The FBI said yesterday it believes that a loosely knit gang may be responsible for as many as 13 bank robberies in the Washington area, the latest of which was a holdup Monday at a Fairfax County bank.

"They're becoming more and more accomplished," said FBI agent Raymond Connolly Jr., who is heading the investigation of the Northern Virginia robberies. "They've gotten away with every one of the robberies." Although FBI and local police have refused to reveal the amount of money taken in the robberies, Connolly said the total has been "sizeable."

In the latest holdup, two men robbed the Arlington Boulevard branch of the First American Bank of Virginia near Falls Church about 11:30 a.m. Monday, police said.The two men wore ski caps and heavy jackets. One man vaulted over the counter and began grabbing money from three cash drawers and stuffing it into the large pockets of his hunting jacket, according to police.

Connolly said officers have linked the same group of suspects to as many as 13 robberies in D.C., Northern Virginia and Maryland since November.

He said the bank camera photographs taken during several of the robberies showed the suspects to be "similar in appearance." He said investigators believe the gang could consist of up to six members.

The robbers' methods have been similar in each of the holdups, investigators said. Each of the targeted banks has been located near an interstate highway, Connolly said.

The robbers have worked in pairs, with one waiting in the bank lobby while the other leaps over the counter and crams money into coat pockets, paper bags or briefcases, police said.

A customer at a drive-in window witnessed Monday's robbery through a plate-glass window and began chasing the robbers after they sped off in what witnesses said was a late-model silver Ford Escort, according to police.

Snow-clogged streets helped the robbers elude the pursuing customer, however. Investigator John R. Stone said two Fairfax County patrol cars had flat tires on the way to the bank and plainclothes police arrived even later after digging their cars out of the snow.

Police have obtained the clearest pictures of the robbers from Monday's holdup, Connolly said. The photographs have been distributed to law enforcement offices throughout the metropolitan area. Police have asked that people with any information concerning the two suspects pictured call the FBI at 683-2680 or the Fairfax County police at 691-2131.