WATCH: Trudeau addresses discovery of remains of over 200 dead children at former BC Residential School

"Sadly this is not an exception or an isolated incident—we're not going to hide from that. We have to acknowledge the truth: Residential schools were a reality—a tragedy that existed here in our country—and we have to own up to it," said Trudeau.

The last of Canada's residential schools closed in 1996.

This mass unmarked grave was found on the ground of what used to be the Kamloops Indian Residential School, founded in 1890 and closed in 1978.

The school throughout its history has been famous for its poor conditions. The facility on various occasions has been described as crowded and unsanitary, with rumors of widespread hunger and disease being prevalent there. One Canadian government worker noted in 1935:

"The sleeping accommodation for 285 pupils in the school consists of five dormitories, which are crowded. During an epidemic it is impossible to properly isolate the patients and contacts."

Comments expressing shock and sadness came from across Canada's political spectrum, including from the Conservatives' Michelle Rempel Garner, who during a Monday press conference called the discovery "a stain on our country."