David Letterman has been targeted by an extremist website frequented by al-Qaida.

The site reportedly urged its followers to “cut the tongue” of the late-night host because of a joke he made about accused terrorist leader Ilyas Kashmiri, who was killed by a U.S. airstrike in Pakistan in June.

The FBI is looking into the threat. Jim Margolin, spokesman for the bureau’s New York office, says, “We take every potential threat seriously.”