Fairfax County fire officials spent yesterday sifting through rubble on the third floor of a Tysons Corner office building, searching unsuccessfully for the cause of a smoky, intensely hot fire that did $2 million damage, mostly to expensive electronic equipment.

The two-alarm fire, extinguished in 20 minutes Monday night, was confined to a suite of 12 offices occupied by United Nuclear Corp. in the Crescent Plaza Building, 7700 Leesburg Pike, according to fire Lt. Leonard Murry.

Murry said the building appeared relatively undamaged, but the fire destroyed computer and electronic equipment of the mining and manufacturing company, which specializes in supplying materials for the nuclear power industry. In addition, smoke did extensive damage to furnishings on the second, third and fourth floors of the four-story building, he said.

It probably will take several days to determine the cause of the fire, he said.

The fire was the second one in four days in the county to cause more than $1 million in damages. The other, which occurred Friday afternoon, destroyed two wood-frame apartment buildings under construction in the Woodlawn area. The cause of that fire remains under investigation.

Murry said fire officials didn't believe there was any significance in having had two costly blazes in the county in such a short period.

"These structures are so far apart, and are two totally different types of occupancies. There is not any significance as far as we can see," he said.

Monday night's fire was detected by a cleaning crew that smelled smoke and summoned firefighters, Murry said.

The building has 30 other tenants, who were not permitted to enter their offices yesterday because the power remained shut off.

Murry said the fire was treated as if the building were a high-rise, with 45 firefighters using 12 pieces of equipment to fight it from inside, partly using the building's own water supply system.

The United Nuclear Corp. offices were temporarily relocated yesterday to the Tysons Corner Marriott Hotel.