Retired carpenter Michael Karkoć is under suspicion of ordering the killing of 44 Poles during the Second World War.
Prosecutor Robert Janicki said he is "100%" certain that the 98-year-old was the leader of a unit in the SS-led Ukrainian Self Defence Legion. He is accused of killing civilians and burning down villages.
"All the pieces of evidence interwoven together allow us to say the person who lives in the U.S. is Michael K., who commanded the Ukrainian Self Defence Legion which carried out the pacification of Polish villages in the Lublin region," Janicki said.
Polish web-portal Kresy.pl has been covering the case for several months, together with British Holocaust researcher, Dr Stephen Ankier, who was the first to positively identify Michael / Mychajło Karkoć as an Ukrainian Waffen-SS officer and war criminal.
Ankier also provided evidence that led Associated Press writing articles on the issue (based on his work). Together with Kresy.pl, the researcher provided Polish investigators with evidence for them to apply a new technique (comparative photographic analysis), to confirm the identity of the Ukrainian Waffen-SS officer Michael Karkoć.
An arrest warrant was sought for Karkoć by prosecutors of the state National Remembrance Institute. Poland would seek his extradition if the warrant is granted because it does not allow trials in absentia.
Karkoć's advanced years would not stop the process of bringing him to justice, Janicki said. "He is our suspect as of today," the lawyer added.
Wartime documents, investigated by AP found that Karkoć commanded the unit, based on testimony from other members of the unit and Karkoć's own Ukrainian-language memoir.
Karkoć's family have denied he took part in any war crimes. "There's nothing in the historical record that indicates my father had any role whatsoever in any type of war crime activity," said his son, Andriy Karkoć.
He called into question the Polish investigation, saying "my father's identity has never been in question nor has it ever been hidden."
Efraim Zuroff, the head Nazi hunter for the Simon Wiesenthal Centre, welcomed the prosecution. "Any legal step that's taken against these people is very important," he said.
"It sends a very powerful message, and these kinds of things should not be abandoned just because of the age of a suspect."
Karkoć's family says he suffers from Alzheimer's disease, although Zuroff suggested Karkoć was examined by independent doctors.
"It is a very common occurrence that elderly individuals facing prosecution for World War II crimes make every effort to look as sick and as infirm as possible," he said.
The identity of the Minneapolis resident was revealed by the Associated Press, which uncovered evidence that in 1944 Karkoć himself ordered the men under his command to attack a Polish village in which dozens of civilians were killed. Buildings were set on fire and more than 40 men, women and children were gunned down.