Serbia's war crimes prosecutors are preparing a case against an American who allegedly served in a Nazi unit that killed 17,000 civilians here during World War II, an official said Friday. Bruno Vekaric, a spokesman for the prosecutors' office, told The Associated Press that it has started gathering information about Peter Egner, 86, a native of Yugoslavia now living in US, in order to try him in Serbia. "We have contacted the Americans, various archives, victims' associations to gather data," Vekaric said. "Once we collect enough material, we will launch a formal investigation and seek his extradition." This week, the US Justice Department asked a US federal court to revoke Egner's American citizenship, saying he had served as a guard and interpreter with the Nazi-controlled Security Police and Security Service in Belgrade from April 1941 to September 1943. In its complaint filed in US District Court in Seattle, Washington, the Justice Department said that Egner had failed to divulge that information when he applied for US citizenship. Instead, he reported serving in a German unit and was granted US citizenship in 1966.