Richelle Nice, 36, of East Palo Alto told People magazine that she and Peterson have exchanged about two dozen letters since August.
Nice said she wrote the first letter as an exercise suggested by her therapist, but she didn't intend to mail it. She said she wanted to tell Peterson how the seven-month trial had turned her life upside down. The mother of four boys also wanted to know why he killed his wife, Laci Peterson.
Then she decided to mail it.
About a month later, she got a response.
"I started shaking and crying and hyperventilating," she told the magazine in an issue hitting newstands Friday. "I didn't know what to do. I wondered, 'Do I call the police? Do I even want to open it?"'
She said she was amazed at the tone of his letter. She said he's polite and charming, often showering her with compliments. He even commented on her choice of a breast cancer awareness stamp.
Peterson also seems more concerned about how the trial affected her than himself.
"He talked a lot about those autopsy photos and how hard that must have been for the jurors to see," Nice said.
He also repeatedly denied killing his wife, she said.
In December, Nice suffered a major breakdown and was hospitalized in a psychiatric ward, she said, explaining she's had a difficult life, including four children by two men. She now lives with her mother.
"I remember thinking, 'Dude, you had all the resources in the world, and you can't hold it together any better than that when the chips are down,"' she said. "'When life begins to get a little uncomfortable for you, what do you do? You commit murder?' What a sorry cop-out."