Giant cranes were brought in yesterday to pick apart the pancaked wreckage of a seven-floor parking garage in Rockville and reinforce what remains of the structure, as state occupational safety officials and company engineers continued searching for clues to the fatal collapse.

Showers of sparks lit the rainy night, as welders strengthened joints and added braces to sections of the parking garage still standing. One towering crane was holding up the only remaining wall of the section of parking garage that fell, said Stanley Manvell, vice president of safety for James G. Davis Construction.

"The plan is to super-reinforce the building," Manvell said. He said it was too soon to say what triggered the collapse Friday. Once the building is reinforced, crews will take apart the one wall still standing. Then officials with Maryland's occupational safety agency and company officials will begin removing debris to search for a cause.

"Until we know what it is, everything is suspect," he said.

Three construction workers were killed and one was critically injured when the structure fell. All were members of a crew based in North Carolina.

The parking garage is part of the Fishers Place project off Twinbrook Parkway near the Twinbrook Metro station. Six floors of prefabricated concrete slabs had been erected before they pancaked to the ground Friday, Manvell said.

The remaining wall, facing Twinbrook Parkway, towered over Twinbrook Research Laboratory, forcing the lab to be evacuated Friday. The lab belongs to the U.S. Department of Health and Human Services and houses the Food and Drug Administration's Center for Devices and Radiological Health.

Construction company officials were told that the lab at 12709 Twinbrook Parkway must open today, so crews were expected to work through the night to stabilize and then disassemble the wall, Manvell said.

As occupational safety officials investigated the building collapse, Officer Derek Baliles, a spokesman for the Montgomery County Police Department, said there were no obvious signs of criminal negligence or criminal intent.

Police identified the North Carolina victims as Hubaldo M. Andrade of Chapel Hill, Carl G. Fisher of Wadesboro and Jose Ramirez of Raleigh.

The injured worker, whose identity was not released by the authorities, was in critical condition at Suburban Hospital, a nursing supervisor said yesterday.

Lt. Oscar Garcia, a spokesman for Montgomery County Fire and Rescue, said there were reports that the injured man might have been on the uppermost floor when the structure collapsed, but investigators were unable to confirm the reports.

Tindall Corp., a family-owned Spartanburg, S.C., subcontractor for the concrete, was sued in federal court in 2000 after a concrete pedestrian bridge at Lowe's Motor Speedway near Charlotte collapsed, injuring more than 100 people.

The suit, which involved several companies, was settled in August, according to news reports. The terms of the settlement, which were confidential, involved lump-sum payments and installments.

In the speedway collapse, investigators found that steel cables inside the precast concrete columns for the bridge had been corroded by a chemical additive used to speed the hardening of the concrete.

Another company, Anti-Hydro International, sold the product to Tindall officials, who said they added the material to the grout inside the beams. Tindall officials said they did not know the material contained calcium chloride when they used it in the bridge's columns.

William Lowndes IV, vice chairman of Tindall, said he doubted that a similar problem could have caused the collapse of the parking garage because the company took steps to make sure that the chemical additive would not be used in its concrete again.

"I can assure you, it's not that," he said, adding that the company is still far from knowing what led to the collapse.

Manvell, Davis's vice president of safety, said he was not aware that Tindall Corp. had been among the parties sued in the collapse of the pedestrian walkway, but he was not sure if others in his corporation knew.