The attorney prosecuting the 26-year-old man accused of killing East Palo Alto police Officer Richard May won a significant victory Tuesday when a judge granted his request to present evidence that suggests a motive for the crime.

San Mateo County Superior Court Judge Craig Parsons listened to about an hour of testimony and arguments Tuesday morning before ruling he would allow defendant Alberto Alvarez's parole officers to tell jurors about a form the accused cop killer signed upon his release from prison.

The document informed parolees that under a federal "Trigger Lock" law they could serve 15 years to life in prison if caught with a firearm.

San Mateo County Chief Deputy District Attorney Steve Wagstaffe has argued Alvarez shot and killed May, a former Lompoc police officer, after fleeing the scene of a fight in East Palo Alto on Jan. 7, 2006 to avoid going back to prison for gun possession - a violation of his parole.

Defense attorney Eric Liberman told the court Tuesday that the evidence shouldn't come in because it's "common knowledge" that federal authorities almost never prosecute anyone under the "Trigger Lock" law, which wouldn't even have applied to Alvarez given his relatively minor previous convictions on marijuana and concealed weapon charges.

During the hearing attorney James Thompson, who has worked as a public and private defender on numerous death penalty cases, confirmed the law wouldn't have applied to Alvarez because it requires three separate serious drug or violent felony convictions - and the defendant didn't even have one, he said.

"A person is presumed to know the law," Liberman heatedly told Judge Parsons after Thompson completed his testimony. "The presumption is he would know this does not apply to him ... There is no evidence before the court that Mr. Alvarez believed he was facing a 15-year sentence or a life sentence."

In a brief Wagstaffe submitted to the court, he called that argument specious, saying "it is not relevant whether the defendant actually faced a potential life sentence under federal law."

"The fact is that he was told twice that he did face a life sentence if he possessed a firearm, and this evidence of his state of mind provides strong evidence of his motive and his intent to kill," Wagstaffe wrote.

Parsons agreed and said he had already allowed evidence about Alvarez's parole status.

The evidence "is highly probative on the issue of motive and the defendant's state of mind when Officer May was killed," Parsons said.

"This motion, in my view, is simply a ratcheting up of the original motive evidence that I've already ruled on. What it does, if admitted, is tell the jury how high the stakes were with respect to being apprehended with a firearm with ammunition."

Lead defense attorney Charles Robinson was clearly shocked and asked Parsons to delay the trial so he could find former inmates and parolees who might have told Alvarez the federal law is virtually never applied.

"I'm absolutely stunned this evidence is being let in the courtroom," Robinson said, adding, "We didn't think we would have to disprove common knowledge."

Parsons denied Robinson's request, and the trial resumed with the testimony of criminalist Carlos Jiron, of the San Mateo County Sheriff's office crime lab, and Alvarez's two parole officers. Jiron has spent numerous hours on the stand since Monday afternoon, giving detailed descriptions of how he collected and analyzed physical evidence and displaying everything from spent shell casings to a pair of Alvarez's bloodstained underpants.

The trial resumes today with cross-examination of Jiron. Alvarez remains in custody on no-bail status.

E-mail Jessica Bernstein-Wax at jbernstein@dailynewsgroup.com.