Clark v. Martinez, 125 S.Ct. 716, 2005 WL 50099 (January 12, 2005) (under 8 U.S.C. 1231(a)(6), the DHS may detain inadmissible aliens beyond the 90-day removal period, but only for so long as is reasonably necessary to achieve removal).
http://laws.lp.findlaw.com/us/000/03-878.html