President Joe Biden delivered an apology Friday for a United States policy that forcibly separated generations of indigenous children from their families for more than 150 years and sent them to federally backed boarding schools for forced assimilation. 

"I formally apologize as president of the United States of America for what we did," Biden said in strident remarks. "It's long overdue."

The president's apology, on tribal land at the Gila River Indian Reservation, came in the wake of a years-long investigation commissioned by Interior Secretary Deb Haaland, a member of the Pueblo of Laguna and the first Native American to serve as a Cabinet secretary. Haaland's grandparents were separated from their families because of the policy. 

"We know that the federal government failed," Haaland said in emotional remarks before Biden was introduced.

"It failed to violate our languages, our traditions, our life ways. It failed to destroy us because we persevered," she added.

The investigation uncovered generations of trauma. It identified the deaths of at least 973 Native American, Alaska Native, and Native Hawaiian children who attended the boarding schools.

During his remarks, Biden acknowledged that "the real number is likely to be much, much higher."

"The federal Indian boarding school policy, the pain it has caused, will always be a significant mark of shame, a blot on American history," Biden said.

In total, the probe identified 417 institutions across 37 states or then-territories that were operational between 1819 and at least 1969. 

"Many Indian children suffered physical, sexual, and emotional abuse at these institutions," the report found. It confirmed that at least 74 marked and unmarked burial sites at 65 school sites.

The president's apology comes more than two years after Pope Francis issued a similar apology on behalf of the Catholic church for similar abuses in Canada. More than 150,000 native children were forced to attend Canadian boarding schools.

Alex White Plume, 73, a former president of the Oglala Sioux Tribe who attended two boarding schools on reservations in South Dakota, told NBC News he would not accept the apology from the president.

"I don't really see any way where we could accept it, because it doesn't change anything," White Plume said.

"We need to survive, and in order to survive we need our territories back so we could bring back our language and perform the ceremonies that are specific to places in our territory," he said. "So I don't want to accept an apology. I want them to be meaningful. And if it's a meaningful apology, he would say, 'Okay, we're gonna investigate the genocide, and we'll establish a process to create protocols on how to go about it.' I think something like that would have been more meaningful."

Cecelia Fire Thunder, 78, who became the first female president of the Oglala Sioux Tribe, and her three sisters attended Holy Rosary Mission School in 1953, now known as Red Cloud Indian School, on the Pine Ridge Reservation in South Dakota. She said an "apology is about making sure that our community is receiving resources for behavioral health if there's a trickle down effect of what happened 50 or 60 years ago."

"There is no word for forgiveness in our language," she added. "Just because you acknowledge somebody hurt you, and you say, 'I forgive you,' that doesn't mean the pain left; the pain is still there."

Fire Thunder said an apology "should open the door for people to ask the question, 'What happened?' It should open the door not just for Native people, but for all of America, because they don't know."

Marsha Small, a 65-year-old specialist in ground-penetrating radar, led the search in 2022 for unmarked graves of children who died while attending Red Cloud Indian school.

"I'm a little angry," she said. "But I'm appreciative of President Biden acknowledging this."

"In their statement, the White House says we must learn from that history so that's never repeated," Small added. "Well, we're still living the nightmare. So get some money into these reservations, into the homelands, into urban areas too."

While Biden's apology was welcomed by much of the crowd in Phoenix on Friday morning, one demonstrator indicated it was not enough.

"There are still babies in mass graves your apology means nothing" read a sign held by the demonstrator, who was escorted out during the president's remarks.