Three videos made public so far show Grant’s final moments from multiple angles in a way that could help investigators, attorneys and use-of-force experts determine whether the shooting was accidental, intentional or inexplicable.

The videos – taken by onlookers – have been downloaded more than 450,000 times from KTVU-TV’s Web site, said William Murray, the site’s managing editor. That’s tantamount to two months’ worth of downloads in a few days, he said. An annotated version of one video uploaded to YouTube on Sunday was averaging more than 1,000 views an hour.

“It’s taken on a life of its own,” said Murray. “It’s one of those phenomenons of the Internet world.”

BART spokesman Linton Johnson said the agency was troubled by the way the video footage had become a media sensation.

“It does a disservice to the integrity of the investigation because people form opinions, and then they can’t give independent recollections of what they actually saw because they are tainted by the videos,” Johnson said. “It’s unfair to (Grant’s) family, it’s unfair to the police officer, and it’s unfair to the public. But that’s the world in which we live, and you have to adjust.”

The Chronicle asked several use-of-force experts, many with years of experience training police officers and military personnel and several with decades of law enforcement experience, to analyze the videos – an endeavor several experts warned can be informative but also inconclusive.

Shooting in context

The videos begin with a chaotic scene: BART officers questioning and restraining several people as a crowd of onlookers – many wielding cameras – shout in protest from a nearby train. Several videos capture, from different angles, Mehserle and another officer speaking with and eventually moving to restrain Grant.

The trainers said the scene as shown in the video moments before the shooting would be as important to understanding what happened as the shooting itself.

“The four officers have to be operating under a high level of stress given the relatively confined setting and the people on the BART train who are expressing, in a very loud vocal fashion, their displeasure with the officers’ actions,” said Frank Borelli, a use-of-force expert in Maryland. “Those officers, should things go bad for them, are vastly outnumbered by a group of people who have already voiced their unhappiness with the police.”

But other experts saw the moment differently.

“It’s clear it was not a use-of-deadly-force situation,” said Gregory D. Lee, a retired supervisory special agent with the Drug Enforcement Administration who has testified in federal use-of-force cases. “You’ve got more than enough manpower there to handle these guys.”

Bruce Siddle, founder of Illinois-based PPCT Management Systems, a use-of-force training and research firm, agreed with Lee’s interpretation.

“If you look at the officer’s body language, it’s clear in my mind he didn’t believe he was in a deadly force situation,” he said. “He’s relaxed.”

Seconds before the shooting, Mehserle and another officer apparently placed Grant on his stomach to be searched or handcuffed.

“Two officers appear to be struggling with Grant prior to the shot being fired,” Borelli said. “This would indicate that, at best, Grant was being uncooperative, or at worst aggressively resisting arrest. I have to emphasize that no one except those two officers knows what happened in that struggle and how the officers perceived it.”

Escalation of force

Several use-of-force trainers said it appeared that the struggle led the officer to change tactics.

“In my mind, what happened here was this guy was resisting,” Siddle said. “And at some point there was a decision made … we can’t control him, so we’re going to use the Taser.”

BART officials say some officers carry a Taser X26, a top-of-the-line model used by several other Bay Area police agencies. A BART spokesman said the Police Department started training to use the devices three or four months ago, but a source within BART said officers had only been using them on regular patrols since mid-December.

Agency police have not said whether Mehserle had a Taser the night of the shooting, although several of the experts said the videos appeared to show that he had the device on his belt.

The experts were split on whether Mehserle should have been drawing a weapon of any kind, lethal or otherwise.

“Why would you draw a firearm in that situation to begin with?” said George Kirkham, a Florida criminologist and consultant. “This doesn’t even seem like a Taser situation.”

Chaos on platform

Other experts said it is possible that given the chaos on the platform and the apparent struggle, Mehserle might have concluded that using a Taser to control Grant was justified.

A source familiar with the investigation has said BART is looking at the possibility that Mehserle had intended to fire a Taser at Grant but drew his service weapon instead.

Several cases have been documented of police officers in the United States accidentally shooting people with handguns when they intended to use Tasers, including a 2002 case in which a Madera (Madera County) police officer shot and killed a 24-year-old man. The officer was not charged criminally; a civil case brought by the dead man’s family is still pending.

Similar confusion could have led to Grant’s death, said several trainers, who noted that Mehserle and the other officers appeared in the video to be startled by the shooting.

Siddle said changes in how the brain processes information in a stressful situation might have led the officer to mistake the butt of his service weapon for the Taser. But other experts found the idea that the shooting resulted from such a mix-up hard to believe.

“That’s as reflexive as you getting in on the driver’s side of the car (instead of) the passenger side if you want to drive it,” Kirkham said. “There’s no remote similarity to a conventional firearm. … The Taser is just like apples and oranges.”

It is more likely, Kirkham said, that the officer and his attorney will argue that the officer saw or felt something on Grant – a cell phone, a belt buckle – that he believed was a weapon and that Grant was about to use it.

“That is the only possible explanation I can come up with,” he said. “Beyond that, you’ve got me.”

BART officials have said Grant was unarmed.

Another expert, Roy Bedard, who has trained police officers around the world, advanced a different theory after his first viewing of the video: that the shooting was a pure accident, a trigger pulled because of a loss of balance or a loud noise.

But in an indication of how the videos might move the investigation, Bedard reached a different conclusion after viewing the shooting from a different angle.

“Looking at it, I hate to say this, it looks like an execution to me,” he said. “It really looks bad for the officer. … We have to get inside his head and figure out what he was thinking when he fired the shot.”

E-mail the writers at mstannard@sfchronicle.com and dbulwa@sfchronicle.com.