A First Nation in southern Saskatchewan said Wednesday that it has discovered hundreds of unmarked graves at the site of another former residential school for Indigenous children.

A statement from the Cowessess First Nation and the Federation of Sovereign Indigenous First Nations, which represents Saskatchewan’s First Nations, said that “the number of unmarked graves will be the most significantly substantial to date in Canada.”

Last month the remains of 215 children, some as young as 3 years old, were found buried on the site of what was once Canada’s largest Indigenous residential school near Kamloops, British Columbia.

Read: More than 200 bodies found at Indigenous school in Canada

Cowessess Chief Cadmus Delorme and Chief Bobby Cameron of the federation planned to hold a news conference Thursday to provide more details about the new find at the Marieval Indian Residential School, which operated from 1899 to 1997 where Cowessess is now located, about 87 miles east of Regina, the capital of Saskatchewan.

Read: Canada: Bodies at Indigenous school not isolated incident

From the 19th century until the 1970s, more than 150,000 First Nations children were required to attend state-funded Christian schools as part of a program to assimilate them into Canadian society. They were forced to convert to Christianity and not allowed to speak their native languages. Many were beaten and verbally abused, and up to 6,000 are said to have died.

The Canadian government apologized in Parliament in 2008 and admitted that physical and sexual abuse in the schools was rampant. Many students recall being beaten for speaking their native languages; they also lost touch with their parents and customs.

Read: Trudeau: Residential schools part of Canada s colonial past

Indigenous leaders have cited that legacy of abuse and isolation as the root cause of epidemic rates of alcoholism and drug addiction on reservations.