The computer game player who killed two rivals after losing at a Florida competition before fatally turning the gun on himself has been described as "weird" and aloof by fellow gamers.
David Katz, 24, from Baltimore, was named by police as being behind the shooting at the Madden NFL 19tournament, which left two victims dead and a further 11 injured.
Katz had been knocked out of the competition but returned with a hand gun before opening fire yesterday.
Police were alerted at 1.34pm local time and arrived two minutes later.
Live-stream footage caught the shocking moment a laser dot appeared on one gamer's chest. Moments later a flurry of shots could be heard before the feed cut out.
Local media named the two men who died as Eli Clayton, 22, from California, and Taylor Robertson, 27, from West Virginia. Both men were known in the local gaming community.
Investigators were attempting to piece together a motive, with heavily armoured FBI officers entering Katz's house in Baltimore and other officers searching his car.
The computer game, which sees players take each other on while controlling different American football teams, is made by EA Sports. The company lists Katz as a 2017 championship winner.
This year's competition was being held in a riverfront mall in Jacksonville and had US$5000 in prize money to be divided among the top finalists.
Katz is believed to have arrived on Sunday and been knocked out of the competition yesterday. He then left before returning with a gun.
Footage from previous competitions and comments from other gamers who had played against Katz meant a picture of the suspect was beginning to emerge.
Dennis Alston, who said he beat Katz during one round of the Jacksonville competition, described how his behaviour was odd.
"He had shades on, he didn't speak to anybody," Alston told the local news station ActionNews Jax.
"Even after we played... I went to shake his hand and just tell him 'good game', and he just looked at me. He didn't say anything."
Madden Champion 2017 David Katz, right, with his Champion 17 shirt. Photo / Twitter
Alston added: "You can't put it into words. It's stuff you see in movies, man. No words for it. There's no place for it in this world."
Shane Kivlen, a friend of one of the gamers who was killed, said Katz did not talk much to fellow players either online or when they met face-to-face for Madden tournaments.
He said that after winning the championship last year, Katz "got up and let out the weirdest scream ever".
Kivlen called Katz smart but said "something was off about him".
An interview with Katz after a victory at a previous tournament captured an awkward, wide-eyed young man who believed in his abilities as a gamer.
He said in that interview: "I think personally, I'm one of the better players - and I like to let my game prove that. I understand the game really well."
CHILLING: Announcers describe suspected Jacksonville gunman David Katz before deadly mass shooting at Madden tournament:
Footage of a commentator from a previous tournament describing Katz as an intense gamer also surfaced.
The commentator says he was "not here to make friends", and adds: "He's all business, he's focused, and to even get him to open up to talk to you about anything - it's like pulling teeth, man."
Of the 13 people injured in the attack, 11 were shot, two of whom died. Two others were hurt fleeing the scene.
A group of firemen who were training close by the scene of the attack were able to rush to tend to those who had been shot.
The shooting has renewed the gun control debate in America and comes just months before the country goes to the polls in the November Midterm elections.
US Donald Trump initially voiced support for a significant tightening in gun controls after the Parkland school shooting in February, in which 17 students and staff were killed, but has since softened his stance.
