This likely will not be a popular position...

A while back, I wrote on the Supreme Court's holding in U.S. v. Comstock.  In that case, the Court held that Congress has the power under Article 1, §8 of the Constitution, to enact a statute which permits states to keep sexual predators in "civil commitment" after the expiration of their federal sentence.  The Court did not decide, however, whether other provisions of the Constitution might be violated by such a statute, such as the right to due process, the prohibition on double jeopardy, the prohibition on cruel and unusual punishment, etc.  Instead, the Court remanded the case to lower courts for a determination on those questions.