Law enforcement boats patrol the St. Johns River at The Jacksonville Landing after a mass shooting during a video game tournament at the riverfront mall. Picture: Will Dickey/The Florida Times-Union via AP

Washington - Before the video-game competition began here on Sunday, David Katz was known as a quiet member of the tightknit world of competitive Madden NFL gaming.

People in this community know one another, they say, describing it as a "brotherhood" and a "family." They are friendly and sometimes collaborate, sharing information and coming up with schemes to win tournaments and competitions involving the popular game. Katz, though, stayed away from that.

"He really kept to himself," Shay "Kiv" Kivlen, who played in the Jacksonville event, said of Katz, whom he knew virtually but never met in person. "In Madden competitions, he was different. . . . He didn't really want to talk to anyone. He was just there to play."

Katz, a 24-year-old from Baltimore, shook that community when he opened fire inside the gathering on Sunday, police say. When it was over, the gunman had shot 12 people, killing two, before turning the gun on himself.

Police said surveillance footage from inside the restaurant hosting the competition showed a lone attacker walking by other patrons and going to the backroom, where the contestants had assembled and were playing.

"The suspect clearly targeted other gamers," Jacksonville Sheriff Mike Williams said Monday.

Authorities said they are retracing Katz's steps leading up to the shooting. They also are speaking with his parents, who the FBI said have been cooperating with the investigation. Katz attended Hammond High School in Columbia, Maryland, and went on to the University of Maryland beginning in 2014. There, he majored in environmental science and technology, but he was not registered for fall classes, the school said Monday.

The FBI said in a recent study of dozens of active shootings that attackers, frequently fueled by grievances in their lives, obtain their guns legally and then target specific victims. What may have motivated the Jacksonville attack remained unknown Monday, investigators said.

"We don't have anything that we can stand up here today and tell you that this is the motive," Williams said at a news briefing.

He said that Katz carried two handguns and extra ammunition into the restaurant, but that officials believe he fired only one of the guns. Katz legally purchased both guns in Baltimore from a licensed dealer "recently," Williams said; one of the guns had a laser sight attached, the sheriff said.

Federal investigators headed Sunday to the Baltimore home where it is believed Katz's father lives, tucked in a quiet, developing part of the city with cranes towering overhead and the Inner Harbor not far away. Neighbours said they had not seen David Katz in weeks.

Charles Spencer, special agent in charge of the FBI's Jacksonville division, declined to say Monday whether Katz had any red flags or warning signs in his background. Spencer said the bureau is helping piece together Katz's activities leading up to the shooting, including where he had been, where he stayed and with whom he was in contact before Sunday's violence.

The rampage turned a Madden NFL 19 competition at the Jacksonville Landing - a popular riverfront collection of restaurants and shops in the core of the South's largest city - into the latest public gathering riven by gun violence. Investigators seeking answers fanned out from the edge of the St. Johns River in Jacksonville to the home, 750 miles away, near Inner Harbor.

What happened in Jacksonville also cast a bright light on the high-stress world of esports competitions - a community of dedicated gamers who play for prize money, often in front of legions of online fans. The worldwide audience for esports will reach 380 million this year, and revenue for the industry could top $1 billion by 2020, according to the research firm Newzoo.

For successful players, significant money is at stake. Prominent players who spend hours controlling a digital Tom Brady or Aaron Rodgers on-screen can earn their own endorsement deals and fan bases off-screen. The Jacksonville competition was a qualifier, offering a chance to move on to an event in Las Vegas that features a $25 000 prize.

Two of those competing Sunday were killed, police said: Elijah Clayton, 22, of Woodland Hills, Calif., and Taylor Robertson, 28, of Giles, W.Va. The 10 people who were shot and survived are all expected to recover, Williams said. He also said one person suffered a non-gunfire injury during the chaos and was treated.

Kivlen played in the event, retiring to his hotel room to watch a live stream of his best friend, Clayton, who was playing under the name "TrueBoy." When he first heard the gunfire over the stream, Kivlen thought something was happening in the bar near the tournament venue, so he started calling people who were there.

"One of my friends ended up answering," Kivlen, 21, said in a telephone interview. "He was crying; he was in the bathroom of the tournament."

Kivlen, who had participated in Madden events for four years, including eight major competitions organised by EA Sports, said the Jacksonville event felt like any other - before the shooting.

People can get intense, Kivlen said, and some trash talk is exchanged, "but it's just competitive banter," he said. "It gets intense sometimes, but I've never been to a tournament where there's been a fight or something like that."

He added: "They might be rivals in the game, but everyone's a family."

Kivlen and others in the gaming community recognized Katz and identified him as a player who went by the username "Bread." Kivlen said he was familiar with him because they had played a few times online, describing Katz as someone who didn't talk much.

Devin Jett of Washington, D.C., who has been playing Madden since 2006 and participates in organized events, said most of the people who compete have played together; some even work on strategy together, a practice called "labbing."

"Everybody knows each other; everyone has friends there," Jett said. "It's just tragic that somebody can lose their life playing a game that they love. . . . Usually, Madden's an escape for people to stay out of trouble, and to think somebody lost their life doing that is beyond me."

Timothy Anselimo, 26, who was shot three times Sunday, forged an esports career after spending "10, 12, 13 hours" a day playing video games while growing up in Queens, said his mother, Sujeil Lopez.

Lopez said she was initially distressed by the amount of time he played, but her husband pointed out that Anselimo could have picked up worse hobbies.

"He started in New York to stay out of trouble, and we let him," Lopez said. "We let him play 20 hours on TV because he wasn't in the street."

What started as a childhood passion eventually gave way to playing NBA 2K and Madden competitively - which brought him to Jacksonville over the weekend.

The first bullet tore through his right thumb and middle finger. A second bullet entered the front of his chest and lodged near his right armpit.

Anselimo took off running, trying - with dozens of others - to leave through an emergency exit in the rear of the building, his mother said Monday. As he fled, he fell forward and was trampled in the chaos.

When Anselimo got up again and looked back, a third bullet struck him in the left hip. A worker at a nearby Hooters restaurant helped pull him to safety, Lopez said Monday at University of Florida Health Jacksonville, where her son was undergoing surgery.

Anselimo was the lone patient from the shooting still at that hospital on Monday, officials said, and he was in stable condition.

His mother is glad Anselimo is alive, but the family is worried about what happens if he loses full use of his right hand - and with it, potentially, his competitive-gaming career.

Lopez said her son did not know Katz and did not recall playing against him. She said Anselimo did not feel as though he was targeted, but he noted that Katz seemed removed from such a connected world.

"He was just very quiet," Lopez said. "Everybody there was having a great time, and he was off to the side."

After the shooting, Williams said Jacksonville authorities would be examining security at the Landing. The most high-profile shooting rampages in the United States often target places where crowds gather, including churches, schools, college campuses, movie theatres, nightclubs and offices. In the wake of this latest attack, gamers who participate in competitive Madden tournaments said they hoped the organizers would increase security going forward.

Kivlen said he believes that what happened in Jacksonville "didn't have to do with video games. It was something else with him." Even though the gunman was known to this part of the gaming community, "someone random could have come up and done it," he said.

"The Madden community is filled with people who used to play sports. It's normal people who play sports games," Kivlen said. "A lot of us, we play Madden to get out of the real world. . . . To have this when you're supposed to be in your safe zone, it really sucks."