On New Year's Eve in Oakland, a 22 year-old supermarket worker was pulled off a Bay Area Rapid Transit district train by police investigating a fight. Though unarmed, he was thrown face down onto the platform and shot in the back by a BART security guard, an event recorded and spread on the Internet by other passengers with cell-phone video cameras. Today the guard, Johannes Mehserle, was arrested and charged with murder. Henry Lee reports from Oakland for the San Francisco Chronicle.