Aug. 27, 2011  -- The Internet is known as a breeding ground for illicit affairs between people often hiding behind fake names and handles. But most such virtual relationships aren't dangerous as this -- when "Talhotblond" and "MarineSniper" struck up a relationship online, it ended in murder.

MarineSniper was 46-year old Thomas Montgomery, a married father of two. In May, 2005, posing as a young, handsome Iraq-bound Marine, he entered a teen chat room the popular game site "Pogo."

When 18-year-old 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.

In return, Jessi 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The picture was 30 years out of date.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, Marinesniper, was a nostalgic harkening back to the six years he spent in the military as a young man.

He has hinted 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."

Instant messages recovered from his computer show that the online relationship began to consume Montgomery. He told "20/20" that this relationship "became more real to me than real life."

The feeling seemed to be mutual. Jessi and "Tommy" exchanged gifts, phone calls and love letters.

"I love you always and forever, Tommy," wrote Jessi.

"I have never felt this way," Montgomery responded.

The relationship had become more than a flirtation, Montgomery said.

"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."

Online Fantasy World Crashes

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-mailed one of Montgomery's co-workers, a 22-year-old, good looking, part-time machinist and college student named Brian Barrett, to see if it was 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.

"Brian will pay in blood," Montgomery instant messaged Jessi at one point.

His messages became increasingly violent, as he was forced to watch their romance blossom in the same chat rooms he used to frequent with Jessi.

"He was enraged," said former prosecutor Ken Case. "I mean, it oftentimes shocked me when I saw the names that he would call her."

Jessi and Berrett took to the Internet to make sure everyone knew Montgomery (who recently had a birthday) was a liar.

"They were then going into these chat rooms, and letting people know that he was actually 47 years old," said Case. "They almost made him out to be a pedophile."

But the IMs that came from Talhotblond showed her to be torn --- mad one instant, desperate to return to a love with a man who she knew didn't exist ... teasing him.

She continued her talking to Montgomery online:

Talhotblond: i ache to be with tommy

Talhotblond: do you miss it tom

Marinesniper: more then u will ever know

Marinesniper: my heart aches to hear you call me your tommie

Marinesniper: i wish i could be that 19 yr old marine for u

Talhotblond: i know tom

Talhotblond Rekindles Cyberaffair

Jessi took up with Montgomery again.

"In his mind, this was the jackpot," said Barbara Schroeder, who documented the bizarre relationship in a documentary called "Talhotblond." "He was being accepted for being 47, and he still had this hot young girl who wanted him."

Montgomery knew he was in way over his head, but he couldn't bring himself to end things with her.

"It was like a drug. I was addicted to it," he said. "I couldn't just, just end it."

At one point, when his wife actually told him to get off the computer and talk to her, Montgomery couldn't. "I just told her I'll get off when I'm done," he recalled.

Montgomery says nothing sexual happened between them after Talhotblond found out how old he was, but their IMs tell a different story:

Marinesniper: wish you were nude

Talhotblond: what would ya do?

Marinesniper: stare

Talhotblond: that all

Marinesniper: nope

Marinesniper: u might get the magic

Talhotblond: mmmmm

Talhotblond: make love to me tommy

But it didn't last. Jessi told Montgomery they were through, and seemed to take up with Barrett again. Montgomery began to go into a downward spiral.

"The obsession turns into jealousy, and then the jealousy turns into betrayal and revenge," said District Attorney Frank Sedita. "You really start to get a, a sense of this person going into an abyss. And it's, it's kind of frightening."

And then ... the tipping point. Barrett said he was going to meet Talhotblond -- in person.

"He actually drove down to, I think North Carolina," said Case. "And on his way back, he was saying, 'I'm going right past your house. I'd love to get together.'"

Jessi texted him at the last minute not to visit, but Montgomery, who had learned of the plan to meet, was incensed.

On Sept. 15, 2006, as Barrett left work, three shots rang out. Brian Barrett was found dead in the parking lot where he worked, shot three times by a military rifle.

Police quickly learned of the Internet love triangle from co-workers. And when they couldn't find Thomas Montgomery, they feared they knew just where he was headed.

"At three in the morning," Capt. Ron Kenyon told "20/20," "our first concern was talking to Jessi and making sure she was still alive.

But when police arrived at her home, they were in for another surprise: A woman named Mary Shieler opened the door.

Talhotblond's Shocking Secret

As police questioned her, she revealed a shocking truth: She was the one who had been sending messages to Montgomery and Barrett under the handle Talhotblond. The pictures she sent Montgomery were actually those of her daughter, the real Jessi, who had no knowledge of her mother's cyberlife.

Montgomery was charged and later plead guilty to the murder of Brian Barrett. In exchange for his plea, he received a 20-year sentence. Prosecutors said their discovery of Montgomery's DNA on a peach pit found at the crime scene and a photo of Montgomery family's gun cabinet -- which showed the type of old military rifle that police believe was used to shoot Barrett -- were key to their case against Montgomery.

Prosecutors in Buffalo, meanwhile, looked for a way to charge Mary Shieler for something -- anything -- in the case but concluded she may have tramped all over the moral and ethical line, but never crossed the legal one.

"Shame on her -- she not only didn't do anything about it, I think she provoked it," said Ken Case. "Unfortunately in New York State, you have to do a little something more to be criminally liable."

Mary Shieler didn't come away unscathed. Her husband divorced her over her deception and her daughter Jessi cut ties too, moving in with relatives in Virginia, where she was attending college.

Mary Shieler also pursued an education -- she took classes at a community college in West Virginia. She has spoken publicly about the deadly love triangle just once, to the BBC.

"It was stupid. It should have never happened. I just never thought it would go anywhere," she said. "That it would end, fall off and that would be the end of it."