Mistress Will Be Key in Peterson Trial

May 9 -- 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.