What is the burden of proving beyond a reasonable doubt the use of prohibited communications between defendants and the government?

The government, of course, always bears the burden of proving beyond a reasonable doubt the use of the mails and wires. The government must introduce evidence to show that the defendant "caused" each use of the mails or wires. Showing the defendant's knowing participation in the scheme at the time of the prohibited communications is sufficient. Cf. Van Riper v. United States, 13 F.2d 961, 964 (2d Cir.) ("[T]he conspiracy count adds nothing of substance to the [mail fraud] charge, except as it relieves the prosecution of the necessity of showing the connection of all the defendants ... at the date of the posting of the letters laid in the indictment."), cert. denied, 273 U.S. 702, 47 S.Ct. 102, 71 L.Ed. 848 (1926). When the defendant has introduced prima facie evidence of withdrawal from participation in the scheme, however, the government, to overcome a motion for acquittal, must rebut the showing of withdrawal. To do so, the government's evidence must be sufficient to support a finding that the defendant did not withdraw or that each charged use of the mails or wires was the foreseeable result of actions taken by the defendant or his co-schemers before the defendant's withdrawal.