Sources: Scott Peterson Admitted Involvement in Laci's Disappearance to Amber Frey

MODESTO, Calif. – Accused killer Scott Peterson (search) admitted to girlfriend Amber Frey (search) that he was involved in the disappearance of his 8-months-pregnant wife before changing his story, Fox News has learned.

In a Fox News exclusive, sources close to the case said that during a taped phone call between Peterson and Frey, Frey asked her former lover whether he'd had anything to do with his wife Laci Peterson (search)'s disappearance.

Responded Peterson, according to the sources: "Yes … uh … uh … but no. But I know who did and I'll tell you later when I see you."

What Peterson didn't know was that Frey was already cooperating with authorities, who were taping all the phone conversations between the two.

It is the first time an account has surfaced suggesting Peterson initially admitted playing a role in his wife's abduction.

Sources told Fox News that Frey has apparently turned over to Modesto, Calif., police between 30 and 40 tapes of her conversations with Peterson -- who she says told her he was married but had lost his wife.

Peterson reportedly called Frey constantly after Laci vanished, even after Frey publicly announced their affair, according to sources. He phoned her about six times on Valentine's Day alone, begging to see her, the sources said.

Laci, 27, disappeared on Christmas Eve from the Petersons' Modesto home, a little over a month before she was scheduled to give birth to the couple's first child. Peterson said he'd gone fishing that day and returned to find her missing.

Her body and that of her unborn son Conner washed ashore along San Francisco Bay in April. The coroner released the victims' remains on Friday, but officials wouldn't say whether or not they were given to Laci's family, the Rochas.

Authorities have focused on Peterson since the start of the investigation, but didn't arrest him and charge him with the murders until April. He has pleaded not guilty to the charges and could face the death penalty if convicted.

Those familiar with the prosecution's case told Fox News that it will hinge on Peterson's love for Frey as the motive in the double murders. Frey will play a lead role and lawyers are currently preparing her to take the stand at the Sept. 9 preliminary hearing, sources close to the prosecution said.

Cameras won't be allowed at that hearing, in compliance with a judge's ruling last week.

Peterson's defense team has said it will argue that members of a local satanic cult kidnapped and killed Laci.

The trial is likely at least a year away.

Fox News' Rita Cosby and Catherine Donaldson-Evans contributed to this report.