The shooting occurred at the Equinox Fitness Club in Coral Gables, Florida, according to ABC's Local 10 News and the Miami Herald newspaper.
Miami-Dade police said the second victim was in critical condition at a hospital. Coral Gables Police also confirmed two other victims were injured and were airlifted to Ryder Trauma Center.
Miami-Dade police say the shooting happened during a dispute between the ex-worker and the manager.
The mall is within a couple miles of the University of Miami campus.
At Yard House, a restaurant inside the mall, many patrons were outside dining or waiting for their lunches when police told restaurant employees to get everyone inside. General Manager Tim Hartog said supporters kept running inside and covered up under tables while others hurried out the indirect access before police set the eatery on lockdown. "It was never chaotic".
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Employees were allowed to clean and shut down the restaurant for the day, and were told to go home afterward.
Officials have reported that the scene has been secured.
Authorities have not provided further details about the three people involved in the shooting because they are in the process of notifying next of kin.
"I had just left Equinox, " he said. "The entire mall is locked down and the road in front is closed too".
"Our thoughts and prayers are with the families impacted by this awful tragedy".