The mass death occurred when Congressman Leo Ryan of the Bay Area went to visit Jonestown to investigate reports of child abuse and people being held against their will.
When he arrived, the ensuing paranoiac reaction triggered an apocalyptic response. The Congressman was murdered by Jones followers and Jones, fearing retribution, oredered his followers to kill themselves and their children.
This is where the phrase "drink the kool-aid" in referring to mind control arose. The poison was mixed in big cats of kool-aid which all drank.
There is a good history of the rise and descent into madness and the tragic events in the Washington Post with a photo gallery.
There was a ceremony in San Francisco commenorating the event held by relatives and some of the survivors of the Jonestown settlement.
Many Jonestown survivors and their families believe that the lessons of Jonestown are to remember and guard against demagogues who use religion as a cover for fraud, deception and imposing their own sometimes dangerous social and political beliefs on their naive and unsuspecting followers.
It was that theme that dominated Tuesday's memorial service at the mass grave in Oakland. In an emotional and highly charged address, the Rev. Amos Brown, bishop at San Francisco's Third Baptist Church and president of the San Francisco NAACP, warned the mourners to beware of religious leaders who claim to have all the answers and insinuate themselves into politics, as Jones did so effectively in San Francisco.
"Good religion elevates folk, it teaches people to think for themselves. Good religion isn't authoritarian. Good religion isn't bigoted," he said. "Open up your eyes, America. America isn't a theocracy, it's a democracy. . . . And that is the lesson we must learn from Jonestown."