By that point, Montgomery said the relationship was more than a flirtation. "There was virtual sex going on in there between her and Tommy," he said.
While Montgomery said the virtual sex made him "feel kind of dirty," he was in too deep to sever ties with her.
"If I was smart, I would've just ended it, but it was like a, a drug that I needed every day," he said.
Montgomery seemed to be losing touch with reality. He wrote a note to himself: "On January 2, 2006 Tom Montgomery (46 years old) ceases to exist and is replaced by a 18-year old battle-scarred marine ... He is moving to West Virginia to be with the love of his life."
Fate finally took a hand. In March 2006, Montgomery told "20/20" one of his daughters was using his computer when Jessi happened to instant message him. Montgomery's wife, alerted by her daughter, found a trove of love letters, photos and mementos from Jessi, including a pair of red panties. She sent Jessi a photo of her family and a letter.
"Let me introduce you to these people," she wrote. "The man in the center is Tom, my husband since 1989. ... He is 46 years old."
Montgomery said Jessi was horrified, and broke off the relationship immediately. "She sends me a text message and says, she hates me ... you should be put in jail for this," he told "20/20."
But Jessi also e-mails one of Montgomery's co-workers, a 22-year-old, good looking, part-time machinist named Brian Barrett, to see if it's really true.
Brian's screen name is "Beefcake" and as he consoled Jessi online, she seemed to find a better fit with him -- and perhaps a way to strike back at the combat Marine who wasn't.
Before long, Jessi was sending Brian her photos and the two had become a cyberitem. Marinesniper became consumed with jealousy -- and he wasn't about to take it lying down.
Marinesniper: Brian will pay in blood.