FLAGSTAFF, Ariz. — A first-of-its-kind federal study of Native American boarding schools that for over a century sought to assimilate Indigenous children into white society has identified more than 500 student deaths at the institutions, but officials say that figure could grow exponentially as research continues.

The Interior Department report released Wednesday expands to more than 400 the number of schools that were known to have operated across the U.S. for 150 years, starting in the early 19th century and coinciding with the removal of many tribes from their ancestral lands. It identified the deaths in records for about 20 schools.

The dark history of Native American boarding schools — where children were forced from their families, prohibited from speaking their languages and often abused — has been felt deeply across Indian Country and through generations.

Many children never returned home, and the Interior Department said that with further investigation the number of known student deaths could climb to the thousands or tens of thousands. Causes included illness, accidental injuries and abuse, officials say.

“Each of those children is a missing family member, a person who was not able to live out their purpose on this Earth because they lost their lives as part of this terrible system,” said Interior Secretary Deb Haaland, whose paternal grandparents were sent to boarding school for several years as kids.

The agency is in the process of poring through thousands of boxes containing more than 98 million pages of records, with help from many Indigenous people who have had to work through their own trauma and pain. Accounting for the number of deaths will be difficult because records weren’t always kept.

A second volume of the report will cover burial sites as well as the federal government’s financial investment in the schools and the impacts of the boarding schools on Indigenous communities, the Interior Department said. It has so far identified at least 53 burial sites at or near boarding schools, not all of which have marked graves.

Tribal leaders have pressed the agency to ensure that any children’s remains that are found are properly cared for and delivered back to their tribes, if desired. The burial sites’ locations will not be released publicly to prevent them from being disturbed, said Bryan Newland, the Interior Department’s assistant secretary for Indian Affairs.

Haaland choked back tears Wednesday as she described how the boarding school era perpetuated poverty, mental health disorders, substance abuse and premature deaths in Indigenous communities.

“Recognizing the impacts of the federal Indian boarding school system cannot just be a historical reckoning,” she said. “We must also chart a path forward to deal with these legacy issues.”

Haaland, who is Laguna, announced an initiative last June to investigate the troubled legacy of boarding schools and uncover the truth about the government’s role in them. The 408 schools her agency identified operated in 37 states or territories, many of them in Arizona, New Mexico and Oklahoma.

The Interior Department acknowledged the number of schools identified could change as more data is gathered.

The U.S. government ran some of the boarding schools.

Catholic, Protestant and other churches operated others with federal funding, backed by U.S. laws and policies to “civilize” Native Americans. The federal government still oversees more than 180 schools in nearly two dozen states that serve Native Americans, but the schools’ missions are vastly different from the past.

The Interior Department report was prompted by the discovery of hundreds of unmarked graves at former residential school sites in Canada.