More than 200 firefighters responded Tuesday morning to a three-alarm fire at an apartment complex that was under construction just off Shady Grove Road in Rockville. There were no reported injuries in the incident.

The blaze was reported to rescuers at 4:15 a.m. and likely “burned for awhile before it was detected” because no one lived in the garden-style apartments at the Gables Upper Rock complex at 40 Upper Rock Circle, according to Pete Piringer, a spokesman with the Montgomery County Fire Rescue Services department.

When firefighters arrived, they found the fire had broken through to the roof, he said. The cause of the fire is under investigation, but appears to have started in the upper floors, according to Piringer. The buildings were about 90 percent complete and suffered “extensive damage,” fire officials said, because of water and smoke.

“The biggest issue was that it burned a while before it was detected,” Piringer said. “It was an unoccupied building, and nobody noticed the fire.”

“Open areas in there lend itself to an intense fire,” he said.

Residents lived in at least two nearby buildings, he said, and utilities in those units had to be shut off so firefighters could access the back side of the complex and avoid overhead power lines. Those residents were not evacuated.

It took firefighters an hour to get the blaze under control, and crews would continue to deal with “hot spots” in the building throughout the day, Piringer said.

The fire caused officials to close Shady Grove Road between Gaither Road and Interstate 270 southbound during the morning commute.

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