The US Supreme Court today decided Smith v. United States, No. 11-8976:

Upon joining a criminal conspiracy, a defendant's membership in the ongoing unlawful scheme continues until he withdraws. A defendant who withdraws outside the relevant statute-of-limitations period has a complete defense to prosecution. We consider whether, when the defendant produces some evidence supporting such a defense, the Government must prove beyond a reasonable doubt that he did not withdraw outside the statute-of-limitations period.

Answer: no.

Having joined forces to achieve collectively more evil than he could accomplish alone, Smith tied his fate to that of the group. His individual change of heart (assuming it occurred) could not put the conspiracy genie back in the bottle. We punish him for the havoc wreaked by the unlawful scheme, whether or not he remained actively involved. It is his withdrawal that must be active, and it was his burden to show that.

Justice Scalia delivered the unanimous opinion.