Specialist researchers in Poland say they have found two mass graves containing the ashes of at least 8,000 Poles killed by the Nazis during World War II in forest executions, which the Nazis later tried to hide by cremating the bodies and planting trees in the burial pits. Researchers from a national historical institute marked the find this week with speeches and a wreath-laying at the site in Bialuty Forest, 160 kilometers (100 miles) north of Warsaw. Beginning in March 1944, the corpses that the Nazis had secretly buried in the forest were “removed, burned and pulverized so that this crime would never be known, in order to prevent anyone from taking responsibility for it,” Karol Nawrocki. the head of the Institute of National Remembrance said on Wednesday. “These efforts were not successful,” Nawrocki said. The Nazis used other prisoners, mostly Jews, to do the cover-up work. These prisoners were also killed. Institute experts said at least 17 tons of ash were found in two 3-meter (10-foot) deep pits, meaning the remains of at least 8,000 people are buried there. The victims were mostly inmates of the Nazi German Soldau prison camp in the Polish town of Dzialdowo, who were executed in the forest between 1940-44, experts said. An estimated 30,000 people, mostly Polish elites, military, resistance fighters and Jews were inmates of the camp and a large number of them were killed or died in the Nazi extermination plan. The forest was known to be the burial place of the slain prisoners, but the exact location of the mass graves and the number of victims were not known until now. The institute’s archaeologists and anthropologists identified the two mass graves this month. The institute investigates Nazi and also communist crimes against Poles and has the power to indict the suspects if they are still alive.