It seems misprision of felony could apply.
https://www.law.cornell.edu/uscode/text/18/4
"Whoever, having knowledge of the actual commission of a felonycognizable by a court of the United States, conceals and does not as soon as possible make known the same to some judge or otherperson in civil or military authority under the United States, shall be fined under this title or imprisoned not more than three years, or both."