WARSAW, Poland (AP) — Polish prosecutors say American authorities are moving ahead with their request to extradite a 99-year-old Minnesota man to be tried on allegations he was involved in a World War II massacre of civilians.

Polish authorities issued an arrest warrant last year after opening a case following a series of 2013 stories in which The Associated Press identified the man as Ukrainian-born Michael Karkoc, an ex-commander in an SS-led Nazi unit that burned Polish villages and killed civilians during the war.

Karkoc's family denies that he was involved in any war crimes.

National Remembrance Institute prosecutor Robert Janicki said Thursday that in response to Poland's extradition request, American authorities have now chosen three medical experts to determine whether Karkoc can "consciously take part in a court trial and travel by plane."