On December 6, 1969, in a shocking act of violence, a Hells Angel biker stabs to death an 18-year-old concertgoer a during a set by the British rock group Rolling Stones at the Altamont music festival.

Altamont, a new festival in Northern California, was the brainchild of the Stones, who hoped to cap off their U.S. tour in late 1969 with a concert that would be the West Coast equivalent of Woodstock, in both scale and spirit. Unlike Woodstock, however, which was the result of months of careful planning by a team of well-funded organizers, Altamont was a largely improvised affair that did not even have a definite venue arranged just days before the event.

It was only on Thursday, December 4, 1969, that organizers settled on the Altamont Speedway location for a free concert that was by then scheduled to include Santana, the Jefferson Airplane, Crosby, Stills, Nash and Young and the Grateful Dead, all in support of the headlining Stones. The event would also include, infamously, several dozen members the Hells Angels motorcycle gang acting as informal security staff in exchange for $500 worth of beer as a “gratuity.”

It was dark by the time the concert’s penultimate act, the Grateful Dead, was scheduled to appear. But the Dead had left the venue entirely out of concern for their safety when they learned that Jefferson Airplane singer Marty Balin had been knocked unconscious by one of the Hells Angels in a melee during his band’s performance. (Crosby, Stills and Nash's Stephen Stills was repeatedly stabbed in the leg with a sharpened bicycle spoke.)

It was during the Rolling Stones’ set, however, that a 21-year-old Hells Angel named Alan Passaro stabbed an 18-year-old attendee named Meredith Hunter to death just 20 feet in front of the stage where Mick Jagger was performing “Under My Thumb.” Unaware that someone had died, the Rolling Stones completed their set without further incident, bringing an end to a tumultuous day that also saw three accidental deaths and four live births.

The killing of Meredith Hunter at Altamont was captured on film in Gimme Shelter, the documentary of the Stones’ 1969 tour by Albert and David Maysles and Charlotte Zwerin, which opens with Jagger viewing the footage in an editing room several months later. In the years since, Jagger has not spoken publicly about the killing, for which Passaro was tried but acquitted on grounds of self-defense.