Here's everything we know about Katz so far.
MADDEN TOURNAMENT SHOOTING
[Ads /]
EA Sports confirmed that the shooting took place during a "Madden NFL 19" competition. The competition was being held at GLHF Game Bar, located at the Jacksonville Landing outdoor mall complex.
Authorities were called to the scene around 1:30 p.m. local time after they say Katz opened fire with a single firearm.
In a video stream of the competition, a sound that appears to be gunshots can be heard before the stream cuts off.
A witness told local television station WJXX that the shooting started after an argument over the tournament. ABC has not been able to independently verify that claim.
[Ads /]
Authorities said Katz died of a self-inflicted gunshot wound.
MENTAL ILLNESS TREATMENT
Court records obtained by The Associated Press show that a man accused of killing two people and then himself at a video gaming competition in Florida had been hospitalized for mental illness.
Divorce filings from the parents of 24-year-old David Katz of Baltimore say that as a teenager he was twice hospitalized 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.
WEAPONS, INVESTIGATION
Investigators have found the suspect's vehicle and say he stayed in the area the night before. The FBI searched a south Baltimore home on Sunday evening belonging to Katz's father, the Associated Press reports.
Chief Mike Williams with the Jacksonville Sheriff's Department revealed on Monday that David Katz was armed with two firearms - a .45 caliber, a 9mm and extra ammo when he committed Sunday's shootings. He only used one of those firearms in the shooting though, and it was outfitted with an aftermarket laser sight. ATF agents said Katz legally purchased both weapons within the month of August in Baltimore from a licensed dealer.
There is video surveillance from inside the restaurant that reveals Katz targeted the gamers and then took his own life.
Authorities have searched his hotel room and retrieved items from it but did not reveal what hotel he stayed at on Saturday night. They don't know yet if Katz traveled to Jacksonville alone or with someone else.
There is no indication that he planned this attack before he committed it on Sunday. So far, they have found no notes from Katz.
FBI Special Agent in Charge Charles Spencer said Katz's parents are fully cooperating with FBI agents in Baltimore.
BALTIMORE HOME RAIDED
Katz lives in south Baltimore, according to local media reports.
An upscale townhouse raided overnight by the FBI belongs to Katz's father, the Associated Press reports. The home is located near Baltimore's Inner Harbor. Reporters who gathered outside the house did not see the FBI removing any bags of evidence.
KATZ DESCRIBED AS 'OFF' BY FELLOW GAMER
[Ads /]
EA Sports' website listed a player named David Katz as a 2017 championship winner.
Police confirmed that Katz had won a Madden tournament in the past. They said he was staying in town for the game.
A fellow gamer on Monday described Katz as "kind of different" to the Associated Press.
"Something was off about him," said Shane Kivlen, whose best friend was killed in the shooting.
Kivlen described Katz as smart but said that he did not communicate much with other gamers, either online or face-to-face at competitions.
The Associated Press contributed to this report.
RELATED STORIES
Jacksonville shooting: Suspect, 2 others killed in Madden tournament shooting
What are competitive gaming eSports tournaments?