OAKLAND , California ( CNN ) -- A former transit police officer charged with murder was released from custody Friday after posting a $ 3 million bail . Video shows Johannes Mehserle shoot Oscar Grant III in the back as another officer knelt on him . Dozens of demonstrators gathered in downtown Oakland to protest the release of Johannes Mehserle , 27 , charged with killing an unarmed man on New Year 's Day . iReport.com : Watch the protest . The former Bay Area Rapid Transit police officer 's release funds came from unknown sources , Alameda officials told CNN . Video taken by a bystander showed Mehserle pulling his gun and fatally shooting Oscar Grant III , 22 , in the back as another officer kneeled on Grant . Mehserle may have intended to draw and fire his Taser instead of his gun , according to a court filing by his attorney . In January , protests turned violent after a judge decided to allow bail for Mehserle 's release . A preliminary hearing in the case is set for March 23 , authorities said . CNN 's Jackie Castillo contributed to this report .
Former BART officer accused of assault on $ 3 million bail . Richard Mehserle charged with murder of Brandon Grant ##ham . Mehserle may have intended to use knife instead of gun , attorney says .