ROCKVILLE -- A portion of a partially constructed parking garage in a commercial area collapsed yesterday afternoon, killing two workers and critically injuring a third, authorities said.

A fourth worker was missing, and authorities last night were using canine units to search the wreckage.

"We're still hopeful" the missing worker will be found alive, said Pete Piringer, a spokesman for the Montgomery County Fire Department.

Early last night, the dogs picked up a scent, and rescue teams were bringing in heat and sound detectors in an effort to pinpoint the location, Piringer said.

Two large cranes were added last night to one that was already on site to help stabilize the walls and columns of the structure, said another official, Montgomery County fire administrator Gordon Aoyogi.

Once the walls were stabilized, it could take up to six hours to remove the debris, said Tom Carr, assistant chief of the county's fire and rescue service, who said rescue teams were working as fast as possible.

"Once you get beyond 24 hours, the rate of survival is extremely low, less than 15 percent," he said.

Montgomery County Executive Douglas Duncan visited the scene, telling reporters late last night, "The good news is they're one of the best search-and-rescue teams in the country."

The search for the missing worker was halted for nearly an hour after the 4 p.m. collapse because of concerns that a large crane holding a concrete slab might topple. It resumed after the slab was removed.

About 75 rescue workers and officials from the Maryland Occupational Health and Safety agency and the U.S. Occupational Health and Safety Administration were on the scene of the collapse in the 12700 block of Twinbrook Parkway.

Piringer said he did not know what caused the six-story structure to crumble, saying that the "floors pancaked down."

The collapsed floors of the garage were piled on top of each other at one end of the structure.

Fifteen construction workers were working on the site at the time of the collapse, Piringer said.

Eleven scrambled to safety. Many of them returned to the site to search for the other members of their crew, finding two who were pronounced dead on the scene and a third who had life-threatening injuries, officials said.

The names of the two dead workers and of their injured and missing colleagues were not immediately released.

The injured worker was taken to Suburban Hospital in Bethesda.

Many nearby buildings were evacuated for fear of an additional collapse, and Twinbrook Parkway was closed to traffic.

An official of the Rockville-based James G. Davis Construction Corp. said last night that the garage was about 60 to 70 percent complete and was part of a commercial complex being built that also included two office towers.

"We're still trying to determine what happened," said Stanley Manvell, the company's vice president for safety.

Manvell said the workers on the garage were employed by a subcontractor of Davis, but said he was not sure of the name of the company.

"Our thoughts and prayers go out to the families of the victims," he said.

The James G. Davis Construction Corp. is one of the state's largest contracting firms.

Last year, it had contracts worth $450 million, placing it 85th nationally in a listing of top contractors published by Engineering News-Record.

The garage is near a Metro subway station in an area of light commercial buildings that includes a restaurant and insurance company.

The Associated Press contributed to this article.