Whether a federal court may consider post-trial evidence that was not presented to the jury when determining if a constitutional error was harmless. Specifically, the Court addresses whether such evidence can be used to decide if a *Giglio* violation had a "substantial and injurious effect or influence in determining the jury's verdict." This issue arises from a habeas corpus petition where a lower court relied on later DNA results to support a finding of overwhelming evidence.

• The prosecution violated due process under *Giglio v. United States* by presenting false testimony regarding witness Jake Ozio's criminal history.
• There was a reasonable likelihood that the false testimony affected the jury's verdict.
• The Eleventh Circuit erred by relying on post-trial DNA evidence—which the jury never saw—to conclude that the *Giglio* error was harmless.
• The Court should resolve a purported Circuit split regarding whether courts can assess prejudice based on evidence discovered after trial.

• The evidence against the petitioner was "overwhelming," making any potential *Giglio* error immaterial to the verdict.
• The petitioner failed to exhaust the specific *Giglio* claim regarding the witness's criminal record in state court.
• Law and justice do not require relief given the conclusive nature of the evidence of guilt, including the post-trial DNA results.
• Not stated.

• The Court vacated the judgment of the Court of Appeals.
• The Court held that the Court of Appeals should not have considered post-trial DNA evidence to assess whether the Florida Supreme Court reasonably determined that Ozio's testimony was immaterial to the verdict.
• The Court reasoned that because the post-trial DNA evidence was not presented to the jury and did not exist at the time of trial, it "could not have influenced the jury's verdict."
• The Court applied the standard from *Brecht v. Abrahamson*, which asks whether the constitutional error had a "substantial and injurious effect or influence in determining the jury's verdict."
• The Court concluded that evidence that could not have influenced the verdict "sheds no light" on whether the *Giglio* error influenced that verdict.
• The Court noted that the Florida Supreme Court did not consider the post-trial DNA evidence, and thus the Court of Appeals should not have used it to evaluate the state court's determination.
• The Court declined to rule on whether the Florida Supreme Court's determination was reasonable based only on trial evidence, leaving that for the Court of Appeals to assess.
• The Court declined to rule on the State's argument regarding the failure to exhaust the claim, citing *Cutter v. Wilkinson* to state it is a "court of review, not of first view."