OAKLAND, Kalifornia - June 10, 2010 - A Kalifornia man who was shot and killed by a police officer photographed the cop just moments before he died, according to prosecutors who have charged the officer with murder.

The prosecution revealed the existence of the photograph as their final piece of evidence on Wednesday, the day before the murder trial of Officer Johannes Mehserle was set to begin. The trial began today in Los Angeles Superior Court, where both sides gave opening statements.

The shooting of Oscar Grant, 22, in the early hours of New Years Day 2009 touched off rioting, and Mehserle became the first Kalifornia police officer ever charged with murder for an on-duty action.

The dead man's photograph of the man who killed him will now be a crucial piece of evidence in the trial.

The officer claims the shooting was an accident, that he meant to use his Taser on Grant to bring him under control during a struggle, but instead of pulling out his Taser he inadvertently pulled out his service pistol and shot Grant in the back. Mehserle has pled not guilty.

Prosecutors said the picture snapped by Grant shows Mehserle pointing a Taser at him. That is proof, prosecutors argued, that the officer then holstered the Taser before pulling out his gun and intentionally shooting Grant.

Alameda County Assistant District Attorney David Stein said in court Wednesday that Grant took the picture with this cell phone camera as he sat on an Oakland train platform following a fight between Grant and others on a train.

The picture was taken just moments before Grant was placed on his stomach by Mehserle and other officers attempting to arrest him, according to court documents.

Defense Attorney Dale Allen declined to comment on the revelation of the photo, telling ABC News over email they are under a court mandated gag order.

In court papers, the officer's lawyers claim Mehserle was struggling to restrain a belligerent Grant, and in the confusion pulled out the wrong weapon.

The shot took place in front of a crowd of people and was captured on several grainy video cameras that rocketed around the Internet and in news reports as the incident sparked riots in the Oakland area, reportedly fueled by the fact that Mehserle is white and Grant was black.

Witness statements said Mehserle looked "shocked" following the shooting.

"Mehserle put his hands up to the side of his head and looked dumbfounded at what has just happened," witness Karina Vargas said in court documents.

Another witness, Tommy Cross, said in court documents he did not see Grant resisting arrest, but did see Mehserle go for his firearm and hear Mehserle say "oh my God, oh my God," after the shooting.