A 50,000-pound beam collapsed on the third deck of a mall parking garage, killing one man working on the second floor and trapping another for hours.

As crews labored on a new parking garage at the Westfield Montgomery Mall in Bethesda, a center concrete panel collapsed at about 1:45 p.m. Thursday. About 20 other construction workers were working on the structure at the time.

Montgomery County fire crews responding to the call found one man crushed under the concrete and cable slab. He apparently was killed instantly. The other man was trapped under the slab, alert and conscious for almost four hours before he could be freed.

Crews were brought in from around the county, including trauma units that waited in a nearby high school field, as police and fire officers worked to secure the scene. Once they knew the concrete wouldn't shift anymore, they began a rescue operation.

After almost two hours of removing cement off the surviving man, he was finally freed shortly after 6 p.m. He was transported to the Shock Trauma Center in Baltimore by helicopter with severe life-threatening injuries.

Assistant Fire Chief Scott Graham said officials did not know why the concrete structure collapsed. He declined to give details about how severely the surviving man was injured. Graham said after the rescue was completed that the investigation would be turned over to police.

Mall shoppers slowly exited the building and crews tried to get as many cars as possible out of the parking lot near Macy's. Westlake Drive was blocked off and traffic trickled slowly down Democracy Boulevard as police and fire crews pulled in to aid in the rescue mission.

Bany Alavi, 38, of Germantown, said she was grabbing a quick bite to eat at the mall's food court on Thursday afternoon. As she headed back to her car -- which was parked directly in front of where the construction site was cordoned off -- she heard a loud boom.

At first she thought it was a bomb, but after noticing the construction in the parking garage, she figured it was related to the work being done.

"I thought it was on purpose," she said. "I didn't think that something had collapsed, or that someone as hurt."

When she went to a nearby gas station to get water, she said a clerk told her what had happened. She returned to the scene to find first responders already there.

"I was parked right there," she said. "I can't believe that happened."

The mall is building a new movie theater adjacent to the mall's food court, and the garage is part of that project.