Thomas died five days after a confrontation with police in Fullerton in July 2011. Former officer Manuel Ramos is charged with second-degree murder and involuntary manslaughter, and former corporal Jay Cicinelli is charged with involuntary manslaughter and use of excessive force.
The central piece of evidence is surveillance video of the beating that prosecutors have synced up with audio from the data recorders officers are required to carry with them. Prosecutors will argue the officers continued to pummel Thomas, even though he was bleeding profusely and unable to breathe or resist.
Officers responded to a report of a man breaking into vehicles at a transit center on the night of July 5, 2011. Police say Thomas was questioned at the scene and did not cooperate, leading to a violent confrontation with officers.
"They crossed the boundaries of legitimate police work and broke the law," said Orange County District Attorney Tony Rackauckas.
"Every single thing that he did was appropriate given the circumstances," said John Barnett, Ramos's defense attorney.
"My client took two jabs, basically, as described by the witnesses, two jabs to Kelly Thomas's forehead, really to get him distracted to stop from trying to grab the weapon," said Michael Schwartz, Cicinelli's attorney.
The beating sparked months of local protests that led to the resignation of Fullerton's police chief and a recall of some Fullerton City Council members.
Defense attorneys are expected to play up Thomas's history of drug abuse and violence, including a restraining order his mother sought against him, and an attack on his grandfather with a fire poker.
The judge has banned the family's supporters, a group that calls itself Kelly's Army, from wearing any pins or T-shirts that might inflame the jury.
The jury is made up of eight women and four men, with two male and two female alternates. The trial is expected to last about six weeks.