"I thought he was going to come and shoot me next," Marco Marquez, a key witness who rode along with Officer Richard May the day he was killed, testified Tuesday afternoon in the murder trial of Alberto Alvarez.

Marquez, 19, an Explorer Scout with East Palo Alto Police Department, testified for more than three hours in the San Mateo County Superior Court trial in Redwood City, sometimes choking up and having to dab his eyes as he recalled the Jan. 7, 2006, killing.

Alvarez, 26, of East Palo Alto is charged with first-degree murder under special circumstances (killing an on-duty police officer). He could face the death penalty if convicted on that charge.

Marquez wore a blue police Explorer uniform complete with a badge and East Palo Alto Police Department arm patches during his testimony -- the same uniform he wore on the ride-along . The Explorers program allows youths aged 14 to 21 to experience police work.

Under direct testimony with questions from prosecutor Steve Wagstaffe, Marquez said he arrived early at the East Palo Alto Police Department and asked May if he could ride along.

"Officer May always liked to have a ride-along with him, so we would always ask him," Marquez said. He recalled listening to a NASCAR auto race with May that day, which was uneventful until May received a call at about 4:30 p.m. regarding a fight at the Villa Taqueria restaurant at 2380 Cooley Ave., near University Avenue.

As May drove toward the restaurant, they saw a man in a black jacket walking on the sidewalk. May slowed down and "tried to make contact" but the person took off running across University into Weeks Street, Marquez said.

Marquez identified that man as Alvarez.

He said they then saw Alvarez trying to unlock a gate at an apartment complex but he ran across the street when he saw May's car stop behind him.

Marquez said he remained in the car when May got out to pursue Alvarez to a driveway on Weeks Street. May caught up with Alvarez and struck him twice with his retractable baton but Alvarez kept running, Marquez said.

Marquez said he opened the car door and leaned out to see what was happening. There was a vehicle parked on the sidewalk near the driveway that partially obscured his view, he said.

May placed his right hand on his gun but did not draw it, Marquez said. Marquez then saw Alvarez reach into his jacket and pull out a silver object he thought was a gun.

Marquez said he ducked when he saw the object. He heard the first gunshot as he was getting down behind the car door, he said.

"I stayed down. More shots went off," Marquez said. He said he did not see either May or Alvarez shoot.

Then there was silence. Marquez said he raised up and saw Alvarez limping towards the squad car, a gun in his right hand.

"I thought he was going to come and shoot me next," Marquez said, tearing as he recollected the incident. He said they made eye contact but Alvarez walked past toward the apartment complex. He opened the gate and walked through.

Marquez said he then radioed dispatch to report that an officer was down before going over to May.

"I just ran over to where Rich was, and I saw him lying there on his back," Marquez said, his voice cracking. He could not see May's wounds because there was too much blood, he said.

But during cross-examination defense attorney Eric Liberman challenged Marquez' testimony based on inconsistencies with earlier testimony to a grand jury in August 2006 and statements to investigators.

Liberman said Marquez told San Mateo County sheriffs in earlier interviews that he saw May take out his gun and point it at Alvarez' chest.

Marquez told deputies he saw Alvarez fire the first shot at May before he ducked, Liberman said. Marquez also told East Palo Alto police officers on Jan. 7 that Alvarez fired three times and May fired once, Liberman said.

Marquez said he was mistaken then and maintained he did not see May draw his gun or witness any shooting.

"You're in a stressful situation. There's stuff you see you forget," Marquez said. "There's so much going on, you really can't tell."

Marquez' testimony is critical because prosecutors are using it to claim Alvarez executed May, Liberman said.

Defense has maintained and will establish that May shot first and Alvarez responded, he said.

"I very clearly understand this was a terrible and traumatic event for him, but I do not understand why he would repeatedly make things up to three separate police officials the night this happened," Liberman said after the trial was adjourned for the day. "That concerns me greatly because our client's life is at stake."

Four other witnesses testified Tuesday morning: Rafik Shuman, owner of the nearby Pal Market on Cooley Avenue; two San Mateo County 911 dispatchers, John Carillo and Linda Pegliese; and Maria Loya, another customer who witnessed the fight at the taqueria.

The trial is expected to resume Monday. Alvarez remains in custody without bail.

Related story:

■ Defense disputes 'execution' murder of May