Two construction workers were killed, one was injured and a fourth was reported missing yesterday after part of an unfinished concrete parking structure near Rockville Pike collapsed, Montgomery County officials said.

The pancake collapse came about 4 p.m. in a building that is part of the Fishers Place project south of Rockville, at Twinbrook Parkway and Fishers Lane near the Twinbrook Metro station.

The heap of concrete rubble was being searched for the missing worker last night, authorities said.

Rescue workers wore hard hats and headlamps while using search dogs to look for signs of the missing worker.

"We're hoping to find him in viable shape," county fire and rescue spokesman Pete Piringer said. "The search and rescue team is prepared to move every piece of concrete if necessary."

Fifteen construction workers were in the unfinished gray structure when they began to hear a noise, Piringer said.

"The witnesses indicate that they heard a popping noise," Piringer said, "not even a loud noise," as the building began coming down, one floor atop the next.

Eleven of the workers helped each other to safety. Some of them sat nearby after the dust had settled, in sweat shirts, bandannas and jeans.

They told a reporter that they had been told not to speak about the incident.

Workers at a beauty salon across Twinbrook Parkway heard "a big bang all of a sudden, and then it was over," according to their employer, Mark Manning.

The cause of the collapse could not be immediately determined. Piringer said it would be investigated by state and federal occupational safety agencies as well as the county police department.

Robert Poole, an insurance company risk controller, said that seven floors had been at least partially completed when the collapse occurred.

"We don't know what happened," Poole said. "We're trying to find out right now."

Promotional materials for the Fishers Place project said it is to ultimately provide more than 700,000 square feet of office and laboratory space within walking distance of the Twinbrook Metro station.

The materials, posted on the Internet, indicate that the first three buildings are to include more than 400,000 square feet. According to the posting, the project is to include an above-ground parking structure for 900 cars.

The general contractor for the project was listed as James G. Davis Construction Corp. Officials of the corporation could not be reached immediately.

Robert J. MacDaniels, an official of Oncore Construction, which worked on the foundation for the building, had high praise for the Davis firm.

"Davis is an excellent construction company," he said. "It breaks my heart that this has happened to them. . . . They are good, competent people."

Internet sites listed a third firm as a concrete contractor for the garage, but officials of that firm could not be reached.

Just after the collapse, a large slab of concrete, apparently one of the precast slabs used in the building's construction, dangled from a crane at the site.

Gray dust rose from heaps of jagged concrete, witnesses said.

Two of the workers were found dead at the scene, Piringer said.

He said the injured man was removed immediately and taken to Suburban Hospital with what appeared to be life-threatening injuries. A hospital spokeswoman described his condition last night as critical.

None of the workers' names was released.

Search and rescue workers also took precautions to stabilize the crane and dangling slab, Piringer said.

For a time, the airspace above the collapse site was barred to low-flying aircraft and helicopters, according to county police, who said they feared possible hazards from the dust.

As part of a force of more than 100 emergency workers dispatched to the scene, the county's urban search and rescue team was swarming over the debris last night to find the missing worker.

"Right now, I can't say anything," said Steve Miller, a supervisor with the Maryland occupational safety agency. He said that an agency investigator was on the scene.

Staff writer Clarence Williams contributed to this report.

In addition to their search and rescue mission at the site, a concrete parking structure under construction, Montgomery County emergency workers tried to stabilize the building.Fifteen construction workers who were in the structure, at Twinbrook Parkway and Fishers Lane, heard a popping sound before it began to collapse, an official said. Rubble fell into a construction pit behind the structure, and the missing worker was believed to be in that pit.