concurring:
In addition to challenging their convictions, the defendants, Sean Thomas Sullivan and Kenneth Campbell, seek relief from the life sentences imposed on them in the District of South Carolina in July 2003 under the then-mandatory Sentencing Guidelines regime.1 Sullivan and Campbell repeatedly objected to the sentencing procedures utilized by the court, contending that those procedures contravened the constitutional principles enunciated by the Supreme Court in Apprendi v. New Jersey, 530 U.S. 466, 120 S.Ct. 2348, 147 L.Ed.2d 435 (2000). During the pendency of these appeals, the Court decided Blakely v. Washington, 542 U.S. 296, 124 S.Ct. 2531, 159 L.Ed.2d 403 (2004), and United States v. Booker, 543 U.S. 220, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005). In the wake of those decisions, we now know that a sentencing court commits statutory error if it treats the Guidelines as mandatory, rather than as advisory. See Booker, 543 U.S. at 245-46, 125 S.Ct. 738; United States v. White, 405 F.3d 208, 215 (4th Cir.2005). As explained below, Sullivan and Campbell preserved their claims of such statutory error by raising timely Ap-prendi objections at sentencing. Because the district court committed statutory error in sentencing Sullivan and Campbell, and its error was not harmless, we vacate the defendants’ sentences and remand for resentencing.
I.
A.
Campbell was convicted on two charges: Count 1 for conspiracy to possess with intent to distribute and to distribute fifty grams or more of crack cocaine, in contravention of 21 U.S.C. § 846 (the “drug conspiracy offense”); and Count 49 for conspiracy to use, brandish, discharge, and possess firearms during and in relation to drug trafficking crimes, in violation of 18 U.S.C. § 924(o) (the “firearms conspiracy offense”). Meanwhile, Sullivan was also convicted of Count 1 for drug conspiracy and Count 49 for firearms conspiracy, and was further convicted on six other charges: Counts 29, 30, and 31 for distributions of less than five grams of crack, and Count 32 for possession with intent to distribute more than five grams of crack, all in contravention of 21 U.S.C. § 841 (collectively, the “drug distribution offenses”); Count 34 for using and carrying of a firearm during and in relation to a drug trafficking crime, in violation of 18 U.S.C. § 924(c)(1)(A)(i) (the “firearm use offense”); and Count 35 for possession of a firearm with an obliterated serial number, in contravention of 18 U.S.C. §§ 922(k) and 924(a)(1)(B) (the “obliterated serial number offense”).
Campbell’s presentence report (“PSR”) recommended a base offense level of 38 on the drug conspiracy offense, predicated on the finding that the offense involved 1.5 *263kilograms or more of crack cocaine. See USSG § 2Dl.l(c)(l) (2001). The PSR also advocated a two-level enhancement for possession of a dangerous weapon, id. § 2Dl.l(b)(l), and a four-level enhancement for being an organizer or leader of the conspiracy, id. § 3Bl.l(a), which would result in a total offense level of 43— the highest possible offense level under the Guidelines (carrying with it a mandatory life sentence no matter the defendant’s criminal history category).2 The PSR further recommended imposing the cross-reference for first-degree murder, resulting in a mandatory offense level of 43. Id. §§ 2A1.1, 2Dl.l(d)(l).3 Campbell’s PSR therefore recommended assigning him a total offense level of 43 on the Count 1 drug conspiracy offense.4 Finally, based on the finding that he had at least two prior felony convictions of either a crime of violence or a controlled substance offense, the PSR characterized Campbell as a career offender and ascribed to him a criminal offense history category of VI. Id. § 4B1.1.5
As for Sullivan, his PSR recommended a base offense level of 36 on the combined drug conspiracy offense and drug distribution offenses, predicated on the finding that these offenses involved between 500 grams and 1.5 kilograms of crack. See USSG § 2D1.1(c)(2) (2001). As recommended for Campbell, however, the PSR proffered that Sullivan should be subject to the cross-reference for first-degree murder and its corresponding offense level of 43. Id. §§ 2A1.1, 2Dl.l(d)(l).6 On the firearm use offense, the PSR maintained that Sullivan was subject to the minimum term of imprisonment required by the statute of conviction, 18 U.S.C. § 924(c), that being not less than five years, consec*264utive to any other sentence imposed. Id. § 2K2.4(a)(2). Lastly, the PSR assigned Sullivan a criminal history category of I.
B.
After receiving evidence on the proposed murder cross-references in March and July of 2003, the sentencing court conducted a final sentencing hearing on July 15, 2003. Throughout the sentencing proceedings, the defendants repeatedly contended — both orally and in writing— that the court was constitutionally barred from finding facts outside the jury’s verdict in applying the then-mandatory Guidelines. Sullivan and Campbell relied principally on the Supreme Court’s decision in Apprendi v. New Jersey, 530 U.S. 466, 490, 120 S.Ct. 2348, 147 L.Ed.2d 435 (2000) (holding that, “[o]ther than the fact of a prior conviction, any fact that increases the penalty for a crime beyond the prescribed statutory maximum must be submitted to a jury, and proved beyond a reasonable doubt”). The sentencing court correctly concluded, however, that the defendants’ interpretation of Apprendi had previously been rejected by this circuit in United States v. Kinter, 235 F.3d 192, 198-202 (4th Cir.2000).
