Is probable cause to make a warrantless arrest based on probable cause?

The question of whether a warrantless arrest was based on probable cause is a mixed question of law and fact. United States v. Greene, 783 F.2d 1364, 1367 (9th Cir.), cert. denied, 476 U.S. 1185 (1986). "The underlying facts as found by the district court are to be accepted unless clearly erroneous. The court's ultimate conclusion, however, is reviewed de novo." Id. Probable cause to make a warrantless arrest exists "if, at the moment of arrest, facts and circumstances within the arresting officers' knowledge and of which they had reasonably trustworthy information were sufficient to warrant a prudent person in believing that the suspect had committed ... an offense." United States v. Pinion, 800 F.2d 976, 979 (9th Cir.1986), cert. denied, 480 U.S. 936 (1987).