The Baltimore native was behind “the most exciting moment” of the 2017 series, according to the game’s publisher, pulling off an “unbelievable pass as time expired to be crowned Buffalo Bills Champion” in the New York tournament in April of that year.
The NFL team later posted an image to its Twitter account congratulating the gamer, while an announcer admitted there was “a lot of pressure on these young men” during the live-streamed events. “You have to give it up to Bread,” he added, using Mr Katz’s nickname.
The Madden series of games allows players to create a team of NFL players to take on the league. In competitions, both online and in person, gamers compete for cash prizes.
Commentators described Mr Katz as “a man of business”, and “very, very focused”.
Little more than a year later, the 24-year-old is accused of shooting dead two fellow gamers and wounding nine other people before turning the gun on himself at a tournament in Jacksonville, Florida.
Sheriff Mike Williams said final confirmation of the suspect’s identity was still pending.
At 1.34pm on Sunday, police started receiving panicked calls saying people had been shot at the Chicago Pizza restaurant in the Jacksons Landing complex. Officers arrived two minutes later, Mr Williams said.
Mr Katz’s body was found near those of the two victims, the sheriff added, saying he had taken his own life.
Mr Williams declined to discuss a potential motive for the attack, and investigators are keeping an open mind.
Unconfirmed reports suggested, however, that Mr Katz had been unsuccessful during the tournament. Witnesses said he had been eliminated, while others claimed he was disqualified, although why this may have happened is still unclear.
Police have not identified the victims, but family members told local TV station WJAX that the two people killed were Eli Clayton and Taylor Robertson, both fellow contestants.
The Miami Herald also identified the victims, saying Mr Robertson, 27, was from Ballard, West Virginia while Mr Clayton, 22, lived in Woodland Hills, California.
FBI officers and police have raided Mr Katz’s family home, in the Inner Harbour area of Baltimore.
Bureau spokesman Dave Fitz confirmed agents had gone to Mr Katz’s father’s house. He declined to release specific details, saying the investigation was ongoing.
T J Smith, chief spokesman for the Baltimore Police Department, said the force was assisting other agencies “with some information that has led authorities to Baltimore”.
Police also seized Mr Katz’s vehicle parked near the tournament site.