sideboards on the pickup truck was highly unusual: they were freshly painted, swollen in appearance, and spongy to the touch. The Government maintains that a person familiar with the usual condition and appearance of a stake bed truck would have suspected that these atypical sideboards concealed contraband. This may well be true. However, the Government failed to introduce at trial any evidence demonstrating that Diaz-Car-reon himself was familiar with the type of stake bed truck involved in this case.
. In the instant case, evidence introduced at trial has cast additional doubt on the plausibility of Diaz-Carreon's account of the events. Diaz-Carreon’s wife, for example, testified that she overheard a man (presumably Ruben) tell Diaz-Carreon that he would give him $400 if Diaz-Carreon drove the truck across the border. This testimony suggests that Diaz-Carreon might have been compensated for his role in the attempt to smuggle the marijuana across the border.
.
See Berger v. United States,
. A prosecutor may not "misstate the jury’s function or the burden of proof.”
United States v. Cantu,
.
See, e.g., United States v. Vargas,
. Throughout closing argument, the prosecutor acknowledged that he had the burden to prove guilt beyond a reasonable doubt. At one point, the prosecutor recognized:
[A]s the Government representative, I have the burden of proof in this particular case, and that burden of proof is to prove to you beyond a reasonable doubt.... This means that I have to prove to you and leave you satisfied through you using your own common sense, your own reason, but you’re left without any doubt that this really happened.
Record at 230.
. Diaz-Carreon also complains that the prosecutor used a sarcastic tone of voice when he stated the jury might disbelieve the Government's witnesses. Diaz-Carreon contends that the prosecutor’s use of sarcasm was an oblique attempt to invoke national loyalty as a reason to return a guilty verdict. Diaz-Carreon relies upon
United States v. Herrera,
.See Fed.R.Crim.P. 52(b) ("Plain errors or defects affecting substantial rights may be noticed although they were not brought to the attention of the court.”).
. Diaz-Carreon’s reliance on
United States v. Garza,
. A prosecutor may not challenge the integrity or impugn the character of defense counsel.
United States v. De La Rosa,
. In evaluating the propriety of prosecutorial comments, it is appropriate to examine the comments in context. It might not have been offensive if the prosecutor, without additional comment, had described the defense counsel as a "zealot.” A zealot can be defined as an individual blessed with eagerness and passion. History provides countless examples of revered zealots — zealots for peace, zealots for justice, zealots for righteousness. An advocate in the highest tradition should be so consumed with eagerness and passion for his client’s cause that he becomes, in effect, a zealot.
It is apparent from the record, however, that the prosecutor in the instant case intended no such charitable meaning. A dictionary provides an alternate definition for the word "zealot” — "a fanatical partisan." Webster’s New Collegiate Dictionary 1352 (1979) (emphasis added). That the prosecutor intended to convey this negative meaning is evident from his further comment: "Thank God he’s a defense attorney and not part of the Government.” Read in context, the prosecutor’s comments suggest that defense counsel is a fanatic who would take any measure, including untruths, to secure his client’s acquittal.
.The exact language of the district court’s curative instruction is as follows:
DEFENSE COUNSEL: I object, Your Honor. That’s an unethical argument.
PROSECUTOR: That's not unethical, Your Hon- or. He invited it.
THE COURT: No, he didn’t. It’s improper and the jury is instructed to disregard it. And I’ll instruct you not to do that again.
.
United States v. Murrah
is similar to the instant case, but differs in two significant respects. First, the prosecutorial remarks in
Mur-rah
were egregiously offensive: the prosecutor accused the defense counsel of illegally and unethically withholding a witness.
