A use-of-force expert for Alberto Alvarez, who is on trial for the murder of East Palo Alto Police Officer Richard May, testified Thursday there was no justifiable reason for the officer to stop Alvarez on Jan. 7, 2006.

May violated an East Palo Alto city ordinance against excessive use of force and a police department policy when he followed and attempted to detain Alvarez, 26, defense witness Winthrop Taylor said Thursday afternoon.

May was responding to a dispatch call about a fist fight in Villa Taqueria on Cooley Avenue when he saw Alvarez walking across the taqueria's parking lot.

A police Explorer on a ride-along with the officer testified Alvarez looked in the officer's direction as he crossed in front of the patrol car, then ran across University Avenue.

Alvarez admitted he shot the officer during nearly five hours of testimony Thursday, but said he did so in self defense after being shot in the leg by May and fearing the officer would kill him.

Taylor followed his testimony, saying May had no just cause to stop Alvarez under state and federal law.

Alvarez did not fit the description of the assailant in the taqueria fight, Taylor said.

A police dispatch call clearly noted the perpetrator was a Hispanic male, 22 years old, and wearing a black jacket and no shirt, said Taylor, a former Los Angeles Police Department police use-of-force policy trainer and retired Washington state police chief.

Alvarez was the same age as the perpetrator and wore a black jacket, but he wore two shirts, Taylor said after viewing surveillance videos taken the day of the incident.

The officer had a "clear identifier" that excluded Alvarez from the suspect profile, Taylor said.

"On a Saturday afternoon in January to see somebody wearing a black jacket and no shirt would stand out. It would be key," he said.

U.S. Supreme Court decisions have established an officer cannot detain a person simply for running away. Gut instincts are not reasonable causes, he said.

"There must be a reasonable cause for a lawful detention -- a set of articulable facts. ... Officers have to be able to articulate a set of facts that led that (officer) to believe a person was involved in a crime," he said. Those facts must be able to stand up under review, he added.

May also violated city ordinance and police policy when he struck Alvarez from behind twice with an ASP, an expandable metal baton, Taylor said.

The East Palo Alto Police Department has an escalation-and-continuum policy, which spells out the proscribed actions an officer should take in relation to a suspect's behavior, he said.

The use of a metal baton, considered a defensive weapon, is only to be used if a suspect is assaulting the officer, he said.

"When someone is running away, is that assaultive?" Eric Liberman, defense attorney, asked.

Taylor said it was not.

"Even assuming there was a lawful detention there was no justification for using the ASP and striking Mr. Alvarez in the back," he said.

Taylor's testimony starkly contrasted with the prosecution's expert, David Rose. Rose testified he considered running away resistance and therefore subject to the use of a baton.

"I understand what Mr. Rose said when he said running away is resisting -- that's active resistance. (But) it's different from being combative," he said.

An ASP can be displayed to get a resisting suspect to cooperate, he said. But "as a use of force, to use (it) to strike a person when fleeing, is not appropriate and is not in accordance with policy," he said.

Senior Deputy District Attorney Steve Wagstaffe pointed to discrepancies between Taylor's previous statements in a written February 2009 report and Thursday's testimony.

"In 2009, did you express the view that he had reasonable cause to detain Mr. Alvarez?" Wagstaffe said.

"I said he probably had reasonable cause," Taylor countered.

The earlier answer was based on information he had at the time, he said.

Police Explorer Marco Marquez said Alvarez had jaywalked in front of the patrol car. But videotapes Taylor later reviewed showed Alvarez had not committed any such violation.

He changed his opinion when nothing in the documents Taylor reviewed supported any probable cause for pursuing Alvarez, he said.

Wagstaffe also tried to discredit Taylor's training expertise in the use of batons. But Taylor said he did not train officers in the use of batons, only in the application of force.

Taylor admitted he was fired as police chief in 2004. Wagstaffe tried to link that firing to a no-faith vote police officers took.

But Taylor said he was terminated by the city mayor after launching a corruptions probe of city government and an insurance company found no link between the vote and his termination.

Cross examination of Taylor will continue Monday in San Mateo County Superior Court.

Related story: Alberto Alvarez testifies