What constitutes newly discovered evidence in the context of a motion for a new trial?

Recently, this court considered the question of what constitutes newly discovered evidence in the context of a motion for new trial. In United States v. McKinney, 952 F.2d 333 (9th Cir.), cert. denied, 111 S.Ct. 2797 (1991), the defense became aware of possible juror bias near the close of the trial. Several months later, the court received two letters from a juror, confirming the reports of potential bias and detailing the effect upon the jury and its deliberations. The defendant moved for a new trial based upon the newly discovered evidence contained in the letters. The district court denied the motion and this court affirmed, finding that the evidence was not newly discovered even though the details surrounding the juror misconduct were not revealed until months after trial. The court held that "[e]vidence known or discovered before the trial is over is not newly discovered." Id. at 336.