A year ago this month, the riots at the third Woodstock festival triggered endless, hand-wringing speculation about the moral condition of the nation's youth. As far as the media was concerned, it was the rock & roll Columbine, an occasion in which meaningless violence somehow suggested Hydra-headed meanings, with each new theory delivering few answers and inspiring only more fevered theorizing. More recently, the tragic deaths during Pearl Jam's performance at the Roskilde festival in Denmark have kicked off a frenzy of blame, recrimination and soul-searching about the safety of concerts and the dangerous passions of young fans in roiling mosh pits.

The horrors of such events are often contrasted to the supposed good, old days when audiences communed in hippie splendor and music transported crowds to pure heights of celestial joy. Well, if anyone really believes that was always true, one devastating look at Gimme Shelter, the scarifying documentary about the Rolling Stones' 1970 concert at the Altamont Speedway outside San Francisco, ought to disabuse them of that quaint notion. The sunny Sixties had a very dark side, and it's terrifyingly evident in that movie, which has just been theatrically re-released (with a DVD forthcoming) in honor of the thirtieth anniversary of its initial appearance.

Most likely everyone knows that the central event at Altamont, which took place a mere four months after the original Woodstock festival, was the murder of Meredith Hunter, an eighteen-year-old black man who was stabbed and beaten to death in front of the stage by the Hell's Angels whom the Stones had hired as security guards. The Stones, visibly shaken by the brutal violence that had already taken place at the show, were meandering through a half-hearted version of "Under My Thumb" when the killing occurred.

That a thuggish motorcycle gang could be magically transformed into a loving, mellow security force for an outdoor concert to be attended by more than 300,000 people is only the most blatant of the flower-power fantasies that Altamont destroyed. The Angels might have been clubbing people to the ground with weighted pool cues for routine sport, but Hunter himself was waving around a gun at the moment he was brought down. The Stones are shown sitting in an edit room as the filmmakers -- Albert and David Maysles and Charlotte Zwerin -- show them footage of the slaying. Like us, they are no longer stars but witnesses to an event that happened right before their eyes (though they could not see it) and that they are still struggling, vainly, to comprehend.

It was during the brief American tour that ended at Altamont, which took place on December 6, 1969, that the Stones first claimed the title of "The Greatest Rock & Roll Band in the World." That is how they are introduced as the film opens during one of three shows the band did at New York's Madison Square Garden just a couple of weeks before Altamont. I attended one of those shows, and the Stones made good on their claim. Returning to live performance in the U.S. for the first time since 1966 -- and with guitarist Mick Taylor replacing Brian Jones, who had drowned in his swimming pool the previous year -- the Stones played with feral force. Good as it is, Get Yer Ya-Yas Out!, the live album recorded at the Garden dates, doesn't fully capture the Stones' might at that time. Gimme Shelter does.

That's why it's all the more devastating to watch Jagger beg helplessly for order at Altamont as the Angels glare at him with menacing condescension. Meanwhile, the massive audience -- the vast majority of whom had no idea what was going on near the stage (these were the days before enormous video screens, remember) -- surges and recedes in its own mindless rhythm. "Why are we fighting?" Jagger pleads. Moments before in the film he had strutted the stage like a young, invulnerable god. Now, he is unsure, scared and frustrated, ridiculous in his flouncy stage outfit, as desperate to understand and control what is happening, as he is incapable of doing so. In one seemingly insignificant but absolutely astonishing detail, a dog blithely trots across the front of the stage as the Stones attempt to resume playing and sweep the crowd up into the energy of their music. It's an incredible image of how irreversibly chaotic the event had become.

So the children of the turn of the twenty-first century are hardly the only generation that has had to confront the mortal violence within and around them. Commentators will comment and policies will be created to make concerts safer. That's all good as far as it goes. But part of rock & roll's compelling power is that the music is fully acquainted with the shadowy places within our souls. "I shouted out, 'Who killed the Kennedys?'/When, after all, it was you and me," Jagger sings in "Sympathy for the Devil," one of the songs the Stones performed at Altamont and captured in Gimme Shelter. And, as Woodstock III, Roskilde and this deeply disturbing movie demonstrate, rape, murder and terror of all kinds, alas, are still just a shot away.