(CNN) -- Heavy winds from a severe storm caused significant damage, shattered windows and sent debris raining down on passengers at an airport in St. Louis Friday night.

The Lambert-St. Louis International Airport is closed indefinitely while officials investigate the damage, St. Louis Mayor Francis Slay told reporters.

"There was a reported sighting of a tornado. Although that has not been confirmed, that storm caused significant damage to the airport," he said.

The reported tornado twisted metal and blew out plate-glass windows in the airport's main terminal, CNN affiliate KSDK reported.

Four people were transported to nearby hospitals with injuries, Slay said.

Passengers were hit with flying glass and debris as winds ripped off part of the roof in the airport's C concourse, the station reported.

One witness described a chaotic scene outside the terminal as officials evacuated passengers from at least one aircraft.

"The plane was rocking back and forth," said Brett Knewitz of Albuquerque, New Mexico, who was on a plane that was about to take off from the airport when the storm hit.

Initially officials did not allow evacuated passengers into the airport, he said, because of concerns that the building's roof would collapse.Once he was allowed inside, Knewitz said he saw an injured gate agent.

"She was bleeding like crazy," he said.

Concourse C is used by American Airlines, AirTran and Cape Air, airport spokesman Jeff Lea said.

The National Weather Service said witnesses believe a tornado was on the ground for several miles and observed the twister from a tower at the airport.

KSDK reported that the storm also blew the steeple off a church and forced authorities to close highways.

CNN's Marlena Baldacci, Taylor Ward and Greg Morrison contributed to this report.