Some local outlets are reporting that multiple victims have been shot and at least 4 people were killed - although police have not confirmed the exact number of deaths.
Earlier, the sheriff advised the public to stay away from the area.
'We can't stress enough to stay away.
It happened during a Madden football video game tournament at a Chicago Pizza Sunday afternoon. The event was being livestreamed and showed players reacting as gunshots were heard in the background. "Many blocks away", said the Jacksonville Police Department in a tweet 3 minutes after the initial one. They say they are "working with authorities to gather facts at this stage".
A statement said: "Complexity is not one of the event organizers in Jacksonville". "SWAT is doing a methodical search inside The Landing", the office said.
The Jacksonville Sheriff's Office is in the process of securing the location, and are investigating into the possibility of a second suspect.
Police urged people hiding in locked areas of the complex to stay in place and call 911.
One of the gamers, Drini Gjoka, said he was shot in the hand.
The shooter was a gamer who had participated in the tournament and lost, competitor Steven "Steveyj" Javaruski, told The Los Angeles Times. "We will get to you, please don't come running out".
A mass shooting has been reported at a mall in Jacksonville Florida.
Stay tuned to WWJ Newsradio 950 and LISTEN LIVE for more on this developing story.