From the Jacksonville PD twitter:

IMPORTANT If you are hiding in The Landing. Call 911 so we can get to you. #TheLandingMassShooting

https://twitter.com/JSOPIO/status/1033790582894878725

Reddit live thread

https://www.reddit.com/live/11ihjo37iyebi

We don't normally do megathreads, but we're making an exception in this case.

There's been a mass shooting in Jacksonville, FL at a Madden gaming tournament. What we know so far:

There are multiple confirmed fatalities. At least 4 dead, with 15 or more injured.
The situation is still ongoing. DO NOT GO TO THE AREA if you are in Jacksonville.
Do not report on the activities of police if you are in the area. Don't make them targets.
There is one suspect confirmed dead. Police are searching the area to confirm if there was a second shooter or not.

There is a stream of this on twitch. I didn't watch it, I recommend you don't either.

Jacksonville PD is reporting on twitter:

We are finding many people hiding in locked areas at The Landing. We ask you to stay calm, stay where you are hiding. SWAT is doing a methodical search inside The Landing. We will get to you. Please don’t come running out. #TheLandingMassShooting

https://twitter.com/JSOPIO/status/1033790037840879616