Title 18 of the US Code (18 USC) is the entire federal criminal code.
18 USC Sec. 2382, Misprision of treason. Unfortunately the max sentence is 7 years.
Whoever, owing allegiance to the United States and having knowledge of the commission of any treason against them, conceals and does not, as soon as may be, disclose and make known the same to the President or to some judge of the United States, or to the governor or to some judge or justice of a particular State, is guilty of misprision of treason and shall be fined under this title or imprisoned not more than seven years, or both.