Threatening terrorism against the United States is a class C felony punishable by up to 10 years' imprisonment under 18 U.S.C. § 2332b(c)(1)(g).
The elements of the offense are that someone willfully threatens to commit a crime that will result in death or great bodily harm; the threat is made with the specific intent that it be taken as a threat; the threat is so unequivocal, unconditional, and specific as to convey a gravity of purpose and immediate prospect of execution; the threat actually causes fear in the victim; and the fear is reasonable.
[1] Laws governing such threats were passed after the September 11, 2001 attacks.
The law was amended by the Terrorist Hoax Improvements Act of 2007.
[2][3] False information and hoaxes pertaining to attacks on U.S. officials, government buildings, airplanes, etc.