Chief Deputy District Attorney Steve Wagstaffe, who is prosecuting the case against Alberto Alvarez, told jurors that the evidence in the trial will establish Alvarez's guilt and that he plans to present about 60 witnesses.
Alvarez could face the death penalty for the Jan. 7, 2006, shooting.
Alvarez had been involved in a fight at an East Palo Alto taqueria on University Avenue at about 4:30 p.m. and May responded to the disturbance.
During a foot chase, May was able to corner Alvarez a block and a half from the restaurant but did not know Alvarez was carrying a gun in his pocket, Wagstaffe said.
Wagstaffe said Alvarez fired six shots, striking May four times.
May managed to fire one shot at Alvarez, which hit him in the leg. May died at the scene.
Wagstaffe this morning played a recording of several 911 calls made related to the shooting.
One of the calls was from a resident in the 500 block of Weeks Street, who told police that an officer had been shot.
"It's a cop. Damn, it's a cop on the ground," the caller said. "He shot a ........ing cop right in front of the house. The police is on the ground right now."
Alvarez was arrested the day after the shooting and has been in custody without bail since then.
Wagstaffe also told jurors this morning that just after Alvarez was arrested, he wrote a letter to a friend asking for help. In the letter, Alvarez told his friend he wanted to use mental health problems as his defense, according to Wagstaffe.
Alvarez's defense attorney, Eric Liberman, has said his client has suffered from post-traumatic stress disorder as the result of a violent childhood in East Palo Alto.
Opening statements are expected to continue Monday afternoon.