The defendants in this case were indicted, along with twenty-eight others, for a conspiracy to distribute cocaine and heroin in the Dallas area. The jury found the defendants guilty of conspiracy and attributed at least one kilogram of heroin and at least five kilograms of cocaine to each defendant. On appeal, the defendants challenge the district court’s interpretation of the Sentencing Guidelines and the sufficiency of the evidence against them. We find no error and affirm.
I
Following a series of heroin overdoses in 1996 and 1997, police in Dallas and its surrounding communities enlisted the assistance of the FBI as part of an investigation of heroin distribution networks in the area. The resulting Collin/Denton Counties Drug Task Force identified several large heroin distributors operating in Dallas, including one led by defendant Jesus “Tony” Carbajal and one led by Rogelio Moreno. As part of this investigation, the task force monitored telephone conversations involving Carbajal, Moreno, and Car-bajal’s lieutenant, Rogelio “Oscar” Saenz. The recorded calls indicated that both Moreno and Carbajal obtained most of their heroin supplies from Caesar Rodriguez, a distributor from California. Based on information from these calls, police calculated that the Carbajal and Moreno organizations sold more than seventy-five kilograms of heroin between June 1999 and May 2000.
The task force also conducted ground and air surveillance of suspected conspirators, which revealed the procedures by which Carbajal and Moreno resold the drugs to individuals in the Dallas area. 1 Customers would call Carbajal or Moreno to place orders for heroin and/or cocaine. The customers would be directed to meet a runner at one of several standard locations in and around Dallas. At the designated meeting place, the runner would instruct the customer to follow him to another location to complete the transaction. Some of these customers lived in Plano, Texas, a *282 community north of Dallas in the Eastern District of Texas, and would return home after purchasing the drugs.
In October 2000, a grand jury sitting in the Eastern District of Texas returned a superseding indictment against thirty-two defendants connected with the Carbajal and Moreno organizations. 2 The indictment alleged that each defendant participated in a conspiracy to distribute at least one kilogram of heroin and at least five kilograms of cocaine in violation of 21 U.S.C. §§ 841 and 846. Twenty-eight of the defendants pleaded guilty, and many of these testified against the four remaining defendants: Carbajal, Andres Milan, Julian Soliz Perez, and Favian Ramos. According to the indictment, each defendant played a distinct role in the overall conspiracy: Carbajal was a leader within the distribution network, Milan was Carbajal’s alternate heroin supplier, Perez was an intermediary between the Dallas distributors and their California supplier, and Ramos purchased heroin from the same California supplier.
After a three-day trial in December 2000, a jury found all four defendants guilty of participation in a conspiracy to distribute heroin and cocaine. In response to special issues submitted by the court, the jury specifically attributed one kilogram of heroin and five kilograms of cocaine to each defendant. The district court sentenced Carbajal to life in prison followed by ten years of supervised release. 3 Milan received a sentence of 140 months in prison followed by five years of supervised release. Perez received a sentence of 170 months in prison followed by five years of supervised release. Ramos received a sentence of 148 months in prison followed by four years of supervised release.
On appeal, each defendant raises various issues concerning the propriety of their sentences and the sufficiency of the evidence against them. We now turn to address those issues.
II
Carbajal’s primary argument concerns the district court’s application of Sentencing Guideline § 2D1.1(a)(2) to enhance his sentence. Section 2D1.1(a)(2) establishes a base offense level of 38 if the defendant is convicted of drug trafficking under 21 U.S.C. § 841(b) “and the offense of conviction establishes that death or serious bodily injury resulted from use of the substance.” Based on its finding “beyond a reasonable doubt” that two overdose deaths resulted from the use of heroin sold by Carbajal’s organization, the district court concluded that § 2Dl.l(a)(2) applied to Carbajal and sentenced Carbajal accordingly.
4
On appeal, Carbajal challenges the district court’s determination on two grounds. First, Carbajal argues that the district court employed too lenient a standard of causation in determining whether the deaths “resulted from” heroin purchased from Carbajal’s organization. Second, he contends that the government did not present sufficient evidence connecting Carbajal with the deaths to warrant application of § 2Dl.l(a)(2). We re
*283
view the district court’s interpretation of the Sentencing Guidelines de novo and the district court’s factual findings for clear error. See
United States v. Paul,
A
Carbajal first argues that the sentence enhancement established in § 2Dl.l(a)(2) applies only if the- government can show that drugs attributable to him were the proximate, reasonably foreseeable cause of a death. The government responds, and the district court agreed, that the guidelines impose no such causation requirement. As a consequence, the district court determined that Carbajal could be held responsible for overdose deaths if the government could show a reasonable medical probability that heroin supplied by Carbajal caused the deaths. 5 For the reasons set out below, we agree and hold that § 2Dl.l(a)(2) is a strict liability provision that applies without regard for common law principles of proximate cause or reasonable foreseeability.
It is well established that our interpretation of the Sentencing Guidelines is subject to the ordinary rules of statutory construction.
See United States v. Boudreau,
Carbajal contends that, rather than adopting the reasoning of the cases interpreting § 841(b)(1)(C), we should interpret the plain language of the guideline provision independently. In Carbajal’s view, common law proximate cause and foreseeability principles should apply under § 2D1.1(a)(2) because the plain meaning of the term “resulted from” requires the drugs sold by the defendant “to be a direct cause of death, not a possible or remote cause.” Carbajal therefore argues that § 2D1.1(a)(2) effectively subsumes traditional causation principles like foreseeability and proximate causation.
Like § 841(b)(1)(C), however, the plain language of § 2Dl.l(a)(2) does not impose any sort of explicit causation requirement. Nor can we find a basis to infer that the term “resulted from” incorporates these principles. 8 The Sentencing Commission could have expressly limited the sentencing enhancement to cases in which the drugs sold by the defendant foreseeably caused a death, but it chose not to do so. We therefore hold, in accordance with our sister circuits’ interpretation of § 841(b)(1)(C), that § 2Dl.l(a)(2) is a strict liability provision and does not require proof of proximate causation or reasonable foreseeability. 9
In the alternative, Carbajal urges us to adopt the proximate cause test applied by the Sixth Circuit in
United States v. Swiney,
We therefore hold that the district court did not err in declining to apply a proximate cause or reasonable foreseeability test before enhancing. Carbajal’s sentence under § 2Dl.l(a)(2).
B
The next question is whether, applying the above standard of causation to the circumstances present here, the district court clearly erred in attributing the heroin-related deaths to Carbajal. At the sentencing hearing, the government sought to show that, between December 1998 and July 1999, three people died of heroin overdoses in the Dallas area after using heroin purchased from Carbajal.
The government first presented evidence that Carbajal’s organization sold heroin to Josh Harmon that resulted in Harmon’s death. According to the testimony at the sentencing hearing, Harmon and two friends telephoned Carbajal’s organization on December 19, 1998 to place an order for heroin. Later that day, they met with Carbajal’s lieutenant, “Oscar” Saenz, and purchased a quantity of heroin. 11 Harmon and his friends then mixed the heroin with an over-the-counter sleeping aid and divided the mixture into capsules. Harmon received about forty of the capsules, some of which he consumed later that night at a party. The next morning, Harmon’s friends had difficulty reviving him, and they attempted -to resuscitate him. When these efforts failed, Harmon was eventually left outside a hospital in Dallas, where he was pronounced dead.
At the, sentencing hearing, the government’s medical expert testified that, despite traces of other drugs in Harmon’s blood, there was a reasonable medical probability that heroin was the cause of Harmon’s death. Although Harmon’s friend testified that Harmon could have purchased additional heroin from someone at the party, the friend also testified that heroin users ordinary consume between ten and fifteen capsules over a twelve hour period — considerably fewer than the forty capsules that Harmon received earlier that day. Taken together, we find this evidence sufficient to support the district court’s conclusion that Carbajal, acting through Saenz, sold Harmon the heroin that resulted in his death. 12
Carbajal further argues that Harmon’s death was the product of a superseding cause and therefore did not “result[ ] from” the heroin he purchased from Carbajal’s organization. Specifically, Carbajal argues that Harmon’s death was caused by his friends’ failure to seek immediate medical attention. Even assuming that the concept of superseding causes applies under § 2D1.1(a)(2), however, this argument fails because negligent (as opposed to grossly negligent or intentional) acts by third parties cannot be the su *286 perseding cause of an injury if they are foreseeable. 13 In this case, there is no evidence that Harmon’s friends engaged in grossly negligent conduct or intended to injure Harmon by failing to take him promptly to the hospital. 14 Because the failure to seek immediate treatment for a heroin overdose is clearly a foreseeable outcome, any such delay could not break the causal chain between Harmon’s death and the sale of heroin by Carbajal.
