OAKLAND - IN GRAINY cell-phone videos played over and over on the Internet, police officers force an unarmed black man to the ground and hold him face-down on a crowded train platform.
Suddenly one of the officers draws his gun and fatally shoots the man in the back - then looks up.
The New Year's Day death of 22-year-old Oscar Grant has led to violent street protests amid allegations from the family's attorney that some of the officers used racial slurs.
The officer remains free and has not been charged with any wrongdoing. And some experts have questioned whether he fired his gun deliberately or mistakenly believed he was using his stun gun instead.
At a rally on Wednesday attended by hundreds of people, Shawanda Thomas held a fluorescent yellow sign that read: 'Oscar Grant: Murdered! The Whole Damn System is Guilty.'
Extra police were posted on Thursday at Bay Area Rapid Transit stations after a group of angry demonstrators smashed storefronts late on Wednesday, set fire to cars and clashed with officers equipped with riot gear and tear gas in downtown Oakland. More than 100 people were arrested and about 300 businesses were damaged.
Oakland Mayor Ron Dellums called for calm. 'Even with our anger and our pain, let's still address each other with a degree of civility and calmness and not make this tragedy an excuse to engage in violence,' he said. 'I don't want anybody hurt. I don't want anybody killed.'
At the mayor's request, the Oakland Police Department launched an investigation into the shooting on Thursday. Alameda County District Attorney Tom Orloff, whose office also is investigating, said he probably would decide within two weeks whether to file charges.
Transit police officer Johannes Mehserle was initially placed on paid leave. He resigned from the BART police force on Wednesday, but officials say he has refused to speak with the transit agency's investigators. He has not spoken publicly about the incident.
Mr Mehserle's attorney, Christopher Miller, declined to comment on the investigations.
Mr Grant's family has filed a US$25 million wrongful-death claim against BART, the San Francisco Bay Area's commuter rail system, and relatives want Mehserle to be criminally charged.
'They want justice, but they don't want any more violence,' said John Burris, an attorney for Mr Grant's family. 'That officer hasn't been prosecuted ... That's why people don't have confidence in the system right now.'
Local African-American leaders expressed outrage on Thursday at the shooting. And some Oakland residents have alleged it was racially motivated. Mr Burris said he does not have any evidence that Mr Grant was shot because he was black.
'There were racial slurs made by other officers to the group that Oscar Grant was with, but I have no evidence that this particular officer directed racial slurs toward Oscar Grant,' Mr Burris said.
BART officials said the agency is trying to conduct a thorough investigation, but that the public appears to be making judgments about the case based on raw video they saw online or on television.
'They see the answer before them playing out over and over on TV, but we have to follow the process and have to turn over evidence to the DA, and the DA decides what to do from there,' said BART spokesman Linton Johnson.
The shooting unfolded in front of dozens of train passengers who were returning home after New Year's Eve celebrations.
Police officers arrived shortly after midnight on New Year's Day at the Fruitvale BART station following reports of young men fighting on a train. Mr Grant was one of several who were ordered off the train, questioned and then restrained by Mr Mehserle and other officers.
Videos shot by onlookers show Mr Grant being pushed onto his stomach shortly before Mr Mehserle fired his gun at Grant's back. The bullet ricocheted off pavement and pierced his lung, killing him.
The video footage has led to debate over whether the officer knowingly shot Mr Grant, as the victim's family alleges.
Reports of police officers mistaking a handgun for a stun gun are rare, but not unheard of. In 2006, a sheriff's deputy in Washington state accidentally shot and wounded a disturbed man after mistakenly using his .40-calibre gun instead of his stun gun.
Bruce Siddle, a use-of-force expert who viewed the video clips, theorized that Mr Mehserle was working under stress in a hostile situation and did not realise he was firing his pistol.
'I suspect he thought he was reaching for his Taser,' said Mr Siddle, founder of PPCT Management Systems, an Illinois company that trains law-enforcement officers in the use of force.
'If he was under stress, he would not be able to distinguish between a Taser and his firearm. You have video footage that seems to suggest that this officer made a tragic mistake.'
But George Kirkham, a professor of criminology at the Florida State University who also viewed the footage, said he finds that hard to believe because most Taser stun guns do not look or feel like pistols, and the officer fired in a manner consistent with a handgun, not a Taser.
Prof Kirkham, who works as an expert witness in criminal cases, speculated the officer fired because he thought he saw something in Mr Grant's waistband or pocket that appeared to be a gun or other type of weapon.
'It's not believable that any officer can mix up a Taser and a firearm,' said Prof Kirkham, who has examined almost 500 police shootings over the past 30 years. 'It's like looking for your steering wheel on the right side of your car rather than the left side.'
Outrage over the shooting has been fuelled by raw video clips posted on YouTube and various news websites.
Over the past week, video of the shooting has been viewed more than 500,000 times on the website of KTVU-TV, which has posted exclusive clips of the incident, said Bill Murray, who manages the station's website. That is about twice as many page views as the site typically sees in a full month.
'Once a story gets national momentum, people want to come to it,' Mr Murray said. 'There's always been a certain voyeurism to online video. I think people want to see for themselves.' -- AP
