REDWOOD CITY, Calif. -- Scott Peterson's former mistress is testifying that their first night together began with a blind date at a karaoke bar, and ended in sex.

She said it happened in a hotel room about a month before Laci Peterson's disappearance in December 2002.

Amber Frey testified that Scott Peterson gave her his cell phone number the morning after. She said he never told her he was married.

And according to the slim, blonde masseuse -- her relationship with the accused killer got serious fast.

Frey said she quickly trusted Peterson with her daughter after she started seeing him. The single mother says he even discussed wanting to have a future with them.

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Frey says Peterson went so far as to carry her daughter while hiking on their second date. She testified he also bought the child a children's book and picked her up from day care the next day, and, on their third date, Frey, her daughter and Peterson went shopping for a Christmas tree.

Frey testified Peterson eventually admitted he lied to her by saying he was never married, but she says he claimed that he had "lost" his wife recently, and "absolutely" wanted to be involved in their lives.

One thing in common for each of the first three dates -- Frey and Peterson had sex on each occasion, she testified.

Frey's testimony was marked by short, clipped answers to questions from prosecutors. She spoke so softly that the judge asked her to get closer to the microphone.

Frey's testimony is revealing the first intimate first details of what prosecutors say led Peterson to murder his pregnant wife.

Prosecutors have spent much of their case trying to establish that Peterson's affair with Frey, and hopes for financial gain, were his motives for killing Laci.

Prosecutors allege Peterson killed Laci Peterson, 27, and the couple's unborn child, in their Modesto, Calif., home around Dec. 24, 2002, and then drove to the San Francisco Bay and dumped her body from a small boat.

Although defense attorneys acknowledge Scott Peterson was an unfaithful husband, they say there is no evidence to support the prosecution's assertion that he killed Laci.

Laci Peterson and her unborn child's remains were found on the shore of the San Francisco Bay on April 13, 2003. Scott Peterson was arrested five days later, raising suspicion among some because of his changed appearance (bleached hair and goatee) and location (San Diego). Police also said Scott Peterson was carrying $10,000 in cash and his brother's driver's license.

Laci Peterson was last seen Christmas Eve 2002. Scott Peterson told investigators he returned from a fishing trip in the San Francisco Bay to find her gone and their dog wearing a muddy leash. She was eight months pregnant.