As many as 751 unmarked graves were discovered on Thursday at another former Indian residential school in Canada's Saskatchewan.
The discovery at the Marieval Indian Residential School marks the largest to date, as Canada continues to reckon with its past abuse of indigenous peoples.
In a statement, Prime Minister Justin Trudeau wrote that he was "terribly saddened," telling indigenous peoples "the hurt and the trauma that you feel is Canada’s responsibility to bear."
Cowessess First Nation Chief Cadmus Delorme told reporters it's unclear how many of the remains found belong to indigenous children.
"But there are oral stories that there are adults in this gravesite as well, because it was the Roman Catholic Church that overseen this gravesite. Some may have went to the church and from our local towns and they could have been buried here as well."
Canada's residential school system, which operated between 1831 and 1996, forcibly removed some 150,000 indigenous children from their families.
The mostly Catholic residential schools, run on behalf of the federal government, often subjected the children to malnutrition, physical and sexual abuse.
The Cowessess First Nation's discovery comes a month after the discovery of 215 unmarked graves at the Kamloops Indian Residential School in British Columbia.
Local Catholic officials have given the Cowessess over $50,000 to help restore the Marieval site and identify unmarked graves, and repeated an earlier apology for the "failures and sins of Church leaders and staff".