Leaders of Canadian Indigenous groups said investigators had found more than 600 unmarked graves at the site of a former boarding school for Indigenous children, a find that adds to reports of 215 bodies found at another school.

The bodies were found at the Marieval Indian Residential School, which operated from 1899 to 1997 on the site of the Cowessess Indian Nation, about 85 miles east of Saskatchewan’s capital, Regina.

An underground penetrating radar search returned 751 “hits”, indicating there are at least 600 bodies buried in the area, Cowessess chief Cadmus Delorme said. Radar operators said their results had a 10% margin of error.

“We want to make sure that when we tell our story, we don’t overstate the numbers,” Delorme said. “I mean over 600, just to be sure.”

He added that the search is continuing and that a technical team will evaluate the radar results and verify the figures in the coming weeks.

Delorme said the graves had headstones, but the Catholic Church, which ran the school, removed them.

In a tweet, Canadian Prime Minister Justin Trudeau said he was “terribly saddened” by the latest discovery.

“My heart breaks for the Cowessess Indian Nation following the discovery of Indian children buried at the former Marieval residential school,” Trudeau said. “We will tell the truth about these injustices,” he added.

Saskatchewan Premier Scott Moe said the entire province mourned the discovery of the graves.

Don Bolen, Archbishop of Regina, posted a letter to the cow on the archdiocese’s website.

“The news is overwhelming and I can only imagine the pain and the waves of emotion you and your people are going through right now,” Bolen wrote.

Bolen said that two years ago he apologized to the cow for “failures and sins of church leaders in the past.”

“I know apologies seem like such a small step as the weight of past suffering comes to light, but I offer this apology again and pledge to do all we can to translate this apology into meaningful action. such as assisting with access to information that will help provide the names and information of those buried in these unmarked graves,” he said.

Florence Sparvier, 80, says she attended Marieval boarding school.

“The nuns were very mean to us,” she says. “We had to learn to be Catholics. We couldn’t say our own blessings.”

The nuns at the school “condemned our people,” and the pain inflicted continues generations later, Sparvier said.

“We learned not to like who we were,” he added. “It went on and on and on.”

A month ago, the remains of 215 children, some as young as 3, were found buried in what was once Canada’s largest Indian residential school near Kamloops, British Columbia.

After the discovery, Pope Francis expressed regret and urged religious and political authorities to clarify “this sad affair”. But he did not apologize as requested by Indigenous nations and Canadian Prime Minister Justin Trudeau.

“An apology is a step on the road to recovery,” Delorme said.

“It was a crime against humanity, an attack on Indigenous nations,” said Chief Bobby Cameron of the Federation of Sovereign Indian Nations of Saskatchewan. He said he expects more graves to be discovered on the grounds of the former boarding school.

“We won’t give up until we find all the bodies,” he said.

From the 19th century to the 1970s, more than 150,000 Aboriginal children were forced to attend Christian schools run primarily by Catholic missionaries to integrate them into Canadian society.

The Canadian government has acknowledged that sexual and physical abuse were common in schools where students were whipped for speaking in their native language.