The gunman killed himself and two others, and left 11 people injured in the shooting at the Jacksonville Landing complex in Jacksonville, Florida at around 1:34pm on Sunday, police said.
Police have not yet confirmed the identities of the deceased victims. Local media, including the Miami Herald and ActionNewsJax, have named Robertson and Clayton as the deceased victims, after members of the Madden gaming community posted numerous online tributes.
An official close to the investigation said that the gunman killed himself with the semi-auto handgun used in the shooting, which had a laser sight.
A livestream of the tournament caught the chilling moment that a laser landed on Clayton's chest before shots rang out.