By September 9, 1971, inmates at New York’s Attica Correctional Facility had seized control of the prison and taken guards hostage. Their extreme actions followed months of protests over inhumane conditions, including overcrowding, minimal food, harassment, lack of medical care and rations of one toilet paper roll per month and one shower per week. Then, New York’s governor decided to open fire.

According to a taped conversation with aides, President Richard Nixon thought the decision was justified, saying, “You see it’s the black business...he had to do it.”

Top officials knew there would be a massacre if Governor Nelson Rockefeller abandoned negotiations with the prisoners and sent in state troops, and that’s exactly what happened. Officers dropped tear gas and fired 3,000 rounds, killing 39 people and wounding more than 80 others. Nixon hoped it would send a message to activists, or “the Angela Davis crowd,” as he put it, on his secret tapes.

“I think this is going to have a hell of a salutary effect on future prison riots,” Nixon said of the Attica attack. “Just like Kent State”—where the Ohio National Guard murdered four students during an anti-war protest—“had a hell of a salutary effect.”

Before these tapes became public, “we never really knew that [Nixon] was as deeply invested in what was happening in prisons,” says historian Heather Ann Thompson, who discovered Nixon’s taped conversations regarding Attica. “What we’ve sort of forgotten about this time period was that there was a great deal of activism inside of prisons.” Nixon viewed the protests as dangerous threats that needed quelling.

But it wasn’t just Nixon. Using the tapes and other government records, Thomson revealed in her 2016 Pulitzer Prize-winning book Blood in the Water: The Attica Prison Uprising of 1971 and Its Legacy that officials across the federal government viewed prison protests as dangerous, and that after the Attica massacre, numerous officials would work to cover it up.

The very first day that inmates seized control in Attica, New York, the FBI began sending memos about it to the Army, the Navy, the Air Force, the Marines, the CIA, the attorney general, the vice president and President Nixon himself.

“That was stunning because this is a minuscule town, one random state prison,” Thompson says. “But the instant there is conflict there, the FBI is not only all over it, but so was every branch of the federal government and every branch of the military… That shows that prisons matter in this country.” When prisoners protest, “authorities find that incredibly disruptive.”

These revelations also tell a larger story about the pervasiveness of paranoia within the federal government. Attorney General John Mitchell “was absolutely consumed with this idea of the left and activists kind of taking over the country,” Thompson says. Under J. Edgar Hoover, the FBI’s COINTELPRO, or “counterintelligence program,” surveilled civil rights leaders and sabotaged their organizations.

“This is an entire apparatus of the federal government that is…deeply fearful of and hostile to the civil rights movement,” she says. “Add prison activism into that, and these guys think that the world’s coming apart at the seams.”

On his tapes, Nixon said Governor Rockefeller was worried about Attica because “the word is around that this is a signal for the black uprising.”

“The Nixon tapes give a hint but do not fully reveal just how deeply involved Nixon’s White House was in shutting down prison activism,” Thompson says.

At the time, state government officials spread misinformation about Attica in order to justify the massacre. They said they’d given prisoners an ultimatum to surrender; that the prisoners, who were mostly black, had castrated white hostages; and that prisoners had killed hostages during the retaking of Attica. None of the statements were true. In reality, law enforcement killed the 10 hostages along with 29 prisoners as they raided the prison.

The false statements about what happened at Attica profoundly shaped how Americans saw that uprising in particular and prison activism in general. “On the eve of Attica, right as the prisoner protest was starting, Americans were quite sympathetic to these prisoners; they understood that prisons were hellholes that needed reform,” Thompson says. “And after Attica they are quickly deciding that, no, these guys are animals.”