A federal investigation has found that at least 973 Native American children perished in the U.S. government’s abusive boarding school system over a 150-year period. Interior Department officials released the findings Tuesday and called on the government to apologize for the schools. Officials say the deaths were caused by sickness, accidents and abuse. Interior Secretary Deb Haaland says the schools isolated thousands of children from their families and denied them their identities as Native Americans. Former students at the schools, which operated until the 1960s, previously shared their experiences during a series of listening sessions that highlighted the harsh and often degrading treatment they endured.