More than 900 Native American children died while being forced to attend Indian boarding schools, according to a new federal report that urges the U.S. government to formally apologize for the enduring trauma inflicted by its systematic effort to assimilate the children and destroy their culture.

Many of the children were buried in at least 74 marked and unmarked burial sites at 65 former schools across the country, according to the U.S. Department of the Interior report released Tuesday. The actual number of children who died and the number of potential burial sites are probably greater, the report said.

“For the first time in the history of the country, the U.S. Government is accounting for its role in operating Indian boarding schools to forcibly assimilate Indian children, and working to set us on a path to heal from the wounds inflicted by those schools,” wrote Assistant Secretary for Indian Affairs Bryan Newland in a letter presenting the report to Interior Secretary Deb Haaland, the first Native American Cabinet secretary.