OAKLAND — A Bay Area transit police officer who saw a fellow officer fatally shoot a 22-year-old train passenger said he could have prevented his own death had he followed orders.

Officer Marysol Domenici testified Wednesday that Oscar Grant, who was shot in the back as he lay face down on a platform early New Year’s Day, was uncooperative as officers tried to detain him and his friends following a fight on a train from San Francisco.

“If they would’ve followed orders, this wouldn’t have happened,” Domenici said during testimony in Alameda County Superior Court. “They probably would’ve just been cited and released.”

Domenici was questioned during the fifth day of a hearing to determine whether former transit officer Johannes Mehserle will stand trial for murder in the killing killing Grant.

Mehserle has pleaded not guilty, and his lawyer, Michael Rains, says he meant to pull his stun gun when he accidentally grabbed his pistol.

Domenici’s response about Grant drew gasps and sighs from Grant’s family and friends in the courtroom.

“Are you trying to suggest it was his fault?” Deputy District Attorney David Stein asked Domenici during a contentious cross-examination.

“If he had just sat down, he’d been fine,” Domenici said to more sighs heard in the courtroom.

Grant’s mother, Wanda Johnson, said after the hearing that Domenici’s comments show “she has no regards for human life, for my son’s life.”

Domenici and other officers have described the scene on the Fruitvale train station in Oakland as out of control. During her testimony Tuesday, Domenici said at one point she felt for her life and safety.

On Wednesday, she told the courtroom that she feared she might have to kill somebody after she heard “pop” noise and later realized that Mehserle shot Grant.

She said it then became more chaotic.

“I said to myself, ‘Oh, Jesus Christ, Jesus Christ, if I have to, I’m going to have to kill somebody,’” Domenici said. “That’s when I started thinking, ‘If I have to go to lethal force, I will go to lethal force.’ ... They were coming everywhere. It was so chaotic.”

Stein has disputed BART officers’ testimony that the scene was that crazy. On Wednesday, he showed BART video surveillance of a more subdued scene in an attempt to discredit her account as exaggerated.

Domenici’s partner, Tony Pirone, also took the stand Wednesday. He’s been described by witnesses as being the most aggressive officer at the station New Year’s Day. Pirone told the courtroom how he ordered some of Grant’s friends to sit against a wall and then pulled Grant and another friend off the train.

Pirone said he pulled out his Taser gun as he approached the men, who he said matched a description he was given of the people fighting on the train.

Pirone said he grabbed Grant off the train by his elbow and while Grant was not physically resisting, he cursed at him. Pirone said Grant asked him why he was messing with him and Pirone said he told Grant “it would be a lot easier if you stopped playing games.”

The hearing will resume next Wednesday.