On February 24, 1976, a Jane Doe was found frozen in the Badlands of South Dakota. In the story the FBI told, there was no sign of violence to the body, and an official autopsy found that she had died of exposure. The corpse was so decayed that fingerprints could not be taken at the autopsy, so, in an unusual procedure, the hands were chopped off and shipped to the FBI lab in Washington. A few days later, the rotting body had to be buried for the sake of public health. The grave was unmarked.

Hours after the interment, the FBI lab announced that the severed hands belonged to Anna Mae Aquash, a leader of the American Indian Movement (AIM) and nemesis of the FBI. Aquash’s friends and family, incredulous that the savvy Aquash had taken an underdressed midwinter stroll and succumbed, demanded the body be exhumed. It was, and seconds after seeing the body, a pathologist for the family found a grossly obvious entrance wound at the base of Aquash’s skull. Anna Mae Aquash had died of exposure, alright: exposure to a .32-caliber, copper-jacketed bullet.

In time it would emerge that the FBI had lied about whether police officers and FBI agents had seen the bullet wound, about the impossibility of taking her fingerprints in Dakota (and thus the need to dismember her), about the need to rush her corpse into the ground, and probably about whether agents knew all along that the Jane Doe was Aquash. Indians theorized that the FBI was lying to protect someone or something, perhaps an FBI operative who had been too close to the murder. It was not an irrational suspicion. The FBI had sent any number of spies and provocateurs into AIM, as it had with other activist groups of the era. The job of the provocateurs was to sow dissent, for example by spreading rumors that activists were informers. Often as not, the activists descended into finger-pointing and paranoia. Movements imploded. In some cases, people were killed. Aquash, long dogged with false rumors that she was an informer, appeared to be one such case.

It was this sorry history, or something like it, that Indians suspected the FBI wanted to keep hidden. As decades passed and the FBI failed to solve Aquash’s murder, the suspicion grew more potent. The FBI called the conjecture hogwash: its agents had no clue—almost literally—how Aquash had been killed. But documents I recently won from the FBI in a Freedom of Information Act lawsuit say otherwise.

The documents, which I obtained for my new book, The Unquiet Grave: The FBI and the Struggle for the Soul of Indian Country, show that mere days after Aquash’s murder in late 1975, an informer in Denver told the FBI the details of the killing. AIM, the informer said, had kidnapped an Indian woman in Denver, driven her to South Dakota, interrogated her, and executed her. The informer even named the three triggermen—names the FBI will not release. Independent investigations would eventually show that the informer got the story pretty much right. The only key detail he left out was that the triggermen had acted at the behest of AIM leaders who had become paranoid about FBI infiltrators and ordered Aquash murdered.

The informer’s tip came to the FBI at a time when the bureau was doing everything in its power to cripple AIM. A few months earlier, two FBI agents were killed in a shootout with AIM, prompting the FBI to launch the largest investigation in its history. AIMers everywhere were probed and prosecuted for crimes large, small, and (in some instances) fabricated. Yet when the Denver informer told the FBI that AIM had committed a murder, the FBI apparently did nothing—nothing at all—with the tip. Later, after Aquash’s body was found, agents confirmed that the tip was accurate, but still they did nothing. The inaction is both stunning and damning: for decades the FBI had lied that it could not solve the murder because it had no good leads.

Lately the FBI has been trumpeting its progress in the case. In 2003 federal prosecutors indicted two of the three alleged triggermen. One, Arlo Looking Cloud, was convicted of killing Aquash in 2004, while the other, John Graham, is fighting extradition from Vancouver. (The third suspect is supposedly incapacitated by Alzheimer’s.) But the feds only brought the charges after lay investigators, angered at the FBI’s inaction, publicly named the triggermen in 1999. And even then, the government waited four years to bring the charges and still have brought none against any of the senior conspirators.

Was there an FBI operative among those conspirators? Is that why no charges have been brought against them? It isn’t certain. But two well-placed sources have said that one AIMer who took part in talks about Aquash’s fate had at least offered to spy for the FBI. Since this man was deeply involved with the AIMers who were wanted for killing the FBI agents, it is hard to imagine the FBI turned down his offer—if indeed he made one.

November is Native American Heritage Month. Instead of marking it with the usual limp paeans to Indian culture, perhaps Congress and the president could demand the FBI come clean about Aquash’s murder. It would be one small step to understanding how the FBI helped undermine the foremost Indian rights movement of the last century.