A lunch hour on one of the biggest corporate campuses in the Tampa Bay area turned deadly on Wednesday. A man steered a small sedan across medians and grass, broke through a secured fence and slammed into a patio where Citibank employees were eating outdoors near Brandon. He climbed out holding a rifle, walked toward a woman seated in the dining area and shot her. She died at the scene.

The attack ended within minutes because someone on site was ready for it. The gunman moved into a nearby building and headed for its cafeteria. There an armed security officer employed by Citibank met him at about 12:15 p.m. The two traded shots and the attacker went down. Paramedics rushed him to Tampa General Hospital, where he was declared dead at 1:12 p.m.

Hillsborough County Sheriff Chad Chronister said in a statement Wednesday night that the gunman meant to kill as many people as he could. He credited the security team on the property and his deputies with stopping him before anyone else lost a life. Chief Deputy Frank Losat put it more simply at an afternoon briefing. He said the guard stopped the threat and that more bloodshed was absolutely possible. "Kudos to that security guard," he said.

Thousands of workers inside

Roughly 3,200 people were on the property known as Sabal Center when the first 911 calls came in at 12:11 p.m. The first marked patrol unit pulled up a minute later and deputies went straight inside. In all 367 sheriff's deputies answered the call along with SWAT teams, aviation and drone crews, K9 units, fire rescue and state and federal partners. The sheriff's office said more than 400 officers worked the scene in total.

Searching the campus took about two and a half hours. Teams moved floor by floor through six or seven three-story office buildings, opening locked rooms and checking closets where workers had taken shelter. Employees filed out in long lines and were sent to a triage point at a nearby hotel parking lot while their families gathered at a separate reunification site. A college nearby told students and staff to stay where they were until the all clear.

Four other people were treated at a hospital for injuries that were not life-threatening. Investigators have not said whether they were hurt by gunfire, by the car or while running for safety.

Many questions still open

Neither the woman who was killed nor the gunman had been publicly named by Wednesday night while relatives were being notified. Citigroup chief executive Jane Fraser told employees in a memo that the woman was a member of the company's staff. She called the day a terrible tragedy for the bank's family and thanked the security team for keeping it from becoming even worse. She said the Tampa campus is secure, that extra protection is being added at other company sites as needed and that counseling is available to workers.

Early in the afternoon the sheriff described the shooting as a targeted incident. Losat later said detectives were still sorting through the details and had not established whether the attacker and the victim knew each other. No motive has been released. Federal firearms agents joined the investigation.

With thousands of possible witnesses, detectives planned to conduct interviews late into the night. Losat said the indoor and outdoor crime scenes are large and processing them could run into Thursday. The sheriff's office is asking anyone who recorded video of the attack to send it to investigators.