Ground-penetrating radar was used in the cemetery area of St. Mary's Indian Residential School in Kenora, Ontario, the Wauzhushk Onigum Nation said in a press release.
"With the exception of five grave markers, the remaining are unmarked by any grave or burial markers," the release said.
The radar could have detected tree roots or large stones, so without further tests, it is not known if all the graves contain human remains.
The Wauzhushk Onigum Nation, which still has 50 survivors of the school who attended in the 1940s to 1960s, said more searches will be done at other sites "that have been identified through survivor testimony, archeological assessment and archival investigations that show burial rituals conducted by former residential school staff."
The graves are likely those of children, since Canada's National Centre for Truth and Reconciliation said at least 36 students died while attending St. Mary's. The school was operated by the Catholic Church from 1897 to 1972.
It was one of 139 Indian Residential Schools set up and funded by the Canadian government and run by religious denominations beginning in the mid-1800s. The last one closed in 1996. Indigenous children were forced to attend and the original goal was to stamp out Indigenous culture and replace it with white culture.
About 150,000 children attended the schools and the National Centre for Truth and Reconciliation Commission has estimated that at last 4,100 children died from disease and malnutrition while others suffered physical, emotional and sexual abuse.
The children were often buried at the schools and their parents too often were never informed of their deaths. Close to 1,900 graves have been located in the last few years using ground-penetrating radar.
Prime Minister Justin Trudeau has called the residential schools and what was done to Indigenous peoples a "dark and shameful chapter of our country's history."