When the women were in jail together, Nichols realized that Aquash was terrified about being released. Aquash told her she was aware that some people thought she was a turncoat. Suspicions had arisen when she was quickly let go after an earlier arrest, while others who had been with her stood trial. In Brando’s motor home, Peltier had confessed to both women that he shot an F.B.I. agent, making a gun with his thumb and forefinger and telling them that the man “was begging for his life, but I shot him anyway.” If Aquash ever shared that with the authorities, it would make her a significant threat to Peltier and AIM. Aquash told Nichols that she feared for her life. Soon, the women were escorted by marshals to the back of a commercial airplane, flown to Wichita and then sent to separate jails — Nichols in Kansas and Aquash in South Dakota. Aquash was again swiftly released on bail.

“It was the last time I saw her,” Nichols said. A month later, Aquash was dead.

“For a long time, it was a given among Indians that the F.B.I. engineered Aquash’s murder as a way of scaring and destabilizing AIM,” says Paul DeMain, the editor of News From Indian Country, whose aggressive reporting on the case is often credited with spurring investigators’ interest in it. AIM considered itself at war with the federal government and its proxy, the F.B.I., whose Counterintelligence Program was devised to monitor and take down the radicals of the New Left that the bureau deemed “subversive,” including the Student Nonviolent Coordinating Committee and the Weather Underground. AIM’s concerns weren’t entirely unfounded. A few months before Aquash was killed, one of Banks’s bodyguards, Douglas Durham, appeared on national TV to declare that he was actually a “paid F.B.I. operative” who’d been assigned to infiltrate AIM. Adding to the conspiracy theory was a hasty initial autopsy that somehow missed the bullet in Aquash’s head.

“The great obstacle with Anna Mae all along has been the disconnect of trust from the witnesses we needed,” says Rod Oswald, who became one of the case’s lead prosecutors. “It’s something they’ve passed down to their kids, almost like a legend: The F.B.I. killed her and covered it up, and therefore there was no way the federal government could bring justice to the Native American people.” Investigators needed a collaborator who could, like Nichols, approach AIM members up and down the chain of command.

Nichols was still incarcerated on Dec. 30, 1975, when she gave birth to a daughter, her second with Banks. They named her Tiopa Maza Win, or Iron Door Woman. Soon after, she was released and reunited with Banks, who had been found and arrested but was out on bail. California’s governor, Jerry Brown, granted him asylum, and the family settled in Davis.

“We tried to live as normal as possible,” Nichols said. She took classes at U.C. Davis. Banks became a chancellor at D-Q University, an Indian college, and a guest lecturer at Stanford. Their respite lasted until 1983, when George Deukmejian, elected to succeed Brown as governor, vowed to force Banks from the state. About an hour after Deukmejian was sworn in, Banks’s house was surrounded by the police. But he and his family were already gone, en route to the Onondaga Reservation in upstate New York. “The cops couldn’t arrest Dennis as long as he stayed on Indian land,” Nichols said. “He got a job at the smoke shop on the Interstate. The next year, he said he was tired of running. He finally turned himself in and spent a year in prison.”

In 1989, Nichols told Banks she was fed up with his womanizing and left him. She moved to Santa Fe and left AIM behind, until one day in 1999, when she received a newspaper clipping about the Aquash case in the mail from her mother. Some AIM members were claiming that one of several factions in the organization was responsible. “It started to make sense,” Nichols said. She began asking friends from Pine Ridge what they knew and learned that “for years, everybody had been hearing the stories,” she said.

The contours of the plot were the same in every version: Aquash left the federal courthouse in Pierre on Nov. 24, 1975, when she made bail and was released from jail. She didn’t show up for a court hearing the next day. According to friends, Aquash was desperate to see Banks and made her way to Denver, where she thought she would meet him.