A mass grave containing the remains of 215 indigenous children has been found at the site of a former residential school for indigenous children in Canada, with Prime Minister Justin Trudeau describing the discovery as heartbreaking.

The grim discovery took place at Kamloops Indian residential school near the town of Kamloops, British Columbia, on Friday.

The bodies belonged to the students, some as young as three years old, of the indigenous Tk’emlúps te Secwepemc group and the authorities said investigation into the cause and exact timing of their deaths was underway.

“We had a knowing in our community that we were able to verify. To our knowledge, these missing children are undocumented deaths,” said Rosanne Casimir, chief of the Tk’emlúps te Secwépemc, in a statement. “At this time we have more questions than answers.”

Trudeau wrote in a tweet that the news “breaks my heart – it is a painful reminder of that dark and shameful chapter of our country’s history.”

In a statement, British Columbia Assembly of First Nations Regional Chief Terry Teegee also called finding such gravesites “urgent work” that “refreshes the grief and loss for all First Nations in British Columbia.”

The Kamloops Indian residential school was established in 1890 under the leadership of the Roman Catholic church, and closed in 1978.

The educational facility was part of a cross-Canada network of residential schools created to forcibly assimilate indigenous children by removing them from their homes and communities, and forbidding them from speaking their native languages or performing cultural practices.

A 2015 investigation into Canada’s residential school system concluded that the system, typically run by Christian churches on behalf of Ottawa, constituted “cultural genocide,” with reports documenting physical abuse, rape, malnutrition and other atrocities against the school children.

The same investigation said that the students were exposed to outbreaks of measles, tuberculosis, influenza and other contagious diseases.

It also found that more than 4,000 of 15,000 children died while attending these schools.

In 2008, the Canadian government formally apologized for the system.