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. Egner, who now lives in a retirement community in the Seattle area, said this week that he was unaware of the US complaint and refused to comment on the allegations about his past. Vekaric said a delegation from the Serbian prosecutors' office will travel to Germany next week to check the archives about the Nazi occupation of Serbia during which tens of thousands of civilians were executed. Local historic and security archives also being thoroughly searched for any information about Egner, Vekaric said. "If we make a strong enough case, a warrant will be issued for his arrest," Vekaric said. "There should be no obstacle for his extradition to Serbia to face justice." The Justice Department complaint says that during an interview with federal authorities in February 2007, Egner admitted that he guarded prisoners as they were being transferred to the concentration camp of Semlin and the execution site of Avala, both near Belgrade. He also reportedly admitted serving as an interpreter during the interrogation of political prisoners. His lawyer, Robert Gibbs of Seattle, confirmed that Egner served on a low level in the security police when he was 19 or 20, but said his client denies participating in any persecution. The Justice Department, citing Nazi documents, said that in the fall of 1941, Egner's unit executed 11,164 people - mostly Serbian Jewish men, suspected communists and Gypsies - and that in early 1942, it killed 6,280 Serbian Jewish women and children who had been prisoners at Semlin. In two months, those women and children allegedly were taken from the camp and forced into a specially designed van, in which they were gassed with carbon monoxide.