(CNN) -- A jihadist website is urging its American followers to kill comedian David Letterman, saying his tongue deserves to be cut over his remarks about a terror leader, an online intelligence group said. The writer posted the death threat after he got upset by a Letterman joke about an al Qaeda leader killed in Pakistan, according to SITE intelligence group, which monitors and translates online terror activity. Ilyas Kashmiri, described as al Qaeda's "military brain," died in a drone strike in June, his jihadist group said at the time. Letterman cracked jokes about the killing, and dragged his finger across his neck to show "the way of the slaughter," the message on the jihadist site said. He then said Kashmiri joins terror leader Osama bin Laden, who was killed in Pakistan in May, according to the message posted Wednesday. "This despicable person mocked the leaders of the mujahideen," the post says. The death threat urges jihadist followers to cut off Letterman's tongue and "shut it forever." CNN could not independently confirm the message. A spokesman for the FBI said the agency is looking into the death threats. "In the post 9/11 world , we take all of these threats seriously," spokesman Peter Donald said. A federal law enforcement official said such threats are common. "We see these kinds of threats frequently against people in business and media. Usually nothing comes of them, but we check them all out," the official said. Letterman's publicist declined to comment.