The government also presented evidence that Mark Tuinei, a former offensive lineman for the Dallas Cowboys, purchased heroin from Saenz that led to Tuinei’s death. Specifically, the district court heard testimony that Tuinei and three others—Keelan Murray, Wes Malone, and Nicki Sualua—purchased a quantity of heroin from Saenz at a gas station in Dallas. After receiving some instruction from Murray, Tuinei “cooked” and injected the heroin into his arm. Tuinei immediately passed out on a couch. Although Tuinei had stopped breathing, Murray successfully revived him by performing CPR. Because Tuinei remained unconscious, however, Murray and Sualua carried him to his car. Sualua drove Tui-nei home and stayed with Tuinei while he slept it off. When Tuinei did not wake up the next morning, Sualua called emergency paramedics, who pronounced Tuinei dead.
At the sentencing hearing, the government’s medical expert testified that the cause of Tuinei’s death was the mixed effect of heroin and ecstasy and that there was a reasonable medical probability that heroin was “primarily responsible.” Here again, the district court did not clearly err in attributing Tuinei’s death to heroin purchased from Carbajal’s organization.
Cf. United States v. Cathey,
In sum, we find that the government presented sufficient evidence to support the district court’s determination that the overdose deaths of Harmon and Tuinei “resulted from” the heroin they purchased from Carbajal. 15 As a consequence, the district court did not err in applying § 2Dl.l(a)(2) and sentencing Carbajal to *287 life imprisonment.. 16
Ill
Carbajal next challenges the district court’s decision to exclude evidence that allegedly undermines the pre-sen-tence report’s conclusion that the deaths of Harmon, Tuinei, and Walker were attributable to heroin sold by Carbajal. During the sentencing hearing, Carbajal sought to present cross-examination testimony by the government’s medical expert that Harmon, Tuinei, and Walker may not have died if they had received prompt medical attention. We review for an abuse of discretion the district court’s decision to exclude evidence rebutting information in the pre-sentence report. See Fed.R.Crim.P. 32(c)(3)(A) (providing that the defendant may, “in the discretion of the court, introduce testimony ... relating to any alleged factual inaccuracy contained in [the report]”).
As a general rule, information in the pre-sentence report is presumed reliable and may be adopted by the district court “without further inquiry” if the defendant fails to demonstrate by competent rebuttal evidence that the information is “ ‘materially untrue, inaccurate or unreliable.’ ”
United States v. Parker,
In this case,, Carbajal sought to show that the pre-sentence report was inaccurate because it did not consider whether delays in obtaining medical attention may have contributed to the three heroin-related deaths. As we noted earlier, however, the failure to seek prompt medical attention does not constitute a superseding cause of the victims’ deaths unless that failure is grossly negligent or intended to injure the victim.
See Rodriguez,
IV
Carbajal next argues that the district court erred by imposing a sentence greater than that authorized for a cocaine-only conspiracy. The superseding indictment in this case alleged a conspiracy to distrib *288 ute at least one kilogram of heroin and at least five kilograms of cocaine. Although the jury specifically attributed more than one kilogram of heroin and more than five kilograms of cocaine to each defendant, the district court explicitly declined to consider the'jury’s finding on cocaine in determining their respective sentences. According to Carbajal, the district court effectively granted a motion for judgment of acquittal with respect to the cocaine component of the conspiracy and thus rendered the jury’s verdict on the alleged multi-drug conspiracy “ambiguous.” Carbajal contends that, as a consequence, the district court was required to sentence the defendants within the statutory maximum for the drug carrying the least severe penalty—in this case, cocaine. 17
Carbajal’s argument depends upon his assertion that the jury verdict in this case became ambiguous when the district court disregarded the jury’s finding on the quantity of cocaine attributable to the conspiracy. A jury verdict in a criminal case is ambiguous if the defendant is charged with a multiple-drug conspiracy and the jury verdict does not specify whether the jury found the defendant guilty with respect to some or all of the drugs.
See United States v. Cooper,
The defendants in the present case, however, cannot benefit from this rule because the jury verdict was not ambiguous. To the contrary, the jury specifically found that (1) each defendant participated ,in a conspiracy to distribute heroin and cocaine (2) at least one kilogram of heroin and at least five kilograms of cocaine were attributable to each defendant. The jury’s verdict leaves no doubt that it found a conspiracy with respect to both cocaine and heroin. Although the district court may have implicitly concluded that the jury’s findings on the amount of cocaine attributable to the conspiracy were not supported by the evidence, such a conclusion does not render ambiguous the jury’s findings on the' amount of heroin attributable to the conspiracy. Thus, the district court properly sentenced the defendants based on the jury’s finding that the conspiracy involved at least one kilogram of heroin.
