Two men wearing red baseball caps and brandishing sawed-off shotguns robbed the Rosslyn branch of the First Virginia Bank about 12:30 p.m. yesterday. But the two left the money in an uderground garage after an explosive device planted within the bills was detonated.

"All I saw was the barrel of that gun. It was about 10 feet away fron me," according to a man who was in the bank at the time of the robbery. The witness, who declined to be idnetified said he crawled behind a desk after being ordered to the floor.

The witness estimated that there were 10 customers and 10 bank employees in the bank at the time of the robbery which lasted about two minutes.

There were no shots fired and one aruged with the robbers, he said.

The robbers were last seen running into an underground parking lot in the 1900 block of N. Moore Street, Arlington police reported.

Police and FBI agents cordoned off a sixblack area for three hours while about 50 men conducted a floorbyfloor search of 10and 12story buildings in the 1900 block of N. Ft. Myer Drive, according to the FBI.

Elevator service in the building was stopped and tenants wereadvised to lock their doors. But no one was arrested. The FBI said.

The stolen money, the amount of which was not disclosed, along with the weapons and baseball caps, was discovered in the basement garage beneath the bank the FBI said.