In an exclusive clip, author Gregg Olsen, who has covered the case, details Letourneau’s clandestine communications to Fualaau.
“Mary stayed in touch with Vili in a lot of different ways,” Olsen tells Walters. “One way was to send out messages in the milk bottles that she was — [producing] milk in prison for her babies, so she put little notes in the caps and they went out.”
“She sent audiotapes out,” Olsen explains. “She had prisoners make phone calls for her, because she could not contact him [Fualaau] directly.”
• For more on Mary Kay Letourneau’s shocking romance with her student-turned-husband, Vili Fualaau, subscribe now to PEOPLE or pick up this week’s issue, on newsstands now.
Letourneau, 55, was sentenced in 1997 on a relatively light term, after pleading guilty to child rape, and she was sent back to prison in early 1998, after she was found again having sex with Fualaau. She gave birth to their second child while in custody.
“They courted for 10 years,” David Gehrke, Letourneau’s friend and former attorney, tells PEOPLE. “They had back-channel contact during the time they weren’t supposed to be speaking. Not as regularly as they would have had otherwise, but they were in contact.”
Letourneau has previously spoken about how she stayed in touch with Fualaau from behind bars. She told NBC’s Dateline in 2006 that the couple had “a pretty sophisticated number code” she used to write notes in the packaging of the baby bottles.
“‘104’ was ‘I love you,’ ” she said then. “And if I wasn’t feeling good, or if something was going wrong — there was another one. And then I would write names of songs down. Because the lyrics express how I feel.”
Of receiving Letourneau’s secret messages, Fuaulaau told Dateline, “It just made me feel really good, really happy about it.”
RELATED VIDEO: Mary Kay Letourneau and Vili Fualaau Through the Years
Fualaau filed for legal separation last month, after 12 years of marriage. Friends insist that, despite the illicit way the relationship began, their romance was sincere.
“They loved each other,” says Gehrke. “They were devoted to each other. They had children together, and raised those children.” The couple has two daughters, Audrey and Georgia, now 20 and 18.
Adds family friend Lydia Edgmon: “I can’t even imagine them getting divorced. With his personality and her personality, they are not going to do anything to harm those children.”
“They are always going to be friends,” Edgmon says. “There is nothing that is going to separate them as friends forever.”
The Mary Kay Letourneau episode of Barbara Walters Presents American Scandals is available online now at InvestigationDiscoveryGO.com. Users can view it by logging in with their cable provider information.
• With reporting by CHRISTINE PELISEK