The fatal violence that erupted during the Rolling Stones' set at the Altamont Speedway on Dec. 6, 1969, cast a pall over what was supposed to be a day of celebration — and put frontman Mick Jagger in the crosshairs of an assassination attempt that stayed secret for decades.

As Stones fans are doubtless aware, the Altamont show was dogged with problems from the start, the festival's mood soured by a toxic combination of unruly fans and a security crew consisting of Hells Angels bikers. Fights broke out long before the Stones took the stage, and Jefferson Airplane singer Marty Balin was even knocked out by security. The Grateful Dead, whose manager had been the one to recommend hiring the Hells Angels in the first place, ultimately ended up refusing to perform their scheduled set.

The Stones persevered, however, even after Jagger was punched by a concertgoer while moving between the group's helicopter and the stage. His pleas for the audience to "be cool" fell on deaf ears; meanwhile, the Angels were reportedly not too impressed with Jagger's flamboyant dance moves, and the mood at the show continued to darken. Forced to stop their set during the third song, "Sympathy for the Devil," the band continued to try and connect with the crowd.

It all came to a head when a concertgoer named Meredith Hunter, irate over his rough treatment at the hands of the security crew during a foiled attempt to get on the stage, pulled a gun. Hells Angel Alan Passano intervened, stabbing Hunter and incapacitating him before other members of the crew joined in and delivered a beating. Though the Stones were unaware of it at the time, they were performing during a murder.

The group was understandably appalled, and in the aftermath of the Altamont disaster, Jagger strained to disassociate the Stones from the Angels' hiring and vowed never to use them again. The Angels were outraged, believing Jagger had sold them out — and even though Passano was acquitted of murder on the grounds of self-defense, the singer remained a target. As former FBI agent Mark Young told the BBC during an interview conducted for its The FBI at 100 (via the New York Times), they intended to get their revenge by killing Jagger.

Fortunately, according to Young, weather intervened. "Their plan involved making entry into his Long Island property, going by boat," he recalled. "As they gathered the weaponry and their forces to go out on Long Island Sound, a storm rolled up, which nearly sunk the watercraft they were in, and they escaped with their own lives."

Jagger reportedly remained blissfully unaware for decades. The FBI only turned up the murder plot much later, when an informant offered up details during a widespread investigation into their activities in 1985 — and most members of the public didn't hear about it until The FBI at 100 first aired. Jagger declined to comment.