Investigators believe Katz was in Jacksonville for the Madden 19 tournament, a video game competition and that he might have stayed in town Saturday night.
"[He was here] all weekend", said the gamer.
Katz, a Baltimore native., went by the alias "Bread".
Police confirmed that Katz was in the restaurant for the tournament, but didn't comment on his motive or the events leading up to the shooting.
Since the tournament, which brings together gamers from all across the country, was being streamed live on gaming network Twitch TV, most of the shooting and its chaotic aftermath was caught on video and audio clips, which have since gone viral on the internet.
"Honestly, I felt like I had the ball most of the game", Katz said. He said the FBI was searching the man's home as part of the investigation.
Katz had past success at video game tournaments, winning the Madden Bills Championship in 2017, along with $3,500 and the chance to compete in a later contest in Los Angeles.
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"They were great competitors and well-loved members of the Madden community", Dot City Gaming tweeted, adding that he was a member of the team.
The gunman, Katz, was a professional gamer himself. Police are still searching the area to rule out any other suspects.
"He is not here to make friends".
The gunman who opened fire at a gaming tournament in Florida killing two people before turning the gun on himself, was a well-known champion in gaming circles. He scores a touchdown, and then the feed switches to a kickoff return - and 11 gunshots can be heard. Robertson from Ballard, West Virginia was the victor of the Madden NFL 17 Classic and he also competed in the Madden NFL 18 Club Championship on the Titan's behalf. He suffered a minor injury.
EA Sports described Robertson, who won 72 percent his matches, as the "toughest opponents in competitive Madden". "We are committed to supporting Taylor and Elijah's families through this hard time, and we send our deepest sympathies to their loved ones, to those injured yesterday, and everyone affected". "We are working with authorities to gather facts at this stage".