David Katz, the suspect in the Jacksonville mass shooting, was reportedly introduced at the video game tournament as a "man of business" who "keeps to himself".
The former eSports champion had travelled from Baltimore to Florida intent on taking part in the regional qualifier of the Madden 19 American football tournament, hoping to make it through to the finals in Las Vegas.
Instead the 24-year-old was eliminated, according to witnesses, who say he later returned with a handgun and opened fire.
The outbreak of shooting was captured on a live stream of the event, with several gunshots heard in the disturbing footage before the feed was disconnected.
Just before the shots were heard, a red laser dot appeared on the chest of one of the players, who was wearing white headphones and a red sweatshirt.
Three people, including Katz, were left dead while at least 11 others were shot and wounded in the shooting, which took place at the Chicago Pizza in the Jacksonville Landing shopping plaza.
Local media identified the dead victims as Eli Clayton, 22, of Woodland Hills, California, and Taylor Robertson, 27, of Ballard, West Virginia. Both had been competitors in the tournament, local media reported, citing family of the victims.
Robertson, a husband and father, won the tournament last year and Katz won it the year before, the Miami Herald reported, citing family and friends posting on the Internet.
Sheriff Mike Williams said police believed the suspect was Katz. "There were three deceased individuals at the scene, one of those being the suspect, who took his own life," Sheriff Williams told reporters.
Sheriff Williams said Katz used "at least one handgun" to carry out the shooting.
Listed by games maker EA Sports as a 2017 championship winner, police believe Katz stayed at a North Florida hotel overnight.
“You are not going to see much emotion,” an announcer said earlier as an introduction for Katz, the Miami Herald reported. “David Katz keeps to himself. He’s a man of business. He’s not here to make friends.”
Steven "Steveyj" Javaruski, a professional Madden player for Noble eSports, said the attack happened after the shooter had lost during the competition, the LA Times reported.
Mr Javaruski said he witnessed his fellow competitor target a "few people” killing at least two or three and shooting multiple others.
Katz would sometimes play under the name “Bread” or “RavensChamp” while competing in the national circuit of professional gamers who play “Madden NFL 19,” the popular football game.
In February last year, he won the Buffalo Bills tournament of the Madden NFL football game.
"Madden" competitor Derek Jones, 30, of Santa Fe, New Mexico, said he knew Katz by the gamer tags he used online - often "Bread" or "Sliced Bread" - and had played against him online but had never spoken to him personally.
"You know, I'm glad I lost today," Jones said. "Because if I'd won, I would have been in that game bar right then playing a game and not paying attention. And he could have come and I'd probably be dead right now."
On Sunday evening, the FBI said, its agents searched a family home of the man authorities believed was behind the attack.
Heavily armed agents, some in bulletproof vests and brandishing long guns, could be seen entering an upmarket townhome complex near Baltimore's Inner Harbor.
Cameron Stearns, who lives next door to the house, said he was surprised Katz had been identified as the gunman. “It’s shocking and scary to think only a wall separates you from somebody capable of shooting up a video game tournament,” he told the New York Times.
Marquis Williams was his girlfriend Taylor Poindexter when the bullets started to fly.
As he fled, Williams, 28, said, he could see the back of the gunman's head as the attacker appeared to be walking backwards as he fired.
"We didn't see like a face," Ms Poindexter, 26, told reporters a few hours after the attack, standing on crutches after spraining her ankle trying to escape. "We did see him with two hands on a gun, walking back just popping rounds."
The killings rocked the world of professional electronic gaming, also known as esports, which boasts an estimated 250 million players worldwide in a growing market worth about a billion dollars a year.
Still doesn’t feel real. Saw a lot of things today I wish I hadn’t seen. But I also saw a community of people rally around each other and a massive amount of support from friends and family to check on everyone. I’m thankful for everyone of you guys in the community.I love y’all
— G-tech (@maddenvtech) August 27, 2018
"My heart goes out to the family, friends and people affected by the madden shooting today," one video platform streamer and Internet personality, Ninja, wrote on Twitter. "Evil times we live in, just need to out shine that evil with positivity."
Another high profile gamer, @ProblemWright, one of the top names in Madden competitions according to its maker, Electronic Arts Inc, said he was crying and "in so much pain."
"All over a videogame. Two of our brothers are gone man and its so disturbing. One of the most tragic days ive experienced. This community is like family. Broken," he wrote on Twitter.