An indigenous group in Canada's Saskatchewan province said it had found the unmarked graves of 751 people at a now-defunct Catholic residential school.
It comes just weeks after a similar discovery rocked the country.
Prime Minister Justin Trudeau said he was "terribly saddened" by the new discovery at Marieval Indian Residential School about 87 miles (140 km) from the provincial capital Regina.
He told indigenous people that "the hurt and the trauma that you feel is Canada’s responsibility to bear."
It is not clear how many of the remains detected belong to children, Cowessess First Nation Chief Cadmus Delorme told reporters.
He said the church that ran the school removed the headstones.
"We didn't remove the headstones. Removing headstones is a crime in this country. We are treating this like a crime scene," he said.
The residential school system, which operated between 1831 and 1996, removed about 150,000 indigenous children from their families and brought them to Christian residential schools run on behalf of the federal government.
Canada's Truth and Reconciliation Commission, which published a report that found the country's residential school system amounted to cultural genocide, has said a cemetery was left on the Marieval site after the school building was demolished.
Cowessess First Nation has been in touch with the local Catholic archdiocese and Delorme said he is optimistic they will provide records allowing them to identify the remains.
The Cowessess First Nation began a ground-penetrating radar search on June 2, after the discovery of 215 unmarked graves at the Kamloops Residential School in British Columbia outraged the country.
The Kamloops discovery reopened old wounds in Canada about the lack of information and accountability around the residential school system, which forcibly separated indigenous children from their families.
Pope Francis said in early June that he was pained by the Kamloops revelation and called for respect for the rights and cultures of native peoples.
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