In a blow to prosecutors, a police investigator conceded Thursday that he deliberately failed to mention a witness who contradicts crucial elements of the murder case against Scott Peterson (search).

Detective Allen Brocchini (search) admitted that he excluded from his reports any reference to a woman who recalled seeing Laci Peterson at the warehouse where her husband stored his small boat.

Prosecutors have claimed that Peterson hid the recently purchased boat from his pregnant wife as part of his plan to kill her and dispose of the body in San Francisco Bay.

The woman's story provides an alternate explanation for why a strand of hair that DNA testing indicates might have come from Laci Peterson (search) turned up on a pair of pliers in the boat. That hair is one of the few pieces of physical evidence prosecutors have presented in Scott Peterson's trial.

Prosecutors allege the hair fell from Laci Peterson while her body was in the boat after Scott Peterson murdered her in their Modesto home on or around Christmas Eve morning, 2002. They charge he then weighted down the body and tossed it into the bay, only to have the remains of Laci and the couple's fetus wash ashore four months later.

The bodies were found two miles from where Peterson, 31, claims he was fishing alone the day his wife vanished. Defense lawyers assert someone else abducted Laci Peterson while she walked the dog, then acted on Scott Peterson's widely publicized alibi to frame him.

Defense attorney Mark Geragos (search) on Thursday played for jurors an audio tape on which Brocchini dictated notes from a police interview with the witness, who said Laci Peterson used the bathroom at the warehouse the day before she was reported missing.

Geragos implied Brocchini left out the woman's account because it did not fit with the police theory that Laci Peterson had never been near the boat.

"Can you tell me how that particular piece of information got excised out of your police report?" Geragos asked.

"I excised it," Brocchini replied.

"You did?" Geragos replied, seemingly shocked.

"I guess I did," a flustered Brocchini said.

Prosecutors will have a shot at damage control when they get the chance to question Brocchini again, likely next week.

However, experts say his admission could be tough to counter.

"The prosecution is self-destructing much the same way we saw in the O.J. case where you have police officers just doing stupid things," said Loyola University Law Professor Laurie Levenson. "One good hit like this can cause jurors to question the remainder of the prosecution's evidence."

Geragos spent the day attacking the police investigation as a sloppily executed effort to implicate Peterson, regardless of the evidence.

In a roaming cross examination of Brocchini, Geragos picked apart the prosecution's case — from faulty paperwork to outright omissions of critical details from witnesses.

One witness reported seeing a pregnant woman walking a dog near Peterson's home on Christmas Eve — the day Laci Peterson was reported missing — yet police did not exhaust the lead, Geragos charged during his second day questioning Brocchini.

Geragos asked Brocchini if he had ever shown the man a picture of Laci or the couple's golden retriever, the same kind of dog the witness said he saw in a park near the Petersons' home.

"He said he couldn't see her face," Brocchini said.

"What about the dog, he saw the dog didn't he?" Geragos persisted. "Did you ask him specifically what the golden retriever looked like?"

"No," Brocchini said quietly.

"Wouldn't it have been the prudent thing to do ... to have actually driven to the guy's house and shown him a picture of Laci and shown him a picture of the dog ... to eliminate that lead?" Geragos continued.

"No," Brocchini again replied. "He said he couldn't ID her."

It is a thread defense attorneys have tried to weave throughout the four-week-old trial — that police latched onto Peterson from the beginning while ignoring other leads.

Geragos' cross-examination of Brocchini also yielded an innocent explanation for one of the items police deemed suspicious when they entered the Petersons' home on Christmas Eve.

Police officers have testified that, among other things, a pile of dirty towels on the washing machine struck them as suspicious — that Peterson could have used them to clean up a murder scene. But on Thursday, Brocchini acknowledged the rags were inconsequential, that the Petersons' housekeeper told him she used them to clean the home's windows and fireplace.