Misprision of treason
https://en.wikipedia.org/wiki/Misprision_of_treason
Misprision of treason is an offence found in many common law jurisdictions around the world, having been inherited from English law. It is committed by someone who knows a treason is being or is about to be committed but does not report it to a proper authority.
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United States[edit]
In the United States, misprision of treason is a federal offense, committed where someone who has knowledge of the commission of any treason against the United States, conceals such knowledge and does not inform the President, a federal judge or State Governor or State judge (18 U.S.C. § 2382). It is punishable by a fine and up to seven years in federal prison. It is also a crime punishable under the criminal laws of many states.
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http://www.nytimes.com/1861/01/25/news/treason-against-the-united-states.html?pagewanted=all
In 1790, the Congress of the United States enacted that:
"If any person or persons, owing allegiance to the United States of America, shall levy war against them, or shall adhere to their enemies, giving them aid and comfort within the United States, or elsewhere, and shall be thereof convicted on confession in open Court, or on the testimony of two witnesses to the same overt act of the treason whereof he or they shall stand indicted, such person or persons shall be adjudged guilty of treason against the United States, and SHALL SUFFER DEATH; and that if any person or persons, having knowledge of the commission of any of the treasons aforesaid, shall conceal, and not, as soon as may be, disclose and make known the same to the President of the United States, or some one of the Judges thereof, or to the President or Governor of a particular State, or some one of the Judges or Justices thereof, such person or persons, on conviction, shall be adjudged guilty of misprision of treason, and shall be imprisoned not exceeding seven years, and fined not exceeding one thousand dollars."
just sayin