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 apologise for the schools.

The investigation commissioned by 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, accidents and abuse during a 150-year period that ended in 1969, officials said.

The findings follow a series of listening sessions across 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 — facilitated by the department I lead — took deliberate and strategic actions through federal Indian 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,” Ms Haaland, a member of the Laguna Pueblo tribe in New Mexico and the country’s first Native American Cabinet secretary, said in a news release on Tuesday.

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 children English names, put them through military drills and forced them to perform manual labour, 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 the withholding of food.

Many left the schools with only basic vocational skills that gave them few job prospects.

The new report does not specify who should issue the apology on behalf of the federal government, saying only that it should be issued through “appropriate means and officials to demonstrate that it is made on behalf of the people of the United States and be accompanied by bold and actionable policies”.

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 revitalisation of indigenous languages.