Sentencing is a post-guilt-or-innocence procedure, and particularly where a guilty plea is involved, cannot be used to determine the nature of the offense of which the person was convicted. See Larin-Ulloa v. Gonzales, 462 F.3d 456 (5th Cir. 2006). See also Apprendi v. New Jersey, 530 U.S. 466 (2000).
Thanks to Lisa Brodyaga.