Allen put himself out there as a war hero and a former Marine drill instructor who knew how to get someone in shape. But the real professionals tell the I-Team that some of what Allen did at his gym was abusive and dangerous.
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Joey Lazor is heading off to college soon, but he was just starting his freshman year of high school when he went to San Rafael's House of Steel. His dad thought it would toughen him up. At 4 a.m. on a school day, Lazor made the mistake of looking gym owner Allen in the eye.
Lazor: "He said, 'Don't you dare f****** look me in the face or I'll smash your head into the wall.'"
Noyes: "And you're 14 years old?"
Lazor: "Yeah, it scared me. His bald head and very piercing watery blue eyes, I will never forget that image."
Even though Allen's actual military records show no active duty, just eight months in the Navy before a medical discharge, he demanded respect as a war hero, as seen in a video obtained by 20/20.
"My name is Gregory Bruce Allen, retired first lieutenant United States Marine Corps," he said in the video.
Allen wore a rack of phony medals, including a Purple Heart and Bronze Star with "V" for valor. He also lied about being a Recon Marine, a sniper with two tours in Vietnam.
Kris Hardy also worked out at House of Steel and he tells the I-Team, "He was talking about like they made a movie about him called 'Sniper' and stuff like this."
When asked if Allen meant the movie was about him, Hardy said, "Yeah, he said it was about him, which it really isn't."
Allen inspired such admiration in the young men he trained, several got big tattoos like one that reads: House of Steel -- Lt. Greg Allen.
"I would feel like I got duped if I got that tattoo and found out that this guy was a fraud," says 1st Sergeant Christian Bull, a real Marine drill instructor.
The I-Team asked Bull to analyze Allen's training techniques. The House of Steel workouts were very tough, especially for beginners like Lazor on that first day.
"And I remember trying to do, you had to do 100 pushups," Lazor said. "And if you didn't, you had to start over. And I could not get past the pushups."
Bull explains, "And you can hurt somebody like that. That's exactly how you're going to hurt them. How can I train somebody if I don't know their ability?"
Later that morning, Lazor said Allen held a hose over kids' faces, as they lay outside on the ground.
"You have to scream like non-stop, otherwise the water goes in your mouth and the only way to prevent yourself from choking is to, is just to keep screaming cause as soon as the water gets into your mouth you start choking and then it's just a disaster," he said.
Several sources told the I-Team that Allen often used that technique with the hose.
Bull: "That sounds like something you would do in some kind of POW camp."
Noyes: "Do you use water in any way besides to hydrate, besides a healthy way?"
Bull: "Absolutely not."
Kris Hardy told the I-Team that Allen and his squad leaders sometimes used drinking water as punishment and that in one session, while running laps in a parking lot, he downed 10 bottles.
"We'd have to drink the whole water bottle after one lap," he explained. "And then we would get another water bottle, have to drink that all down in another lap, and do that over and over again until we vomited back up."
Hardy says one of Allen's squad leaders made him do pushups in the mess.
"What kind of person would make somebody else do pushups in their own vomit?" Bull said. "To me, I don't see how the guy slept at night."
Bull says Allen seems to have made up training techniques of what he thought Marines might do because he never attended boot camp himself. Bull is upset about what Allen did in the name of the U.S. Marine Corps.
"It absolutely infuriates me because to me that's un-American," he said. "You don't do stuff like that. We are a nation that's built on honesty, integrity, commitment, dedication. These words, they have meaning. When we say things they mean something."
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Some young men now serving in the military credit Allen's training with giving them a head start before boot camp.
The I-Team spoke with Simon Duvall, a Reconaissance Marine, when he was in the Philippines for a joint exercise."
Getting in shape, a lot of running, a lot of calisthenics, lifting," he said. "It definitely prepared me, gave me an upper edge."
Duvall was stunned when the I-Team broke the news to him that Allen is a fake. Bull said that with all the effort Allen put in, he could have become a real Marine and wouldn't have had to lie about it.
After the first I-Team report in May, Allen shut down the House of Steel. Two weeks ago, we saw a crew patching the walls at the gym and preparing for new tenants.
Allen did not return the I-Team's calls or texts, but we did reach his lawyer who would not comment for this report. The Marine Corps has banned Allen from all functions and facilities and now forbids any Marines from working out with him. The I-Team will keep tabs on the FBI investigation and will let you know the outcome.