TRENTON, Canada 

Prime Minister Justin Trudeau apologized Monday for not responding to invitations from an indigenous tribe in British Columbia to mark Canada's inaugural National Truth and Reconciliation Day event. 

Instead of attending the Sept. 30 ceremony with the Tk'emlúps te Secwe̓pemc tribe, Trudeau spent the day vacationing with his family in another part of British Columbia.

Trudeau admitted his error in person Monday during a joint news conference with Tk'emlúps te Secwe̓pemc Chief Rosanne Casimir.

"It was a mistake, and I understand that it made a very difficult day even harder," Trudeau said. "You didn't have to invite me back, I know that. Thank you for doing so."

The day was set aside as an official federal government commemoration day to acknowledge the historical treatment of the various indigenous tribes in Canada -- forced attendance for about 150,000 children at 139 Indian residential schools beginning in the early 1800s.

Thousands were subjected to cruel treatment and sexual abuse, and some died and were buried in unmarked graves which are just now being probed. The Tk'emlúps te Secwe̓pemc announced in late spring that there were more than 200 unmarked graves at a former residential school where the tribe's children were sent. The last school closed in the 1990s. The day is also set aside to remember the thousands of missing and murdered indigenous women and girls.

While Trudeau attended a special ceremony marking Truth and Reconciliation Day on Parliament Hill on Sept. 29, he did not acknowledge two invitations sent to him by the Tk'emlúps te Secwe̓pemc to speak with residential school survivors on Sept. 30.

Casimir said the tribe was deeply disappointed at the snub.

"The shock, anger and sorrow and disbelief was palpable in our community," she said. "Today is about making some positive steps forward and rectifying a mistake."

Casimir asked Trudeau for three things -- full access to student attendance records from the residential schools, funding for a center for survivors of the schools, and help in exploring for all unmarked graves.

Trudeau said there will be as much money as necessary available to search for graves "right across the country."