The federal government is mapping burial sites at former residential schools as researchers try to identify how many of the estimated thousands of native children who went missing from the schools are buried in unmarked or anonymous graves.

Cemeteries scattered across Alberta, Saskatchewan, Manitoba and Ontario have been identified by researchers. Some of the graves have single white wooden crosses bearing no name. Others do not include even a cross.

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At...the St. John's Indian Residential School in Alberta (also known as Wabasca Residential School), the researchers found a document from 1961 describing how the principal came across an unmarked cemetery. A second letter indicates the unidentified principal ultimately cleaned up the site and erected 110 white crosses.

"The place was a terrible mess, so much underbrush," according to one of the letters. "Even though it is not finished, one can see a great improvement in it all, at least it is not woods now."

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An Indian Residential Schools Resolution Canada (IRSRC) report reviewing Indian Affairs documents describes an incident in 1992, when a construction company uncovered at least 19 graves connected to an unmarked graveyard at the site of the former school.