But after lawyers for four white Los Angeles police officers dissected the video footage and told jurors to put themselves in the shoes of officers under stress, a jury delivered not-guilty verdicts in the 1991 beating of black motorist Rodney King. After six days of riots and 54 deaths, federal prosecutors filed civil rights charges and won convictions against two of the officers.

The King case becomes a cautionary tale for prosecutors now that Alameda County District Attorney Tom Orloff has filed murder charges against Johannes Mehserle, 27, the former BART police officer who fatally shot an unarmed and prostrate Oscar Grant at the Fruitvale Station in Oakland early New Year's Day.

As in the King case, the defendant is white, the victim black. Once again, onlookers' videos appear to show a criminal assault, this time fatal. Two local elected officials, Oakland City Councilwoman Desley Brooks and County Supervisor Keith Carson, have referred publicly to the shooting as an execution. The president of the NAACP in California, Alice Huffman, said Mehserle could have been motivated by racial prejudice.

As in King's case, in which the first trial was held in suburban Ventura County, intense media coverage and public anger might persuade a judge to transfer the Mehserle case to another county.

And, as in King's case, the guilt of the officer, if he goes to trial, will be determined not by what he did but by a jury's assessment of why he did it.

That won't necessarily show up on a video, said a lawyer for one of the officers in the King case.

'Both useful and deceptive'

"Videotapes can be both useful and deceptive," said Harland Braun, whose client, Theodore Briseno, was acquitted at both trials.

"They portray accurately, from that particular angle and time frame, what's going on," Braun said. But he added, "They can deceive you because they show only one angle" - and because they don't show what happened before the camera was turned on or after it was turned off.

Orloff said Wednesday that the videos shot by BART passengers - including "very valuable" footage that the public hasn't seen - show an apparently unlawful and intentional killing, the textbook definition of murder.

"The videos are very powerful on what act was committed," the district attorney said. "The issue likely to be in this case is, what was the mental state at the time that act was committed?"

Range of possible verdicts

His mental state at the time of the shooting will determine whether Mehserle committed murder, manslaughter - voluntary or involuntary - or no crime at all.

An intentional killing without provocation is second-degree murder if it's spontaneous and first-degree murder if it's planned. The sentence is 15 years to life for second-degree murder and 25 to life for first-degree, and 25 years is added to each for use of a gun.

At a news conference, Orloff didn't mention the possibility of a racially motivated murder, which is punishable by a death sentence or life without the possibility of parole.

Officers who shoot unarmed suspects often report that the victim provoked the attack by making a threatening move or appearing to reach for a gun.

Even though Grant had no gun, jurors could acquit Mehserle if they decide he had reasonably believed that Grant posed an imminent danger.

If jurors concluded that the officer's belief was sincere but unreasonable, they could convict him of voluntary manslaughter, punishable by three to 11 years in prison or as much as 21 years for use of a gun.

Stun gun confusion?

The defense also could argue that Mehserle thought he was firing his Taser stun gun and not his pistol. If a jury believed him, legal analysts say he would have a virtually ironclad defense to a murder charge and could be convicted of no more than involuntary manslaughter, which requires proof of grossly negligent conduct showing disregard for human life. That is punishable by two to four years in prison, or up to 14 years with use of a gun.

However, neither Mehserle nor his lawyers have offered any explanation for the shooting, and the Taser possibility remains speculation.

Also, in a court statement justifying Mehserle's arrest, Oakland police said Grant had struggled with officers but had already been restrained when Mehserle shot him in the back. That could raise the question of why Mehserle thought it necessary to pull either his service weapon or a stun gun.

BART police Chief Gary Gee says Mehserle was equipped with a newly issued Taser, which officers were ordered to carry on the opposite hip from their gun to avoid confusion. Robert Talbot, a University of San Francisco law professor and former trainer at police academies in San Francisco and San Mateo County, said the videos he's seen could support a claim that the fatal shooting was an accident.

Didn't look 'murderous'

"Nothing about his body looks murderous," Talbot said of Mehserle. "He looked almost too casual" for a deliberate killing, in which you might expect "somebody is going to put both hands on the gun and put it in the back of somebody's neck."

Besides, Talbot said, "he'd have to be out of his mind ... to stand on a platform full of people and pull the trigger."

A few cases in which officers have confused firearms for stun guns have arisen elsewhere, but apparently none has ever gone to a jury. Steve Tuttle, a spokesman for Taser International, said the company is unaware of any criminal prosecution against an officer who fired a gun in the mistaken belief that it was a Taser.

One who avoided prosecution was Marcie Noriega, a police officer in the city of Madera, who fatally shot Everardo Torres in 2002 with a gun she said she thought was a Taser. After an investigation that produced a 1,100-page report, District Attorney Ernest LiCalsi decided not to file criminal charges against Noriega, who remains on the police force. The Torres family's civil suit against the officer and the city is pending.

Changes in weapons

Changes in police weaponry since the Madera case, though, could make it harder for an officer to argue that he mistook a service weapon for a stun gun.

Tasers, which were about the same size and weight as pistols in 2002, now are smaller and weigh less than half as much. While Noriega held her gun on the same side as the Taser, officers now are told to keep the weapons in different areas.

"A Taser looks different from a gun," said Laurie Levenson, a professor at Loyola Law School in Los Angeles and a former federal prosecutor. "It's hard to believe that it's a reasonable mistake for an officer who's had any training at all."

E-mail Bob Egelko at begelko@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle