At least 973 Native American children died in the US government’s abusive boarding school system, according to the results of an investigation released Tuesday by officials who called on the government to apologize for the schools.

The investigation commissioned by the US interior secretary, Deb Haaland, found marked and unmarked graves at 65 of the more than 400 US boarding schools that were established to forcibly assimilate Native American children into white society. The findings don’t specify how each child died, but the causes of death included sickness and abuse during a 150-year period that ended in 1969, officials said.

Additional children may have died after becoming sick at school and being sent home, officials said.

The findings follow a series of listening sessions held throughout the US over the past two years in which dozens of former students recounted the harsh and often degrading treatment they endured while separated from their families.

“The federal government took deliberate and strategic action through boarding school policies to isolate children from their families, deny them their identities, and steal from them the languages, cultures and connections that are foundational to Native people,” Haaland, a member of the Laguna Pueblo tribe in New Mexico and the country’s first Native American cabinet secretary, said in a Tuesday call with reporters.

“Make no mistake,” she added, “this was a concerted attempt to eradicate the, quote, ‘Indian problem,’ to either assimilate or destroy native peoples all together.”

In an initial report released in 2022, officials estimated that more than 500 children died at the schools. The federal government passed laws and policies in 1819 to support the schools, the last of which were still operating in the 1960s.

The schools gave Native American kids English names, put them through military drills and forced them to perform manual labor, such as farming, brick-making and working on the railroad, officials said.

Former students shared tearful recollections of their experience during listening sessions in Oklahoma, South Dakota, Michigan, Arizona, Alaska and other states. They talked about being punished for speaking their native language, getting locked in basements, and having their hair cut to stamp out their identities. They were sometimes subjected to solitary confinement, beatings and withholding of food. Many left the schools with only basic vocational skills that gave them few job prospects.

Donovan Archambault, 85, the former chairman of the Fort Belknap Indian reservation in Montana, said beginning at age 11 he was sent away to boarding schools where he was mistreated, forced to cut his hair and prevented from speaking his native language. He said the experience led him to drink alcohol heavily before he turned his life around more than two decades later. He never talked about his school days with his children until he wrote a book about the experience several years ago.

“An apology is needed. They should apologize,” Archambault told the Associated Press by phone Tuesday. “But there also needs to be a broader education about what happened to us. To me, it’s part of a forgotten history.”

Haaland said she was personally “sorry beyond words”, but she suggested a formal apology should come from the federal government. She didn’t say if she would push Joe Biden to issue one.

Interior department officials also recommended that the government invest in programs that could help Native American communities heal from the traumas caused by boarding schools. That includes money for education, violence prevention and the revitalization of Indigenous languages. Spending on those efforts should be on a scale proportional to spending on the schools, agency officials said.

The schools, similar institutions and related assimilation programs were funded by $23.3bn in inflation-adjusted federal spending, officials determined. Religious and private institutions that ran many of the institutions received federal money as partners in the campaign to “civilize” Indigenous students, according to the new report.

By 1926, more than 80% of Indigenous school-age children – about 60,000 children – were attending boarding schools that were run either by the federal government or religious organizations, according to the National Native American Boarding School Healing Coalition.

The Minnesota-based group has tallied more than 100 additional schools not on the government list that were run by churches and with no evidence of federal support.

US Catholic bishops in June apologized for the church’s role in trauma the children experienced. And in 2022, Pope Francis apologized for the Catholic church’s cooperation with boarding schools in Canada. He said the forced assimilation of Native peoples into Christian society destroyed their cultures, severed families and marginalized generations.

Legislation pending before Congress would establish a Truth and Healing Commission to document and acknowledge past injustices related to boarding schools. The measure is sponsored in the Senate by Democrat Elizabeth Warren of Massachusetts and backed by Republican Lisa Murkowski of Alaska.