A two-alarm fire ravaged the eighth floor of a 12-story Crystal City office building yesterday morning, causing $500,000 in damage and creating traffic snarls that tied up Route 1 and nearby roads for much of the morning.

The fire, which started around 4:30 a.m., gutted offices on the northeast side of the eighth floor at 2521 Jefferson Davis Highway (Route 1).

The office building is the headquarters for the Naval Sea Systems Command, which oversees the construction and maintenance of Navy ships and weapon systems.

Fire officials said they had not found the cause of the fire, but said it did not appear to be suspicious.

"The probable cause is electrical in nature," said Lt. Steven Hynson, spokesman for the Arlington County Fire Department.

Hynson said most of the damage appeared to have been to office equipment. Upper floors of the office building also were damaged by smoke, he said, while lower floors were damaged when water used to extinguish the fire seeped down.

Office workers and military personnel had been advised in television and radio news bulletins not to show up for work at the high security office facility until after 10 a.m., but scores who had not gotten the word began milling around the office complex around 7 a.m.

"I heard the fire engines early this morning and couldn't sleep so I came over here," said Souren Hanessian, an engineer on the fourth floor of the building who lives in an apartment complex a block away.

"On the backside of the building all the windows have been busted out," Hanessian said, as he awaited word of when he would be allowed to enter the building.

Department of Defense police cordoned off the building, and employees whose offices were on the seventh, eighth and ninth floors were sent home. Employees on the unaffected floors were allowed to enter the building around 11:30. All employees will be expected to report to work today, a Defense Department spokeswoman said.

Hynson said traffic delays occurred on Jefferson Davis Highway when employees arriving for work found that their usual route along Clark Street had been closed.