Institute experts said at least 17 tons of ashes were found in two pits that are 3 meters (10 feet) deep, meaning that remains of at least 8,000 people are buried there.

The victims were mostly inmates of the Soldau Nazi German prisoner camp in the Polish town of Dzialdowo who were executed in the forest between 1940-44, the experts said. An estimated 30,000 people, mostly Polish elites, military, resistance fighters and Jews were inmates at the camp and a large number of them were killed or died, in the Nazis’ plan of extermination.

The forest has been known as the burial site of the slain inmates but the exact location of the mass graves and the number of the victims were not known until now. The institute’s archeologists and anthropologists located the two mass graves this month.

The institute investigates Nazi crimes and also communist crimes against Poles and has the power to bring charges against the suspects if they are still alive.