Peter Egner, 88, is suspected of war crimes against Jews and other citizens during the Nazi occupation of Serbia, court officials said.
Mr Egner is an ethnic German who was born in the former Yugoslavia.
He has denied the accusations and has been opposing US attempts to strip him of his citizenship.
Mr Egner became an American citizen after moving to the US in the 1960s.
During the war, he is accused of serving in an Einsatzgruppe, a Serbian police unit run by the Nazis - though he has said he knows nothing about the unit, the Associated Press news agency reports.
According to the US justice department, the unit killed thousands of Jewish and Serbian women and children in early 1942 by gassing them with carbon monoxide in a specially designed van.
US and Serbian authorities have been co-operating on the case.
Germany and its allies invaded Yugoslavia in 1941, carving the territory up between the different Axis powers.
Serbia fell under Nazi control, with a collaborationist administration installed in Belgrade until it was toppled by Josip Tito's partisans and the Soviet army in 1944.
http://news.bbc.co.uk/1/hi/world/europe/8600810.stm
