ROCKVILLE, Md. -- Part of a parking garage under construction collapsed Friday, killing at least two workers, injuring another and leaving a fourth missing, fire officials said.

Concerns about an additional collapse prompted the evacuation of surrounding buildings, said Pete Piringer, a fire department spokesman.

Derek Baliles, a police spokesman, said a no-fly zone had been established over the area because authorities don't want helicopters to get caught in the dust flying up or create down drafts that might cause more instability below, he said.

Initial searches failed to locate the missing worker.

The injured worker was taken to an area hospital; the worker's condition was not immediately known. All others at the scene are believed to have been accounted for, Piringer said.

About 15 workers were at the site at the time of the collapse.