Serbia will seek the extradition of a naturalized American who allegedly served in a Nazi unit that killed about 17,000 civilians during World War II, the Balkan country's war crimes prosecutor said Monday.

Peter Egner, 87, who was born in Yugoslavia, is now living in a retirement community in Bellevue, Washington. U.S. authorities are trying to revoke his citizenship, a move that would pave the way for his extradition to Serbia.

War crimes prosecutor Vladimir Vukcevic said his investigators have gathered information about Egner in order to try him in Serbia. Vukcevic met with Serbian Jewish community leaders and told them he will seek Egner's extradition once the legal procedure of revoking his American citizenship is completed.

Egner has been trying to stop the U.S. federal government's efforts.

He has denied the accusations, claiming he knows nothing about the Einsatzgruppe, a Nazi-run Serbian police unit that rounded up Jews, political prisoners and other enemies of the Third Reich in the wake of Hitler's attack on the Soviet Union in the early 1940s.

He emigrated to the U.S. in 1960 and received citizenship in 1966.

The U.S. Justice Department filed a lawsuit in July to revoke his citizenship, saying he failed to disclose details from his past on his naturalization application.

The Justice Department, citing Nazi documents, has said that in 1941 Egner's unit executed 11,164 people — mostly Serbian Jewish men, suspected communists and Gypsies — and in 1942 killed 6,280 Serbian Jewish women and children by gassing them with carbon monoxide in a specially designed van.