June 24 (Reuters) - An indigenous group in the Canadian province of Saskatchewan on Thursday said it had found the unmarked graves of up to 751 people at a now-defunct Catholic residential school, 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, adding that oral stories mentioned adults being buried at the site. Delorme 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, mostly Catholic, run on behalf of the federal government. "Canada will be known as a nation who tried to exterminate the First Nations," said Bobby Cameron, Chief of the Federation of Sovereign Indigenous Nations, which represents 74 First Nations in Saskatchewan. "This is just the beginning." Canada"s Truth and Reconciliation Commission, which published a report that found the 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 is optimistic records will be provided to allow the group to identify the remains. "We have full faith that the Roman Catholic Church will release our records. They haven"t told us "No." We just don"t have them yet," Delorme said. Federation of Sovereign Indigenous Nations Vice-Chief Heather Bear, who went to Marieval as a day student in the 1970s, recalled a small cemetery at the school but not of the size revealed on Thursday. "You just didn"t want to be walking around alone in (the school)," she recalled. There was a "sadness that moves. And I think every residential school has that sadness looming." 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. Radar at Marieval found 751 "hits" as of Wednesday with a 10 percent margin of error, meaning at least 600 graves on the site. read more 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 and subjected them to malnutrition and physical and sexual abuse. 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. But he stopped short of the direct apology some Canadians had demanded. read more