The remains of 215 children, some as young as three years old, were found at the site of a former residential school for indigenous children, a discovery Canadian Prime Minister Justin Trudeau described as heartbreaking on Friday.

The children were students at the Kamloops Indian Residential School in British Columbia that closed in 1978, according to the Tk’mlups te Secwepemc First Nation, which said the remains were found with the help of a ground penetrating radar specialist.

“We had a knowing in our community that we were able to verify,” Tk’emlups te Secwepemc First Nation chief Rosanne Casimir said. “At this time, we have more questions than answers.”

Canada’s residential school system, which forcibly separated indigenous children from their families, constituted “cultural genocide”, a six-year investigation into the now-defunct system found in 2015. It documented horrific physical abuse, rape, malnutrition and other atrocities suffered by many of the 150,000 children.