Mistress Will Be Key in Peterson Trial

Amber Frey did more than just publicly expose her affair with Scott Peterson - she also helped police build evidence against him in the death of his pregnant wife, Laci, and ABCNEWS has learned prosecutors will rely on her testimony to bolster their case.

In a hearing in the case today, a judge in Modesto, Calif., refused to unseal the search warrants and sealed additional documents at prosecutors' request, saying that the investigation is ongoing and making any evidence public could jeopardize Peterson's chances for a fair trial.

Since Frey, 28, came forward in a dramatic news conference in January to admit that she was having an affair with Scott Peterson at the time Laci disappeared, he was never seen in the same light by many.

But weeks before that, Frey was already helping police in their investigation of the case. Modesto, Calif., police said Frey contacted them on Dec. 30, six days after Laci disappeared. She met with detectives and gave them information about her relationship with Scott Peterson.

Though she knew nothing about Laci Peterson's disappearance, she tried to get Scott Peterson to talk about it in a series of wire-tapped phone conversations -- conversations that could be played for a jury.

"I believe that she is the weight that hangs around Scott Peterson's neck and could prove pivotal in obtaining a conviction against him," Kimberly Guilfoyle-Newsom, on leave from her job as an assistant district attorney in San Francisco, said today on ABCNEWS' Good Morning America .

Laci Peterson, eight months pregnant, disappeared on Dec. 24, when Scott Peterson said he had gone on a fishing trip to San Francisco Bay. The bodies of his wife and their unborn child were found washed up last month on a beach near the area where Peterson said he had been fishing.

Do Photos Show a Motive?

The photographs that surfaced earlier this week -- of Scott Peterson and Frey celebrating Christmas, which seem to mirror photographs of Peterson with Laci -- almost by themselves suggest a motive, Guilfoyle-Newsom said.

"It tends to show that in fact Laci Peterson was an encumbrance to Scott and the life that he wanted to lead with Amber or with other women," she said. "So compelling are the two photographs of the Christmas tree with Scott, with Laci and then the other one with Amber. He was living two separate lives."

However, she said, the defense could use the photographs to make a case that Peterson was not planning to commit murder, because he was not trying to hide the affair.

The defense, led by high-profile attorney Mark Geragos, will likely be very careful about how he cross-examines Frey, Guilfoyle-Newsom said, because the young woman could be a very sympathetic witness, especially since she cooperated with police once she learned that the man she was seeing had lied to her.

"If Mark is too tough on her, he'll look bad," Guilfoyle-Newsom told Good Morning America . "I think the best he can do is try and argue in motions to keep her off the stand or keep the evidence that she's going to present to be as limited as possible. He better argue to get those pictures out or big trouble for Scott."

A Family Afraid

Frey has apologized to Laci Peterson's family, but has insisted all along that he lied to her about whether he was married.

"I met Scott Peterson Nov. 20, 2002," she said when she publicly announced the affair. "I was introduced to him. I was told he was unmarried. Scott told me he was not married. We did have a romantic relationship."

Peterson himself admitted to the affair and lying to Amber Frey. He first told her he was single, then claimed he was a widower.

These revelations immediately cost him the support of Laci Peterson's family. A jury may also hear reported romantic phone calls Peterson had with Frey that occurred only days after his wife and unborn child vanished.

At first, Frey carried on as if nothing had changed and so did Scott, but a few days later Frey asked him point blank what happened to his wife. According to a report in the National Enquirer , Peterson paused for a moment, then denied he had anything to do with it.

Though Frey was cooperating with police, she and her whole family were terrified at the time, her sister, Ava, said.

"You can't help think that if Scott is guilty of these crimes, God forbid, could that have happened to my sister next?" Ava Frey said.

ABCNEWS' Taina Hernandez contributed to this report.