WordNet (r) 3.0 (2006):

own up
    v 1: admit or acknowledge a wrongdoing or error; "the writer of
         the anonymous letter owned up after they identified his
         handwriting" [syn: make a clean breast of, own up,
         fess up]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "own up":
   acknowledge, admit, admit everything, allow, avow, come clean,
   concede, confess, cop a plea, grant, let on, open up, out with it,
   own, plead guilty, spill, spill it, spit it out, tell all,
   tell the truth