A national working group is looking into the painful and complicated question of what happened to children who died or went missing from Canada's residential schools.

The working group, set up by former Indian and Northern Affairs Minister Jim Prentice, includes members of the federal government, churches and aboriginal groups.

It is expected to make recommendations to the federal government's Truth and Reconciliation Commission on how best to investigate residential school deaths and missing students, including how many children died and where they may be buried.

"This is the most critical, I think, and tragic aspect of the history of the residential schools … that children were taken from their parents and died a long way from home," John Milloy, a Canadian Studies professor at Trent University and a member of the working group, told CBC News in an interview.

"They have been buried without the knowledge of their parents, in places that their parents cannot visit or get to … many children simply disappeared."

The Truth and Reconciliation Commission is part of Ottawa's $1.9-billion settlement with former students that includes compensation payments for the thousands of survivors. The payments started flowing to former students across the country last fall.

Milloy, who wrote the book A National Crime on Canada's residential school experience, said estimates from as far back as 1907 show that 24 to 42 per cent of children in some schools died of tuberculosis infection.

"Nearly every school I know of had a cemetery on the school grounds. We've got pictures of them and stories about them, and children were, we assume, buried there," Milloy said.

"Some of them are marked, and many of them — from reports from survivors and pictures I've seen — are unmarked."

There are no numbers on how many children died in the residential school system. Milloy said it will cost Ottawa millions of dollars to uncover that information, but added it's something that must happen before healing can begin.