V
Carbajal also contends that the government presented insufficient evidence to prove that venue in the Eastern District of Texas was proper. Carbajal did not, however, raise a proper objection to venue before the jury’s verdict and therefore waived this issue on appeal.
19
*289
See United States v. Carreon-Palacio,
VI
Milan, Perez, and Ramos each argue that the government presented insufficient evidence to prove beyond a reasonable doubt that they participated in a conspiracy to distribute cocaine and heroin. Where the defendant has preserved his challenge to the sufficiency of the government’s evidence, we review de novo the district court’s denial of a motion for judgment of acquittal.
See United States v. Sanchez,
To show the existence of a drug conspiracy in violation of 21 U.S.C. § 846, the government must prove: “1) an agreement existed to violate drug laws; 2) the appellants knew of the agreement; and 3) the appellants voluntarily participated in it.”
United States v. Baptiste,
Perez argues that the evidence does not support the government’s contention that he knowingly participated in the alleged conspiracy. We disagree. Rogelio Moreno, the leader of one of the heroin distribution operations involved in the conspiracy, testified at trial that he obtained cocaine from Perez on two separate occasions. Moreno further testified that (1) Perez had unsuccessfully attempted to obtain heroin for Moreno and (2) Moreno purchased heroin from Caesar Rodriguez in Perez’s apartment. This evidence is sufficient to support the jury’s conclusion that Perez was a knowing participant in the conspiracy. 20
*290
Milan argues that the government’s evidence connecting him to the conspiracy is inconclusive or, at most, discloses a single drug transaction. To support its theory that Milan was Carbajal’s back-up heroin supplier, the government presented evidence that Carbajal turned to Milan for heroin supplies when Carbajal began to run low. This evidence, viewed in conjunction with evidence of Milan’s actual sale of heroin to Carbajal, provides a sufficient basis from which the jury could reasonably infer that Milan knowingly participated in the conspiracy.
21
See United States v. Gourley,
Ramos similarly argues that the evidence shows, at most, that he was a “knowing spectator at the scene of [a] crime.” Because Ramos failed to preserve this issue for appeal, however, our “review is limited to determining whether there was a manifest miscarriage of justice, that is, whether the record is devoid of evidence pointing to guilt.” 22 At trial, the government presented evidence that, in the course of searching Ramos’s car after he was arrested, police officers found a package containing 140 grams of cocaine and 126 grams of heroin on the floorboard where Ramos was sitting. The record also supports a reasonable inference that Ramos obtained the package from Caesar Rodriguez, the primary supplier for the Carbajal and Moreno organizations, shortly before Ramos was arrested. In sum, the record is not devoid of evidence connecting Ramos with the conspiracy alleged in the indictment 23
VII
Taking a slightly different tack, Milan argues that the government’s evidence at trial proved, at most, the existence of multiple conspiracies rather than the single conspiracy alleged in the indictment. Because the government could not show that Milan participated in all of the distinct *291 conspiracies, Milan argues that this variance prejudiced him by exposing the jury to evidence of other conspiracies with which he was not connected.
To prevail on this claim, Milan must show (1) an actual variance between the allegations in the indictment and the proof at trial and (2) prejudice flowing from the variance that affected his substantial rights.
See United States v. Morns,
As a general rule, “ ‘where the indictment alleges a single conspiracy and the evidence establishes each defendant’s participation in at least one conspiracy[,] a defendant’s substantial rights are affected only if the defendant can establish reversible error under general principles of joinder and severance.’ ”
United States v. Pena-Rodriguez,
In this case, Milan relies on a general allegation that the evidence concerning his co-defendants’ activities — particularly those involving the sale of cocaine — had a prejudicial “spillover effect” on his case because he did not participate in those activities. But the conspiracy alleged here was not particularly complex, and the district court instructed the jury to consider the charges against each defendant separately. Under these circumstances, it seems clear that' the jury would have no difficulty making an individualized assessment of the evidence against each defendant. Because Milan has failed to “ ‘isolate events occurring in the course of a joint trial’ ” that may have impaired his defense and to “ ‘demonstrate that such events caused substantial prejudice,’ ” 24 we conclude that reversal is not warranted in this case. 25
*292 VIII
For the reasons set out above, we AFFIRM the judgment of the district court.
AFFIRMED.
Notes
. In their testimony at trial, Rogelio Saenz and other conspirators confirmed that Carba-jal’s organization typically followed these procedures.
. The original indictment, which did not include the quantity of drugs involved, was filed in May 2000.
. The district court sentenced the defendants based solely on the amount of heroin attributable to them.
