A gunshot-trajectory expert attempted to demolish assertions on Tuesday morning that Alberto Alvarez, 26, murdered East Palo Alto Police officer Richard May in January 2006 in an execution-style shooting.

Prosecutors in the trial have said Alvarez, who has admitted shooting May, fired the fatal shot into the officer's face as the defendant walked by the downed officer.

Whether that assertion is true could mean the difference between a life sentence for Alvarez or the death penalty.

Alvarez shot May four times, the first three shots disabling the officer and knocking him to the ground, prosecutors have said. Alvarez then walked by May, only to return and fire once into May's face, killing him, according to prosecutors.

But defense expert John Jacobson of Forensics Analytical Sciences Inc., a private Hayward lab, said Tuesday the first shot Alvarez fired was to May's head.

When Alvarez first fired his 9 mm handgun, trajectory data indicate the bullet struck May in the face and May's head was bent down or the officer could have been crouching. The bullet traveled at a 45-degree angle downward, he said.

"If he was standing straight it would have been at a 20-degree angle," Jacobson said, indicating the shooter would have had to be significantly elevated or standing directly over the officer lying on the ground for the bullet to have entered at 45 degrees.

Jacobson said the officer likely shot Alvarez first -- in the right leg -- while Alvarez was moving sideways between two vehicles outside a residence on Weeks Street in East Palo Alto.

He took into account the driveway's slope, the angle and direction of a bullet wound in Alvarez's right upper thigh and the location of a bullet hole in the garage door, he said.

He concluded the officer was standing or slightly crouched near the sidewalk and fired between the vehicles, striking Alvarez.

The bullet passed through Alvarez's thigh, exiting and smashing through the garage door behind him, Jacobson said.

Surveillance video showed Alvarez is left-handed, so having his right side facing the officer could make it implausible he fired the first shot, defense attorneys have said.

The bullet cartridge from May's gun was also found under the driver's side of a white truck, which is consistent with the truck's position and with a gun fired by the officer's right hand. Bullet casings are ejected from the right, Jacobson said.

On cross examination by San Mateo County Senior Deputy District Attorney Steve Wagstaffe, Jacobson said he thought Alvarez had been struck first because the shot to the officer's face would have incapacitated him and he could not have fired his gun again.

"Could (May's gunfire)have been in the middle of an exchange of shots?" Wagstaffe asked.

"I'm of the opinion that the first shot was in the face of Officer May and incapacitated him," Jacobson reaffirmed.

"Could it be simultaneous? ... Would that be possible?" Wagstaffe said.

"Yes, it would," Jacobson said.

A second bullet from Alvarez's gun struck the officer in the left shoulder. But this bullet exited at a 45-degree angle upward, Jacobson said. The angle was consistent with May starting to fall backward after being struck the first time, he said.

May's radio, which was located on his left shoulder, had damage consistent with a bullet strike, Jacobson said.

The second bullet was not fired when the officer was lying on the ground, Jacobson said, because there would have been a ricochet or gouge mark in the pavement as the bullet exited. No such mark was found.

But Wagstaffe questioned the positioning on graphics presented by Jacobson during the trial.

It did not match Alvarez's own testimony or that of another witness, Virginia Rodriguez, both of whom placed May behind a white truck in the driveway, Wagstaffe said.

Jacobson said the trajectory showed May had moved to a position just behind an Oldsmobile to the north of the truck. he could have fallen in that direction after being shot.

May was found lying behind an Oldsmobile just to the north of the truck

If the first bullet incapacitated the officer, how did he move from behind the white truck, where two witnesses said he was located, to behind the Oldsmobile? Wagstaffe asked, indicating the first shot must have been other than fatal.

"The defendant testified Officer May had not moved. It's inconsistent with the whole reconstruction. It calls into question the whole reconstruction, doesn't it?" Wagstaffe said.

Jacobson said he stood by the physical evidence, although he could not say precisely where either man was located or how they were holding their guns.

Was it possible two bullets that hit the officer's bulletproof vest might have caused him to move or threw him backwards, instead of the officer falling after a shot to the face? Wagstaffe said.

Jacobson said he didn't think so. The angles of those bullets were not consistent with the officer standing up, he said.

But wouldn't the force of the impact push the officer back? Wagstaffe said.

"No," Jacobson said.

"It has no effect? ... If I punch someone in the chest several times, it wouldn't move them?" he asked.

Jacobson said the CSI effect and graphics in movies aren't reality.

"It doesn't make one fly back when being struck with a bullet. ... I base that by seeing people being struck several times in the chest while wearing a bullet-proof vest. You do not fly back; you cringe. ... It's very painful," he said.

Jacobson said the trajectories did not try to pinpoint a precise location for either man. Alvarez could have been moving anywhere along the area between the two vehicles, he said.

"You would not dispute that he's running between the cars and could turn to look at the officer. It's possible he's there running, turns, takes out his gun out and the officer shoots him?" Wagstaff asked.

Jacobson conceded it was possible.

The two other bullets fired by Alvarez indicate the defendant was moving around the officer's feet at the time of the shots, he said.

The bullets left waves that disturbed the weave of the Kevlar lining in May's bulletproof vest, indicating the shooter's position, Jacobson said.

It is "similar to shooting into clay water. You can follow the shock wave of the bullet back to the muzzle of the gun and you can clearly see the directionality change," between the two shots into the vest, he said.

East Palo Alto Police Chief Ronald Davis testified about his experience as a racial profiling and use-of-force expert on Tuesday afternoon. His testimony was in rebuttal to that of defense expert Winthrop Taylor.

Davis is under contract with the U.S. Department of Justice Civil Rights Division and reviews hundreds of use-of-force investigations nationwide, he said.

As chief, Davis said he did not have an expectation that his officers should follow tiered escalation-of-force policy to the letter. The policy leaves room for interpretation, depending on the circumstances, he said.

"It's clear you can never apply one permanent or fixed standard," he said.

The goal in chosing any level of force "is not to match resistance but to overcome it," he said.

Contary to Taylor's testimony, Davis said he does not encourage grappling or ground fighting with a suspect. There are too many opportunities for serious injury, he said.

Taylor said in his opinion May should have gone to the taqueria where Alvarez and another man were said to be fighting, rather than stopping Alvarez on the street.

But Davis said the idea of passing a suspect by to go to a scene was "silly."

"To go to a store to see if it was robbed after a suspect is leaving in front of you" would not be something he would expect or want from his officers, he said.

Taylor said May also erred by not assuming Alvarez or any Hispanic suspect would be running away out of fear because they could be undocumented. May also did not take into consideration that Alvarez might not understand English, he said.

But Davis called those assumptions racial profiling.

That's like saying "if I see an African American, he must be on parole," he said.

The defense rested its case. Both sides will present closing arguments on Monday and jurors will receive instructions from the judge. They could begin deliberations by Monday afternoon.