Shots were fired near a ticket counter at Houston's largest airport on Thursday, critically injuring one armed man and sending people in the terminal scrambling and screaming, a Houston police spokesman and witnesses said.

A call that there had been a "discharge of firearms" came into police at 1:35 p.m., John Cannon, the spokesman, said. The shots were fired near the ticket counter in Terminal B at Bush Intercontinental Airport, he said. One person has been taken to an area hospital with life threatening injuries. It was not immediately clear who fired the shots.

Dale Howard, of Tomball, was at the baggage handling area of the airport waiting for his sister to arrive on an incoming flight when he heard two shots fired from the floor above. A few seconds later, he said he heard three more shots.

"People were screaming. I knew exactly what it was — gunfire," Howard said.

Police from an adjacent station rushed in, and Howard said he directed them to the floor above.

Parts of the terminal remained blocked off as police investigated the shooting. The airport announced on its Twitter feed that the terminal had been closed and passengers would be redirected to other terminals.

Darian Ward, a spokeswoman for the Houston Airport System, confirmed the terminal had been closed but said the airport was still operating. Passengers who were scheduled to leave from Terminal B are being rerouted to other terminals, she said.

A "lone armed man" was injured in the shooting, Ward added.

Greg Newburn, who was in the terminal waiting for a flight to Oklahoma City, said he was sitting in a cafe area when he heard two gunshots and after a pause, several more.

"It seemed like quite a few shots. Everyone was scrambling, running left and running right, turning tables up and hiding behind tables. Nobody knew what was happening. I couldn't tell where the shots were coming from," he said.

Newburn, from Gainesville, Fla., said it took him a few seconds to realize that the shots had come from the terminal ticketing area, near the security checkpoint. Newburn said he didn't see who had fired the shots.

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Associated Press writers Juan A. Lozano and Ramit Plushnick-Masti contributed to this report.