The Sex Offender Registration and Notification Act (SORNA) expressly gives the Attorney General authority to specify its applicability to offenders convicted before enactment.  So does the act apply to pre-enactment offenders by its own force, without AG action?  Not surprisingly, the US Supreme Court answered that question no in Reynolds v. United States.  It sent the case back to the Third Circuit to decide if the AG had validly specified such application.