BELGRADE, Serbia-The United States and Serbia are cooperating on an investigation of a U.S. citizen suspected of committing war crimes in Belgrade during World War II, the U.S. Embassy said Friday.

The Serbian war crimes court has opened a genocide investigation against 86-year-old Peter Egner, accusing him of serving in a Nazi unit that killed thousands of Jews, Serb and Roma during the Nazi occupation of the Serbian capital.

Investigators from the U.S. Justice Department met with the Serbian war crimes prosecutor this week, the embassy said in a statement. The two sides are cooperating to help "ensure that justice is served to the victims of Nazi aggression in Serbia during World War II," it said.

Egner, who lives in a retirement community outside Seattle, has denied any wrongdoing through his legal representative.

Egner emigrated to the United States in 1960 and received citizenship in 1966.

A U.S. Justice Department asked a federal judge in July to revoke Egner's citizenship because he failed in his application to disclose details from his past.

Revoking Egner's U.S. citizenship would pave the way for his extradition to Serbia.

The Justice Department, citing Nazi documents, has said that Egner's unit in 1941 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.