Serbia has asked the United States Justice Department for help identifying those who posted a death threat against its pro-Western president on Facebook. The threat against President Boris Tadic on the social networking site said: “Tadic, a bulletproof Mercedes won’t help you. When you open the door, a sniper awaits you.” Serbia’s Justice Ministry said Wednesday that it had contacted the 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 by former Serb paramilitaries after an opposition newspaper published similar threats.