Polish authorities say they have uncovered two mass graves of human ashes near the former Nazi concentration camp of Soldau.
The discovery was made by the Polish Institute of National Remembrance (IPN), which documents war crimes.
IPN prosecutor Tomasz Jankowski has said that "at least 8,000 people likely died there" given the weight of the human ashes found -- 17.5 tonnes.
The remains were unearthed in Ilowo Osada in the Bialucki forest, around 160 kilometres north of the capital Warsaw.
The site is close to the Soldau concentration camp -- now known as Dzialdowo -- which was built in 1939 during the Nazi occupation of Poland in World War II.
An estimated 30,000 prisoners and political opponents were killed at the camp, but some historians say the number could be much higher.
"The victims buried in this grave were probably murdered around 1939 and belonged mostly to Polish elites," Jankowski said.
In 1944, some Jewish prisoners were ordered to exhume bodies and set them on fire to erase the traces of Nazi war crimes. Special investigators believe the Nazis later tried to hide the remains by planting trees on burial pits.
"We have taken samples from the ashes, which will then be studied in the laboratory," Andrzej Ossowski, a genetics researcher at Pomerania Medical University, told AFP.
"We will be able to carry out DNA analyses, which will allow us to learn more about the identity of the victims," he added.
Around six million Poles died during the war, including three million Jews.