What is the standard of review for a finding of vindictive prosecution?

The standard of review of a district court's finding of vindictive prosecution is a little less obvious. We recently examined the question in United States v. Gann, 732 F.2d 714 (9th Cir.), cert. denied, --- U.S. ----, 105 S.Ct. 505, 83 L.Ed.2d 397 (1984), noting that "[s]ome cases have suggested an abuse of discretion standard, while others deem the issue to be one of mixed law and fact, calling for the application of the clearly erroneous standard." Id. at 724 (citations omitted); see United States v. North, 746 F.2d 627, 632 (9th Cir.1984), cert. denied, --- U.S. ----, 105 S.Ct. 1773, 84 L.Ed.2d 832 (1985). In United States v. McConney, 728 F.2d 1195 (9th Cir.) (en banc), cert. denied, --- U.S. ----, 105 S.Ct. 101, 83 L.Ed.2d 46 (1984), we reexamined the standard of review for mixed questions of law and fact.