Misprision of felony.
Title 18 U.S.Code Sect. 3
http://www.law.cornell.edu/uscode/text/18/4
Whoever, having knowledge of the actual commission of a felony cognizable by a court of the United States, conceals and does not as soon as possible make known the same to some judge or other person in civil or military authority under the United States, shall be fined under this title or imprisoned not more than three years, or both.
Make that section 4.