A free concert took place in northern California in December of 1969 featuring The Rolling Stones. It would descend into chaos due to poor planning, rampant drug use and the ill-advised use of the Hells Angels as concert security. 

Resources:
Joel Selvin, Altamont: The Rolling Stones, the Hells Angels and the Inside Story of Rock's Darkest Day, (New York: Harper Collins), 2016. 

Lester Bangs, Reny Brown, John Burks, Sammy Egan, Michael Goodwin, Geoffrey Link, Greil Marcus, John Morthland, Eugene Schoenfeld, Patrick Thomas, and Langdon Winner, Rolling Stone Magazine, "The Rolling Stones Disaster at Altamont: Let It Bleed," January 21, 1970. 

Albert & David Maysles and Charlotte Zwerin, Gimme Shelter (1970), http://watchdocumentaries.com/rolling-stones-gimme-shelter/ Note: Meredith Hunter can be seen in the crowd at timestamp 1:18:50. The attack on him begins at 1:23:26.