Thus, during the final sentencing hearing on July 15, 2003, the district court found, by a preponderance of the evidence, the facts necessary to justify application of the PSRs’ recommended sentencing enhancements. And the court acknowledged that it was “required to follow the law as set forth in the Guidelines, whether I agree with those Guidelines, along with whether I think they’re good or bad.” Sullivan J.A. 855.7 That is, the court recognized that it was constrained to follow the then-mandatory Guidelines regime.
The court calculated Campbell’s total offense level on the drug conspiracy offense as the maximum of 43, either by (1) imposing the murder cross-reference or (2) setting his base offense level at 38, predicated on the finding that the offense involved 1,858 grams of crack cocaine, and then applying the two-level enhancement for possession of a dangerous weapon and the four-level enhancement for being an organizer or leader of the drug conspiracy. See Sullivan J.A. 902-03 (“I just want the record to show that ... in the event that I was in error in cross-referencing the murder, that because of the drug amount and because of his total Offense Level and Criminal History, that the identical sentence would be imposed by the Court.”). Campbell thus faced a mandatory life sentence on the drug conspiracy offense no matter his criminal history category, which the court designated as VI. The court imposed a life sentence on Campbell on the drug conspiracy offense, as well as a concurrent sentence of twenty years on the firearms conspiracy offense.
The sentencing court similarly determined that Sullivan deserved the murder cross-reference and a mandatory life sentence on the drug conspiracy offense. The court imposed a life sentence on Sullivan on that offense, in addition to lesser concurrent sentences on the other drug and firearms offenses, and the mandatory five-year (i.e., sixty-month) consecutive sentence on the Count 34 firearm use offense. The court observed that if the murder cross-reference did not apply, Sullivan would not face a life term. Instead, the court announced that Sullivan would have a base offense level of 36 on the drug conspiracy offense, predicated on the finding that he was responsible for 720 grams of crack. With a criminal history category *265of I, the resulting sentencing range would be 188 to 235 months of imprisonment on the drug conspiracy offense (and other combined counts), for a total sentence of 248 to 295 months including the mandatory consecutive 60-month sentence on the firearm use offense. The court stated that absent the murder cross-reference it would have sentenced Sullivan to a total 295 months of imprisonment. Sullivan J.A. 923.
II.
During the pendency of these appeals, the Supreme Court issued its decisions in Blakely v. Washington, 542 U.S. 296, 124 S.Ct. 2531, 159 L.Ed.2d 403 (2004) (holding that the State of Washington’s mandatory sentencing guidelines contravened the Sixth Amendment), and United States v. Booker, 543 U.S. 220, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005) (applying Blakely principles to the federal Sentencing Guidelines). Under Booker, there are two types of error. First, Sixth Amendment error occurs when a sentencing court enhances a defendant’s sentence beyond the maximum authorized by facts found by a jury beyond a reasonable doubt or admitted by the defendant. See Booker, 543 U.S. at 245, 125 S.Ct. 738; United States v. White, 405 F.3d 208, 215 (4th Cir.2005). Second, statutory error occurs when a sentencing court treats the Guidelines as mandatory, rather than as advisory. See Booker, 543 U.S. at 245-46, 125 S.Ct. 738; White, 405 F.3d at 215. Because we conclude that the district court committed reversible statutory Booker error in sentencing both Sullivan and Campbell, we need not reach the issue of whether Sixth Amendment error also occurred.8
Sullivan and Campbell properly preserved their claims of statutory Booker error by raising Apprendi at sentencing; thus, their claims are subject to harmless error review. See United States v. Williams, 445 F.3d 724 (4th Cir.2006); United States v. Rodriguez, 433 F.3d 411, 415 (4th Cir.2006). Applying this standard of review, they are entitled to relief if the statutory Booker error affected their substantial rights. See Rodriguez, 433 F.3d at 416. And “the burden is on the Govem*266ment to show that such an error did not affect the defendants’] substantial rights.” Id. The Government can make such a showing if the sentencing court indicated that it would not have imposed a lesser sentence under an advisory (rather than a mandatory) Guidelines regime. Id.
The Government is unable to make such a showing here. Although the district court announced “alternative” sentences for both Sullivan and Campbell, these alternatives reflected the court’s calculations for each defendant solely under a mandatory Guidelines regime. Significantly, the court never indicated how it would have sentenced the defendants under an advisory Guidelines regime, taking into account not only the computations spelled out in the Guidelines, but also the factors outlined in 18 U.S.C. § 3553(a). See United States v. Hughes, 401 F.3d 540, 546 (4th Cir.2005) (recognizing that, in the wake of Booker, “a district court shall first calculate (after making the appropriate findings of fact) the range prescribed by the guidelines” and then “shall consider that range as well as other relevant factors set forth in the guidelines and those factors set forth in § 3553(a) before imposing the sentence”). And, under harmless error review, the court’s silence on how it would have sentenced Sullivan and Campbell if the Guidelines had then been advisory must be interpreted in the defendants’ favor. See Rodriguez, 433 F.3d at 416.
