According to the "Full Disclosure - The Internet Dark Age" paper (1) the N.S.A. can route a target who is using Tor into their "own" Tor network where they control all the nodes, thus compromising the security of the target. Does anybody knows how one can be sure that he is in the legitimate Tor network and not a fake one?
(1) http://cryptome.org/2013/12/Full-Disclosure.pdf