JACKSONVILLE, Fla. — A gunman armed with a handgun killed two people at a video gaming tournament on Sunday, turning a football competition waged in a virtual domain into the chaotic scene of a double murder. The shooting suspect, a gamer attending the event, fatally shot himself, the authorities said.

Sheriff Mike Williams of Jacksonville identified the suspect as David Katz, 24, of Baltimore, but said it was too early to know his motive. The sheriff said the F.B.I. and the Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting in the investigation.

A live-stream of the tournament caught the chilling moment when the shooting began. A red laser dot appeared on the chest of one of the players, who was wearing white headphones and a red sweatshirt. The video of the players then disappeared. Twelve gunshots rang out.

Eleven people were injured, nine by gunshots.

The shooting took place at 1:34 p.m. at the Jacksonville Landing, a riverfront collection of restaurants, shops and an open-air performance space that is a gathering place in the city on weekends. President Trump held a packed rally there as a candidate in 2015. By late Sunday, it was surrounded by fire trucks and police vehicles, its perimeter roped off by police tape. SWAT team officers stood at the entrance of a nearby parking garage. A helicopter flew high overhead.