A former San Mateo County forensics expert on Monday gave scathing testimony criticizing the evidence gathering in the shooting death of East Palo Alto Police Officer Richard May.

Celia Hartnett, a former supervising criminalist in the 1990s and current CEO of Forensic Analytical Sciences, a private lab in Hayward, said some evidence gathering destroyed the opportunity to know if May was pointing his gun when he was shot in the face and killed by Alberto Alvarez.

The missing spatters on the gun might have determined that key fact, she indicated.

Alvarez faces the death penalty or possibly life in prison for the officer's shooting death. He has admitted shooting May but said it was in self-defense in testimony last week.

Hartnett's lab is one of the few full-service private criminal labs in the country and handles forensic materials from throughout the country. She said evidence was never gathered from outer areas of the two cars in the Weeks Street driveway where May was killed. Only the driver's side and front ends of the vehicles were sampled, she said.

"It's a critical issue. It's a bad idea to go on the assumption that you know already what happened. You're restricting your options to locate evidence," she said.

A forensics expert for the prosecution previously testified he had not taken measurements for the slope in the driveway.

"When doing a bullet trajectory, it's absolutely critical. A one-foot slope drop off will offset the relative positions of the shooter to the target," she said.

Earlier Monday, East Palo Alto Officer David Carson testified he was the second man on the scene after the call came in that an officer was down.

He testified he saw May lying on the ground with his gun and baton nearby, but could not recall their positions. He did recall seeing splatters of blood on the gun, he said.

Carson took some photos and was laying out numbered markers at different points of evidence, but another officer had already taken away the gun and baton before he could mark them, he testified.

Hartnett said that evidence was not properly gathered and there was very little blood left on the gun by the time she examined it. That evidence was important to showing if the officer was holding the gun up at the time he was struck by bullet to the face and the directionality of the shot, she said.

A coroner's report found many blood splatters on the officer's right hand and some on the left hand, with a few on the right palm, she said. The splatters were consistent with high-velocity blood spatter, which makes a fine spray and is typical of blood ejected from the impact of a bullet, she said.

Hartnett said the coroner's report did not specify where on the officer's palm the blood residue was found, and that could be critical in determining if he was holding his gun.

"If you can see where it shadowed you can tell if he has a gun in his hand at the time he was shot," she said.

Senior Deputy District Attorney Steve Wagstaffe asked Hartnett if blood spatters could also appear in the same way if the officer was shielding his face with his hands held out, palms toward the shooter. Theoretically, that was possible, she said.

Hartnett also criticized the failure of county criminalists to examine a bullet hole in the garage door for evidence to see if it was related to the crime scene.

"In my opinion, what was done at this crime scene was unacceptable," she said.

Also on Monday, Wagstaffe chipped away at the testimony of defense use-of-force expert Winthrop Taylor. Taylor said in testimony last Thursday May violated his department's policy and a city ordinance against excessive use of force when he hit Alvarez twice with his baton as Alvarez allegedly fled.

But Wagstaffe attacked Taylor's credibility, first calling into question a photograph of bruises on Alvarez's upper arm that Taylor had mistaken for Alvarez's buttocks. The photo showed a close-up of what appeared to be two hemispheres with a crack between -- the upper arm and shoulder blade of a then 250-pound Alvarez.

Taylor said the photograph speaks for itself. He was later shown a broader view of the body that clearly shows an arm, he said.

Wagstaffe also questioned why Taylor selectively chose to include testimony from Police Explorer Marco Marquez in his report but not that of Virginia Rodriguez, a resident who said she saw the two men playing or fighting in close contact on the lawn of the home where May was killed.

Marquez said he saw May strike at Alvarez with his baton, but Rodriguez did not see any weapon in the officer's hand, he said.

Taylor testified previously the officer had no justification for striking Alvarez because he was fleeing. But Wagstaff asked if Rodriguez's account was correct, would the officer be justified in using his baton if Alvarez was combative?

Taylor said he would be justified.

Marquez gave three different accounts of events, but Rodriguez gave only one consistently, Wagstaffe said.

"But you chose to pick Mr. Marquez's testimony over Ms. Rodriguez's," Wagstaffe said.

Wagstaffe also questioned Taylor's reading of East Palo Alto's ordinance and police department policy as having to be followed to the letter of the law.

Wagstaffe cited a U.S. Supreme Court case and a Ninth District Court of Appeals decision that allows for "exigent circumstances." In those decisions, the court recognized that officers make split-second decisions that sometimes go beyond following policy, he said.

Witnesses will again take the stand Tuesday in what is expected to be the last day of defense testimony.