Sex-Charged Cyberchat Spirals Out of Control

March 18, 2010— -- In the whirl of cyberspace, it was just one chance encounter...

"How r u doing"
"Hey tall, how r u"

A beautiful 18-year-old girl meets a handsome Iraq-bound Marine in a chat room. Who would have thought it would lead to a two-year affair, a love triangle and murder?

It would turn out that all was not quite as it seemed. That good-looking young Marine was actually balding 46-year old Thomas Montgomery, a married father of two. Montgomery said he was stuck in a dead-end job in Buffalo, N.Y., that was "slowly sucking the life out of me." And, he said, he and his wife were drifting apart.

Montgomery began spending a lot of time on the Internet. "I found it easier to talk to the people online than I could to my own wife," he told "20/20."

To this day, Montgomery can't quite explain what he was doing in a teen chat room on the popular game site "Pogo," in May 2005. But when a girl named "Talhotblond" started instant-messaging him, he decided to pretend he was 18 too.

"I kept thinking, well, we're never going to meet. ... I'll just play the game with her," he said.

Before long, the flirtation became a romance.

Talhotblond's instant messages revealed that her real name was Jessi, a softball-playing high school senior from West Virginia. She sent Montgomery photos that lived up to her screen name ... and then some.

"There were some ... very provocative poses," he said.

As they got to know each other, Montgomery asked for other photos: He wanted to see what she looked like at her graduation, or at that baseball game -- and Jessi would send them off to him. But in return, she wanted to see what he looked like too; so he sent her his photo from Marine boot camp.

The picture was 30 years out of date. Montgomery's screen name was Marinesniper, a nostalgic harkening back to the six years he spent in the military as a young man.

Today, he hints darkly of covert ops and dark deeds best unmentioned, but U.S. Marine records obtained by "20/20" show that although he qualified as a sharpshooter, he never trained as a sniper or saw action.

But for Jessi, he invented a younger, stronger, more virile version of himself, called "Tommy." "He was my height, 6 feet tall, had bright red hair," said Montgomery, "big shoulders, muscles and all that."