The FBI is investigating a threat made against talk show host David Letterman on a popular Internet destination for radical Muslims.

A frequent contributor to the Shumuka al-Islam forum, a jihadist website, issued the threat in response to a joke the 64-year-old comic made on his CBS program ‘The Late Show With David Letterman.’ The contributor, who identified himself as Umar al-Basrawi, urged readers to “cut the tongue of this lowly Jew and shut it forever.” Letterman was raised Presbyterian.

Neither CBS nor a Letterman spokesman would comment on the threat. And the network would not make available a transcript of Letterman’s June 8 monologue, in which he mocked the death of Ilyas Kashmiri, a senior al Qaeda leader and possible successor to Osama bin Laden. However, Charles Strozier, director of the Center on Terrorism at John Jay College of Criminal Justice, told amNew York that Letterman should take any warnings seriously.

“It could be a total hoax, and it probably is, but in this world of mass communication, something that seems frivolous can intensify if it gets picked up by the wrong people,” Strozier said.

[Associated Press]