He's not your standard burglar. And that's what has Fairfax County police concerned.

Detectives believe one man has committed 11 break-ins since mid-September, mostly in the Belle Haven and Mount Vernon areas of the county near the George Washington Memorial Parkway. By itself, that's unusual--burglars normally don't return to the same neighborhood after more than a couple of visits, especially in a residential area, said Lt. Brian Boykin, head of criminal investigations in the Mount Vernon district.

Even more unusual, this burglar sometimes hits late at night, when residents typically are home. In three instances between 11 p.m. and 4 a.m., the residents were home, Boykin said. Twice, the burglar stole valuables and vanished without waking the residents. But on a third occasion, he was confronted by a resident and ran.

"Your typical burglar, they're not looking for a confrontation," Boykin said. "They're looking to get things easily, and generally they strike during the daytime when people aren't home. Obviously, someone like this, just by the hours, ups the ante. They will have a confrontation."

There's no indication that this burglar is armed, "but in the situation he's put the residents and tenants in," Boykin said, "he's prepared."

After the same area of 10th Street just south of the Belle Haven Country Club was victimized for the third time, police released a composite sketch obtained from at least one of the residents. The suspect is described as a black man in his twenties or thirties, about six feet tall with a medium build, sometimes wearing a baseball cap backward. Boykin said he may be driving a large, light-colored, older model car, possibly a Cadillac.

"He's cunning," said Boykin, "due to the fact this has gone on so long."

Boykin said police are not ruling out more than one suspect. But detectives said probably just one man is responsible because the suspect descriptions have been similar, because of the recurring tendency to enter through unlocked windows or doors and because a large white car has been spotted near three of the crime scenes.

In six of the burglaries, the man was able to enter the houses or apartments through unlocked doors or windows. But Boykin said he forced entry into the other five. He has stolen jewelry, silverware, computers and personal items.

Neighborhood and condominium associations in the victimized areas have been spreading the word about the break-ins and warning residents to lock up. But Paul Carlisle, who lives in the Belle View area, said the burglar himself might have provided Carlisle's own personal wake-up call.

"He may have tried to get in my place," Carlisle said, recalling a recent night when he was in the bathtub at 9 p.m., and heard his dog growling. Carlisle said he normally keeps his balcony door open, and that the dog began backing out of the room after growling. Carlisle said he called out to the dog, then climbed out of the bathtub to check around. He found nothing, but two hours later, one of his neighbors was victimized.

Both police and residents said the Belle View area is not normally subject to much crime. "I've lived here 12 years, and go for walks at night all the time," Carlisle said.

"This is really an aberration of sorts," said Stephen Kent, another Belle View resident. "It's very, very safe."

He said that the rash of burglaries seems to have rejuvenated the crime watch program and that neighbors are exchanging information about what they've seen, or lost. Kent said one of his neighbors had credit cards stolen from a desk drawer, but his wallet was untouched.

"I love to keep my windows open," Kent said. "I haven't been able to do that until they get this resolved."

Burglaries attributed to the same suspect:

* Sept. 20: 6000 block of Fort Hunt Road.

* Sept. 24: 6500 block of 10th Street.

* Oct. 6: 3400 block of Wessynton Way.

* Oct. 12: 2600 block of Londonderry Road.

* Oct. 13: 8200 block of West Boulevard Drive.

* Oct. 13: 6700 block of West Wakefield Drive.

* Oct. 15: 6500 block of 10th Street.

* Oct. 16: 8200 block of East Boulevard Drive.

* Oct. 16: 6600 block of Boulevard View.

* Oct. 19: 8800 block of Linton Lane.

* Oct. 21: 6600 block of 10th Street.

CAPTION: A composite sketch of the suspect sought in 11 burglaries.