WARSAW, Poland — Special investigators in Poland say they have found two mass graves containing the ashes of at least 8,000 Poles slain by the Nazi during World War II in forest executions that the Nazis later tried to hide by incinerating the bodies and planting trees on the burial pits.

Investigators from a national historical institute marked the finding this week with speeches and wreath-laying at the site in the Bialuty Forest, 100 miles north of Warsaw.

Starting in March 1944, the bodies that the occupying Nazis had secretly buried in the forest were “brought out, burned and pulverized in order to prevent this crime from ever being known, in order to prevent anyone taking responsibility for it,” Karol Nawrocki, the head of the Institute of National Remembrance, said Wednesday.

“These efforts were not successful,” Nawrocki said.

The Nazis used other inmates, chiefly Jewish, to do the cover-up job. Those inmates were also killed.

Institute experts said at least 17 tons of ashes were found in two pits that are 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.