SANTA ANA (CNS) - Testimony is scheduled to begin today in the trial of two former Fullerton police officers, including one charged with murder, who  are accused of fatally beating a transient.

Former Officer Manuel Anthony Ramos is charged with second-degree murder and involuntary manslaughter and former police Cpl. Jay Cicinelli is charged with involuntary manslaughter and excessive force in the death of Kelly Thomas.  Ramos, the first OrangeCounty police officer charged with murder, could be sent to prison for 15 years if convicted.

The trial began in earnest with opening statements yesterday, when Orange County District Attorney Tony Rackauckas -- believed to be personally prosecuting a case for the first time since 1999 -- told jurors the defendants killed a ``harmless'' homeless man on July 25, 2011; defense attorneys responded that their clients did not cause Thomas's death and did not violate the law as they confronted an unruly, combative man.

During his 45-minute opening statement, Rackauckas told the jury that Ramos and the 37-year-old Thomas knew each other from previous encounters.

``This was a routine encounter for both Kelly Thomas and Manuel Ramos,’’ he said.

``But within the space of 30 minutes, Kelly was lying in a pool of blood, unconscious and dying.''

Rackauckas said that during the confrontation at the FullertonTransportationCenter, Cicinelli ``pummeled'' Thomas with the butt of a stun gun.

``Cpl. Cicinelli recklessly and repeatedly beat Kelly Thomas in the face and head with his Taser,'' Rackauckas said, adding the defendant also used his weight to pin Thomas down so he could not breathe.

 ``This is an important case,'' Rackauckas said. ``In the decision you make in this case you'll be speaking as the voice and conscience of the community.''

Police went to the Fullerton Transportation Center the night of the  beating in response to a 911 call from the nearby Slidebar nightclub that  someone -- investigators later determined it was not Thomas -- was trying to  break into cars outside the club.

Ramos and Officer Joe Wolfe -- who was indicted for involuntary manslaughter and excessive force and will be tried separately -- confronted Thomas at the transportation center while Wolfe went through a backpack Thomas had with him.

 At one point, Ramos ``stood over him in this menacing manner,” made a show of donning latex gloves,'' held up his fists to Thomas and threatened to ``f---  you up,'' Rackauckas said.

As baton blows rained on Thomas, the transient repeatedly apologized and  cried out for help from his father and God, Rackauckas said, citing a  videotape of the beating. Thomas also repeatedly said he could not breathe, he  added.

Thomas was pronounced dead five days later at UC Irvine Medical Center  in Orange. An autopsy showed he died from a lack of oxygen to the brain because  of the pressure on his chest and bleeding in his nose, Rackauckas said, adding  there was no evidence of alcohol or drugs in Thomas' system at the time.

Ramos' attorney, John Barnett, told jurors that Thomas struggled with  alcoholism and abused methamphetamine for years starting in the 10th grade and  that the drugs left him a ``time bomb'' who would periodically ``explode'' into  violence.

'`This is a case about a man who made bad choices in his life,'' said  Barnett, one of OrangeCounty's best known attorney. ``This is not a case about  a bully cop who targeted homeless men.''

Ron Thomas, a former sheriff deputy, says his late son was diagnosed  with schizophrenia. Defense attorneys say there's no proof of the diagnosis.

Cicinelli's attorney, Michael Schwartz, said the evidence will show  Thomas had an ``enlarged heart'' due to years of drug abuse and that he lapsed  into cardiac arrest when he over-exerted himself. The blows and chest  compression did not kill Thomas, he said.

``A tragedy? Yes,'' Schwartz said. ``A crime? No''