On June 10, 1944, the Germans carried out a massacre in Oradour-sur-Glane in central France, just four days after the D-Day landings. They killed 642 men, women and children.
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The troops herded them into barns and into a church, blocked the doors and then set fire to the entire town. Those not killed in the blazes were shot as they tried to flee, but a handful managed to escape.
Dortmund prosecutor Andreas Brendel has opened an investigation of the men earlier based on a precedent set by the trial of John Demjanjuk.
The 91-year-old was convicted in May of 28,060 counts of accessory to murder after a Munich court found he was a guard during the war in the Nazis' Sobibor death camp in occupied Poland.
The case was the first time someone was convicted in Germany on evidence of being only a guard, without evidence of a specific killing.
Mr Brendel said it is known that the six men under investigation, all aged either 85 or 86, were part of the 3rd Company of the 1st Battalion of the "Der Fuehrer" Regiment of the fanatical SS's "Das Reich" Division.
Oradour-sur-Glane today remains a phantom village, with burned-out cars and abandoned buildings left as testimony to its history.
The killings were believed to have been ordered by battalion commander Sturmbannfuehrer Adolf Diekmann, whose SS rank is equivalent to major, in retaliation for the kidnapping of a German soldier by the French Resistance.
Diekmann apparently believed the perpetrators were in or connected to Oradour, but he was killed in action a short time later, and little more is known about his reasoning.
The six men being investigated now, whose names Mr Brendel would not release citing German privacy laws, were all low-ranking.
When questioned, two of them denied being part of the massacre and four were unable for health reasons to give statements to prosecutors - although medical experts are examining them to determine if they are really incapable of understanding questions put to them.
Authorities are now trying to determine exactly what role the men played that day, with evidence of direct participation in, or knowledge of, the planned massacre being necessary to bring charges of murder or accessory to murder.
"We know that all the members of the 3rd Company were in Oradour," Mr Brendel said. "Naturally they had different functions - some of them secured the area, for example. Our problem at the moment is finding out how much they knew and what they might have done to facilitate the crime."
The homes of all of the suspects were searched over the last two months or so, but no evidence was found that could help the investigation.
Prosecutors also are awaiting French documents that they hope could aid the investigation, and have some statements from previous trials related to the massacre, Brendel said.
So far they have not been able to find any eyewitnesses.