A previously unknown group posted the threat against President Boris Tadic on the social networking site saying, "Tadic, a bulletproof Mercedes won't help you. When you open the door, a sniper awaits you."
Serbia's Justice Ministry said Wednesday it had contacted the U.S. Justice Department to help determine the I.P. addresses from which the threat was posted.
Serbia's first pro-democracy prime minister, Zoran Djindjic, was assassinated in 2003 outside Serbian government headquarters by former Serb paramilitaries after similar threats were published by an opposition newspaper.