A total of 367 sheriff's deputies responded to the scene to clear the building and secure the area, the Hillsborough County Sheriff's Office (HCSO) wrote in a post on X, Xinhua reported.
Authorities believe the shooting was "a targeted incident."
At least four other people were being treated at a local hospital with no life-threatening injuries, said HCSO chief deputy Frank Losat in a press briefing.
The suspect, armed with a rifle, drove through a secure fenced part of the building and crashed into an outdoor dining area, where multiple individuals were having lunch, Losat said.
An armed Citibank security guard confronted the gunman and shot him, according to Losat.
He said that around 3,200 Citibank employees work at Citibank Center in Brandon, a suburb of Tampa, and that police spent more than two hours clearing the scene.
Police are still conducting interviews with witnesses, added Losat.