LOS ANGELES (AP) — The star witness in a trial of a former transit officer accused of killing an unarmed black man at an Oakland train station can’t take the stand.

But the videos recorded by several people aboard a Bay Area Rapid Transit train that chronicle the events leading up to the 2009 New Year’s Day shooting are being called upon by attorneys on both sides to help prove their case.

During opening arguments Thursday, prosecutors said the tapes show former transit officer Johannes Mehserle, who is white, losing his calm amid the chaos, while a defense attorney argued his client made a horrible mistake by pulling out his gun instead of a Taser.

Mehserle, 28, is accused of killing 22-year-old Oscar Grant while he was lying on the ground. Mehserle has pleaded not guilty to murder.

“These videos go a long way in explaining how this happened and why this happened,” said Deputy District Attorney Dave Stein.

The trial is tinged with racial overtones and being watched closely by the Oakland community, where tensions boiled over into violence after the shooting. The case was moved from Alameda County to Los Angeles and is somewhat reminiscent of the fallout from the Rodney King beating that eventually resulted in the acquittal of four Los Angeles police officers in 1992.

Two of the prosecution’s first witnesses were a pair of women who shot separate video at the Fruitvale station. Karina Vargas pulled out her camera as she said she saw BART officer Tony Pirone yelling to get Grant and a friend off the train while several others were being detained nearby.

“It was the aggressiveness that compelled me to record,” she said.

On a video shot by Margarita Carazo, a man on the train repeatedly says “That’s (expletive) up” as a group of BART officers, including Mehserle and Pirone, dealt with Grant and his friends. Carazo said what she saw was “wrong.”

Mehserle’s attorney told jurors the videos prove his client accidentally shot Grant.

Rains said his client struggled with Grant for 12 seconds to try and handcuff him before the shooting and when he couldn’t, Mehserle told Pirone, ”’I’m going to Tase him!’” Rains said.

He said on one of the videotapes that his client’s thumb was in an upward position, as if to flick a switch activating the stun gun charge.

“Mr. Mehserle is getting ready to fire his Taser, believing he was going to incapacitate Mr. Grant,” Rains said.

Also proving Mehserle mistook his gun for a Taser was his visible reaction after the shooting, putting his hands to his head in apparent disbelief, the lawyer said.

“The weight of his mistake just crushed down on him,” Rains said as jurors watched the video.

Stein said Mehserle was fueled by misguided emotions as he arrived to a chaotic scene that escalated when Pirone used a racial epithet at Grant.

“The shooting death of Mr. Grant was the result of emotions taking over for discipline,” Stein said during his opening statement. “This was a result of aggression taking over judgment and training. For that, the defendant must be held responsible.”

In one video snippet, Stein noted that Mehserle looked down at his holster before pulling his gun. Mehserle would have had to reach for his Taser gun across his body on the left side to pull that weapon.

“Was the defendant confused?” Stein asked. “Look where he’s looking. He’s looking at what he’s doing.”

The prosecutor also said that Mehserle told another officer “I thought he was going for a gun” as more evidence showing Mehserle’s intent to use his firearm.

Stein showed for the first time a photo taken by Grant before he was shot. It is of Mehserle with his stun gun out near his right waistband. He is looking to his right and not at Grant.

Grant’s mother, Wanda Johnson, said outside of court — surrounded by supporters waving flyers bearing her son’s photo — that she was moved by the image shown in the courtroom.

Grant was “letting me know, ‘Mamma I wasn’t doing anything,’” said Johnson, who led a prayer with friends and family in the courtroom before the proceedings began. “That was what was heartbreaking, that he’s still speaking to me.”

Copyright 2010 The Associated Press