On the basis of proof which included two eyewitness identifications, numerous pieces of circumstantial evidence, and petitioner Herreras handwritten letter impliedly admitting his guilt, Herrera was convicted of the capital murder of Police Officer Carrisalez and sentenced to death in January, 1982. After pleading guilty, in July, 1982, to the related capital murder of Officer Rucker, Herrera unsuccessfully challenged the Carrisalez conviction on direct appeal and in two collateral proceedings in the Texas state courts, and in a federal habeas petition. Ten years after his conviction, he urged in a second federal habeas proceeding that newly discovered evidence demonstrated that he was "actually innocent" of the murders of Carrisalez and Rucker, and that the Eighth Amendments prohibition against cruel and unusual punishment and the Fourteenth Amendments due process guarantee therefore forbid his execution. He supported this claim with affidavits tending to show that his now-dead brother had committed the murders. The District Court, inter alia, granted his request for a stay of execution so that he could present his actual innocence claim and the supporting affidavits in state court. In vacating the stay, the Court of Appeals held that the claim was not cognizable on federal habeas absent an accompanying federal constitutional violation.

Held:

Herreras claim of actual innocence does not entitle him to federal habeas relief. Pp. 398-419.

(a) Herreras constitutional claim for relief based upon his newly discovered evidence of innocence must be evaluated in light of the previous 10 years of proceedings in this case. In criminal cases, the trial is the paramount event for determining the defendants guilt or innocence. Where, as here, a defendant has been afforded a fair trial and convicted of the offense for which he was charged, the constitutional presumption of innocence disappears. Federal habeas courts do not sit to correct errors of fact, but to ensure that individuals are not imprisoned in violation of the Constitution. See, e.g., Moore v. Dempsey, 261 U.S. 86, 87-88. Thus, claims of actual innocence based on newly discovered evidence have never been held to state a ground for federal habeas relief [506 U.S. 390, 391] absent an independent constitutional violation occurring in the course of the underlying state criminal proceedings. See Townsend v. Sain, 372 U.S. 293, 317. The rule that a petitioner subject to defenses of abusive or successive use of the habeas writ may have his federal constitutional claim considered on the merits if he makes a proper showing of actual innocence, see, e.g., Sawyer v. Whitley, 505 U.S. 333, is inapplicable in this case. For Herrera does not seek relief from a procedural error so that he may bring an independent constitutional claim challenging his conviction or sentence, but rather argues that he is entitled to habeas relief because new evidence shows that his conviction is factually incorrect. To allow a federal court to grant him typical habeas relief - a conditional order releasing him unless the State elects to retry him or vacating his death sentence - would in effect require a new trial 10 years after the first trial, not because of any constitutional violation at the first trial, but simply because of a belief that, in light of his newfound evidence, a jury might find him not guilty at a second trial. It is far from clear that this would produce a more reliable determination of guilt or innocence, since the passage of time only diminishes the reliability of criminal adjudications. Jackson v. Virginia, 443 U.S. 307, Ford v. Wainwright, 477 U.S. 399, and Johnson v. Mississippi, 486 U.S. 578, distinguished. Pp. 398-407.

(b) Herreras contention that the Fourteenth Amendments due process guarantee supports his claim that his showing of innocence entitles hi