Impact of Mistress on Peterson Trial

M O D E S T O, Calif., Oct. 29 -- Scott Peterson's extramarital affair with Amber Frey could come back to haunt him in a way he never expected. As his preliminary hearing gets under way in Modesto, some legal experts say Frey could turn out to be the prosecution's secret weapon.

The blond massage therapist from Fresno helped police record phone calls between her and Peterson for more than three weeks.

ABCNEWS' legal contributor Kimberly Guilfoyle Newsom says it's very likely that the phone calls will reveal some very important conversations between Frey and Peterson.

"The prosecution wouldn't be seeking to introduce them unless there was something, some kind of admission, some incriminating evidence, statements that he made in those phone conversations, otherwise they're just irrelevant and we should just deal with the affair itself," Newsom said.

Weeks after his wife Laci disappeared, the 31-year-old salesman admitted to having an affair with Frey.

Frey, a 28-year-old single mother, came forward in a dramatic news conference to admit that she was having an affair with Scott Peterson at the time Laci disappeared.

"Scott told me he wasn't married. We did have a romantic relationship," Frey said in the January news conference.

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.

Partial phone records reveal a staggering volume of communication between Frey and Peterson even after her announcement that she was cooperating with the investigation.

The transcripts of those calls have been sealed, but ABCNEWS has learned that Frey asked Peterson point blank in one of those calls if he had anything to do with his wife's disappearance. Peterson said he did not, but had information about who did.