A Canadian First Nation tribe said it had found a mass grave housing 751 bodies at a Saskatchewan boarding school for Indigenous children, similar to one unearthed in early May that held the remains of 215 children. Members of the Cowessess First Nation used radar to scan the grounds of the Marieval Indian Residential School, which operated from 1899 to 1981 under the supervision of the Roman Catholic Church, beginning this year and made “a horrific and shocking discovery of hundreds of unmarked graves,” according to a press release.

Bobby Cameron, the chief of the Federation of Sovereign Indigenous Nations, described the revelation as a horrible confirmation of his fears. “There was always talk and speculation and stories, but to see this number—it’s a pretty significant number,” he told The New York Times. “It’s going to be difficult and painful and heartbreaking.”

There are nearly 150 such schools across Canada, which separated indigenous children from their families in an often brutal attempt to assimilate them. The U.S. Interior Department announced a federal initiative to excavate and catalogue graves at similar schools in the U.S. on Tuesday.