A tweet from the Buffalo Bills in February 2017 shows a photo of Katz holding up a Bills jersey with the number 17 and "Madden Champ" embroidered on the back. The football team's Twitter account names Katz as a winner in the tournament as he stands next to former Bills player Cyrus Kouandjio.
"Congrats to David Katz, the Madden 17 Bills Championship winner!" the tweet reads.
The Associated Press previously reported that Katz was listed by EA Sports as a champion in the 2017 Madden NFL tournament.
On Sunday, Katz was identified as the suspected shooter who killed two people and injured several others before shooting and killing himself in Jacksonville at the video game tournament. He reportedly used a single handgun in the attack.
In addition to the two fatally shot victims, nine other victims were transported to nearby hospitals for treatment, seven of whom were suffering from gunshot wounds. Two other victims drove themselves to the hospital.
The Jacksonville Sheriff's Office impounded Katz's vehicle pending a search while heavily armed FBI agents entered the upscale Baltimore townhouse of Katz's father, according to FBI spokesman Dave Fitz, who did not release any other specific details concerning the home search. Reporters from outside the residence said that agents could be seen inside the home Sunday morning, wearing vests and carrying long guns. They reportedly did not carry out any evidence bags.
Katz is believed to have been staying in the Jacksonville area for the night as part of the Madden NFL 19 tournament. A Jacksonville reporter shared video footage of a Madden tournament in which Katz was playing, where announcers described him as "very, very focused" and "a man of business."
"David Katz keeps to himself. He's a man of himself. He's not here for the experience ... He's not here to make friends. He's all business," one of the announcers said in the video.
It's unclear if the footage is from this year's tournament or from a previous year. Watch the video here.
The shooting occurred inside the Good Luck Have Fun video game bar room inside the Chicago Pizza restaurant at a mall in downtown Jacksonville. The tournament was being live-streamed on Twitch. Audio of the tournament, appearing to include sounds of gunfire and screams, circulated on Twitter before the Twitch stream was ended.