What was the Hells Angels’ role at the Altamont music festival in 1969?

The Hells Angels were contracted by the Rolling Stones’ road manager to provide security at the music festival held at the Altamont Speedway in Livermore, California, in 1969. A member of the Angels killed a Black teenager, Meredith Hunter, during the Rolling Stones’ performance. The Altamont festival became a notorious event of the 1960s.