A man accused of killing two people and then himself at a video gaming competition in Florida had been hospitalised for mental illness, court records show.

Divorce filings from the parents of 24-year-old David Katz of Baltimore say that as a teenager he was twice hospitalised in psychiatric facilities and that he was prescribed anti-psychotic and anti-depressant medications.

The records show Katz’s parents disagreed deeply on how to care for their troubled son.

Katz’s father claimed his estranged wife was exaggerating symptoms of mental illness as part of the couple’s long-running and acrimonious custody battle. They divorced in 2007.

Authorities say Katz killed two people and wounded nine others on Sunday before fatally shooting himself at the Madden NFL 19 video game tournament in Jacksonville.

Jacksonville Sheriff Mike Williams says Katz used at least one handgun in the attack.

Seattle gamer Shay Kivlen, 21, was competing at the tournament and said the shooting didn’t make sense.

“In Madden, you never get so mad at a loss that you would want to do that,” he said.

The Jacksonville tournament had $US5000 ($6800) in prize money to divide among the top finalists.

Mr Kivlen said some gamers rely on that money to make ends meet.

But he and other competitors insisted most players take losses in stride and, even with cash on the line, still view it as being just a game.

“No one deserves to die over playing a video game, you know?” said Derek Jones, 30.

“We’re just out here trying to win some money for our families and stuff.”

The game’s maker, EA Sports, lists a David Katz as a 2017 championship winner.

Mr Kivlen said that even when Katz showed up at in-person competitions, he never seemed to socialise and would brush off attempts at conversation.

“We’ve always known he was a little off and stuff just because he wasn’t social at all,” Mr Kivlen said, adding that Katz’s odd behaviour extended to his game play.

“He would do kind of weird stuff online that other people wouldn’t do.

“He would catch a ball and just start jumping out of bounds and stuff when he could have gotten more yards, just hurting himself. I don’t know what he was doing.”

Kivlen, who said he had once beaten Katz for a coveted spot in a tournament, heard secondhand from a friend that Katz had been asking where Kivlen had gone shortly before the shooting.

After losing his game on Sunday, Mr Kivlen said he left to take a nap at his hotel about 20 minutes before the attack. He was watching a live stream of the tournament online when the gunfire erupted.

A friend hiding in a bathroom at the venue answered his phone. When he said Mr Kivlen may have been a target, Mr Kivlen called police and an officer was sent to his hotel room for about 90 minutes until they received word that the gunman was dead.

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