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 U.S., 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."