A second witness recalled that Mehserle raised his hands and pressed them to his head, appearing "dumbfounded," while a third witness quoted the officer as blurting out, "Oh my God! Oh my God!"

The accounts are from witnesses called by Alameda County prosecutors, who rested Tuesday after putting nine people on the stand during a preliminary hearing for Mehserle. The 27-year-old former officer is accused of murdering Oscar Grant, 22, after forcing him to his chest while trying to arrest him at Fruitvale Station in Oakland.

Mehserle's attorneys, who say the officer meant to subdue Grant with a Taser when he fired his pistol and hope to get the charge reduced, are set to begin calling witnesses this morning. After the hearing in Oakland, Superior Court Judge C. Don Clay must decide whether to send the case to jury trial.

The shooting, caught on film by five of the prosecution witnesses, has been shown repeatedly on a flat-screen television in court. But nearly as important as the moment that Mehserle's gun fires may be his reaction - and what it means.

His attorney, Michael Rains, has said Mehserle was shocked because he had intended to squeeze a Taser trigger. But prosecutors and Grant's family members contend Mehserle made up the Taser story in an effort to avoid punishment.

Mehserle has never told investigators why he drew a weapon or why he expressed apparent shock after he shot Grant. And he is not expected to testify at the hearing. But Michael Cardoza, a defense attorney and former prosecutor who has been following the case, said the officer would have no choice but to testify at trial.

"He's got to take the stand in this case to explain what was going on," Cardoza said outside court. "That's the only way a jury would, in my opinion, even consider bringing this down from second-degree murder."

Grant, a Hayward resident and supermarket worker, had been detained after a fight aboard a Dublin-Pleasanton train about 2 a.m. One of Grant's friends, 16-year-old Jamil Dewar, testified Tuesday that Grant had been involved in the fight - which he described as a tussle with a second man that Dewar and others broke up.

Soon, BART police Officer Tony Pirone, cursing and pointing his Taser, detained Grant and four of his friends, Dewar and other witnesses said. On the videos, passengers can be heard shouting and cursing at officers.

Mehserle shot Grant while trying to handcuff him and after Pirone decided that he should be arrested for resisting officers.

Among those who witnessed the shooting was Tommy Cross Jr. of San Leandro, a San Francisco State University student who took the stand Tuesday and described how he had captured the clearest video of the incident from the doorway of a BART car.

Cross said Grant had not resisted officers and had his hands behind his back when he was shot. In court, though, it was difficult to see exactly where Grant's hands were on Cross' video, which was played only once at full speed.

Cross said Mehserle had twice said, "Oh my God!" but his voice did not appear to be captured on tape.

Another witness, Daniel Liu, who said he had sold his video of the shooting to KTVU-TV for $1,000, testified that Grant "didn't look like he was resisting" as Mehserle tried to handcuff him. But under cross-examination, he said, "His left arm was moving up and down. I'm not certain if he was doing that to resist arrest or what."

Rains gave a peek into the defense's case in his questioning. He suggested that Grant had been drinking alcohol and had, during a chaotic scene on the platform, fought Mehserle's attempt to handcuff him.

Defense witnesses are expected to include officers who were with Mehserle when he shot Grant. Rains has also subpoenaed Grant's friends who were detained with him, as well as other witnesses who were on the BART train.

Rains plans to slow down and analyze the video footage in court, and, according to sources, will call into question the adequacy of the training BART provided officers before giving them Tasers last year.

E-mail Demian Bulwa at dbulwa@sfchronicle.com.

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