United States v. Watts
In United States v. Watts, 519 U.S. 148, 156 (1997) (per curiam), the U.S. Supreme Court held that at sentencing, unlike at trial, the government need prove its case only by a preponderance of the evidence, and it is unconstrained by the Federal Rules of Evidence. See also Fed. R. Evid. 1101(d)(3).