Accordingly, we are obliged to conclude that Sullivan and Campbell were prejudiced when the district court committed statutory Booker error in connection with their sentences by treating the Guidelines as mandatory. We thus vacate the defendants’ sentences and remand for such further sentencing proceedings as may be appropriate.9
. Judge Duncan and I are pleased to concur in Judge Widener's opinion in these appeals, insofar as it relates to the issues raised concerning the defendants' convictions. I write separately, with the concurrence of Judge Duncan, on the sentencing issues only.
. Significantly, a life sentence was authorized under the applicable criminal statute for the Count 1 drug conspiracy offense. See 21 U.S.C. § 841(b)(1)(A).
. The proposed murder cross-reference was based on evidence that Sullivan and Campbell were involved in the murders of rival drug dealers Elshawndra Jones and Toby Bing in February 1999.
. On the firearms conspiracy offense in Count 49, Campbell’s PSR recommended a base offense level of 24, predicated on the determination that Campbell had at least two prior felony convictions of either a crime of violence or a controlled substance offense. See USSG § 2K2.1(a)(2) (2001). The PSR further recommended, however, that the firearms conspiracy offense be combined with the drug conspiracy offense, and that Campbell be assigned the highest offense level of the counts in the group, i.e., offense level 43. Id. §§ 3D 1.2(c), 3D1.3(a).
. Campbell's classification as a career offender — to which he posed no objections — required that his offense level on the drug conspiracy offense in Count 1 be at least 37. See USSG § 4B1.1 (2001) (providing that "[i]f the offense level for a career criminal from the table below is greater than the offense level otherwise applicable, the offense level from the table below shall apply,” and setting forth an offense level of 37 for a controlled substance offense — such as Campbell's — with a statutory maximum sentence of life). Of course, the PSR recommended an offense level greater than 37 on the drug conspiracy offense predicated on other findings and enhancements.
.On Sullivan’s combined firearms conspiracy offense in Count 49 and obliterated serial number offense in Count 37, the PSR recommended a base offense level of 12, see USSG § 2K2.1(a)(7) (2001), a two-level enhancement for the obliterated serial number, id. § 2K2.1(b)(4), and a four-level enhancement for use or possession of a firearm in connection with another felony offense, id. § 2K2.1(b)(5). With these enhancements, the total offense level on Sullivan’s combined firearms conspiracy offense and obliterated serial number offense was 18. The PSR ultimately recommended combining these counts with the drug conspiracy and drug distribution offenses, and assigning Sullivan the highest offense level of all the counts in the group, i.e., offense level 43. Id. §§ 3D1.2(c), 3D1.3(a).
. Our citations to "Sullivan J.A. — ” refer to the contents of the Joint Appendix filed by Sullivan and the Government in Sullivan’s appeal.
. Although we do not decide the Sixth Amendment issue, we observe that the Government initially conceded that such error occurred in sentencing both Sullivan and Campbell. The Government's concessions of Sixth Amendment error followed the defendants’ assertions that—without any improper fact-finding by the court'—Campbell’s offense level on the drug conspiracy offense would have been 32 (resulting in a sentencing range of 210 to 262 months) and Sullivan’s offense level on that same count also would have been 32 (carrying a sentencing range of 121 to 151 months). Cf. United States v. Hughes, 401 F.3d 540, 547 (4th Cir.2005) (recognizing that Sixth Amendment error occurred when maximum Guidelines sentence authorized by jury verdict was twelve months, but court imposed forty-six-month sentence). During oral argument, however, the Government sought to withdraw its concession with respect to Campbell. The Government explained that it had previously overlooked Campbell’s designation as a career offender, which carried with it a minimum offense level of 37. At that offense level, with a criminal history category of VI, the resulting sentencing range would have been 360 months to life. Thus, according to the Government, no Sixth Amendment error occurred when the sentencing court imposed a life sentence on Campbell. In response, Campbell contended that the Government could not withdraw its previous concession of Sixth Amendment error. Campbell did not challenge the assertion that no such error actually occurred or his classification as a career offender. Cf. United States v. Washington, 404 F.3d 834, 842-43 (4th Cir.2005) (recognizing that sentencing court may only look to certain sources in making factual findings about prior convictions without violating Sixth Amendment). Campbell’s response leads us to doubt that his sentence contravened the Sixth Amendment. Nevertheless, as explained above, we need not reach this issue.
. We acknowledge that the defendants have raised other appellate contentions with respect to their sentences. Specifically, Campbell asserts that the sentencing court’s findings with respect to the leadership role enhancement, the murder cross-reference, and drug quantity were clearly erroneous. Sullivan contends that the court’s finding on drug quantity was clearly erroneous, and also that the court erred in applying the murder cross-reference based on facts found by a preponderance of the evidence (rather than by clear and convincing evidence). We decline to exercise our discretion to address these other alleged Guidelines-application errors, and leave those issues for the district court. See Hughes, 401 F.3d at 556 & n. 15.