Correction: An earlier version of this article referred incorrectly to the burial site discovered at Kamloops Indian Residential School as a mass grave. The Tk’emlúps te Secwépemc First Nation says the remains were found spread out; it considers it an unmarked, undocumented burial site, not a mass grave. The article has been corrected.

TORONTO — The remains of 215 Indigenous children, including some as young as 3, have been found on the grounds of a former residential school in British Columbia — a grim discovery from one of the darkest chapters of Canadian history, and one that an Indigenous leader called “an unthinkable loss.”

Chief Rosanne Casimir of the Tk’emlúps te Secwépemc said in a news release Thursday evening that the “stark truth of the preliminary findings” was unearthed last weekend with the help of a ground-penetrating radar specialist who surveyed the site of what was once the Kamloops Indian Residential School.

“We had a knowing in our community that we were able to verify,” Casimir said. “To our knowledge, these missing children are undocumented deaths.”

She said it’s possible more remains could be discovered. The First Nation hopes to complete preliminary findings by mid-June.

Perry Bellegarde, the national chief of the Assembly of First Nations, said on Twitter that “while it is not new to find graves at former residential schools in Canada, it’s always crushing to have that chapter’s wounds exposed.”

Canadian Prime Minister Justin Trudeau said the discovery of the remains “breaks my heart.”

“It is a painful reminder of that dark and shameful chapter of our country’s history,” Trudeau tweeted. “I am thinking about everyone affected by this distressing news.”

From 1883 to 1996, nearly 150,000 Indigenous children were separated from their families, often by force, and sent to the government-funded, church-run schools in an attempt to assimilate them. There, many faced neglect and physical and sexual abuse. Speaking Indigenous languages and practicing their traditions were forbidden.

For many, the schools have left lasting scars and trauma that has been passed down from one generation to the next. Canada’s Truth and Reconciliation Commission concluded in its 2015 report that what happened at the schools constituted “cultural genocide.”

It also identified at least 3,200 students who died at the schools during that time — a rate that was far higher than for students elsewhere in Canada — though it said that the figure was probably greater and merited further investigation. It has since revised that number up to more than 4,100 children.

Students at residential schools often died of illnesses such as tuberculosis, which spread quickly in cramped and unsanitary living quarters and because the children were often malnourished. Others died by suicide, in fires, in accidents while enduring hard labor or by freezing to death while trying to escape.

Officials working to find the graves and to compile a register of the children who died at the schools told The Washington Post in 2018 that a lack of resources and missing documents was impeding progress, raising fears that unmarked graves could be destroyed.

The 2015 report said that school records were often destroyed. In some cases, school officials failed to note the names of the students who died, the cause of death or to report the deaths to the parents. Those who died were sometimes buried on school grounds, in part because the schools were long distances from Indigenous communities.

The Kamloops Indian Residential School, the largest in the residential school system overseen by the department of Indian Affairs, was set up in 1890 under the administration of the Roman Catholic Church. It operated as a school and, later, as a residence for students, for more than eight decades, shutting down in 1978.

In 1927, a medical health officer visited the school at the request of the principal, according to the Truth and Reconciliation Commission’s report. He found that a recreation room for younger boys was “most unsanitary” and had contributed to “numerous infections, colds, bronchitis and pneumonia during the past winter.”

The outside toilets, the inspector said, were “a distinct menace to the health of the children” and should be destroyed.

The commission also cited statements from George Manuel, a student at the school in the 1920s. He recalled being forced to speak English and having been called “a heathen” because of his grandfather.

Manuel had one enduring memory.

“Every Indian student smelled of hunger,” he said.

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