. The district court also found by a preponderance of the evidence that heroin sold by Carbajal caused the overdose death of a third customer. In addition, the district court increased Carbajal’s base offense level by four points because Carbajal organized and led the drug distribution operation.
. Perez, Milan, and Ramos were not sentenced under § 2D1.1(a)(2).
. In
United States
v.
McIntosh,
.
See United States v. Robinson,
. We note that a leading dictionary defines “result from” as "to proceed, spring, or arise as a consequence, effect, or conclusion.” Webster's Third New Int’l Dictionary 1936 (1993). This definition supports our conclusion that § 2D 1.1 (a)(2) does not impose a proximate causation or reasonable foreseeability requirement.
. Carbajal further argues that the definition of the term "offense of conviction" should inform our interpretation of the standard of causation under § 2D 1.1 (a)(2). The guidelines do not define the term "offense of conviction,” but the Third Circuit has suggested in
dicta
that "offense of conviction” should be interpreted to "include[] only the facts underlying the specific criminal offense for which the defendant was convicted.”
United States v. Pressler,
. See McIntosh,
. The government also presented testimony that Saenz obtained his heroin solely from Carbajal.
. Because the district court found that Harmon’s death "resulted from” heroin sold by Carbajal beyond a reasonable doubt, we need not address whether the district court could have applied a lower standard of proof in this context.
.
See United States v. Rodriguez,
.
Cf. Rodriguez,
. The government also presented evidence that Kyle Walker, a regular heroin user, died after ingesting heroin he obtained from his girlfriend. An officer testified that, before she fled the country, Walker’s girlfriend told the officer that she had purchased heroin from Saenz on the day of Walker's death. As noted above, § 2D 1.1 (a)(2) applies if the government can prove that one death resulted from drugs sold by the defendant. Because the government has satisfied this requirement with respect to Harmon and Tuinei, we need not address the sufficiency of the evidence connecting Carbajal to Walker’s death.
. Carbajal also contends that the sentencing enhancement based on his prior conviction and the enhancement based on the heroin-related deaths are elements of an aggravated offense that must be submitted to the jury and proven beyond a reasonable doubt under
Apprendi v. New Jersey,
. As Carbajal concedes, the appellants forfeited this argument by failing to raise it at the sentencing hearing.
See United States v. Olano,
.
Cooper,
. Carbajal responds that his motion for judgment of acquittal at the close of the government’s case was sufficient to raise an issue concerning venue. Carbajal's motion for acquittal argued that "the Government has not presented sufficient evidence that would allow a rational trier of fact to find [Carbajal] guilty beyond a reasonable doubt of what has been charged against him.” This motion, standing alone, is not adequate to put the
*289
government or the district court on notice that Carbajal challenged venue in the Eastern District of Texas. Although "[w]aivers of venue rights by silence are not to be readily inferred” and a pre-trial objection to venue is not required in all circumstances, Carbajal failed to preserve this issue for appeal by specifically raising the issue in his motion for acquittal or by requesting a jury instruction on venue.
Carreon-Palacio,
. Perez also argues that the district court abused its discretion by denying his motion to sever his trial from that of the other defendants. Specifically, he contends that the joint trial with Carbajal created an unacceptable risk that the jury would find him guilty by association. To show that the district court abused its discretion in denying his motion to sever, Perez must demonstrate prejudice from a joint trial (1) "to such an extent that the district court could not provide adequate protection” and (2) that outweighed the government's interest in judicial economy.
United States v. Richards,
.Milan also argues that the jury's finding that he was responsible for at least one kilogram of heroin and at least five kilograms of cocaine is not supported by the evidence. As Milan acknowledges, however, the district court did not consider the jury’s cocaine finding in sentencing him. He further argues that
Apprendi v. New Jersey,
.
United States v. Delgado,
. We need not decide whether the government’s evidence would have been sufficient to sustain Ramos's conviction if he had renewed his motion for acquittal at the close of all the evidence.
.
United States v. Posada-Rios,
. We similarly find no merit in Milan’s argument that the district court erred by declining to instruct the jury on the difference between a single conspiracy and multiple conspiracies. A multiple conspiracy instruction is required “where the indictment charges several defendants with one (1) overall conspiracy, but the proof at trial indicates that some of the defendants were
only
involved in separate conspiracies unrelated to the overall conspiracy charged in the indictment.”
United States v. Greer,
Moreover, although Perez adopted Milan’s arguments on appeal, Perez has not identified evidence indicating that he was “only involved in separate conspiracies unrelated to the overall conspiracy charged in the indictment.”
Greer,