It's a bird, it's a plane, it's . . . it's a plane, all right, but nobody's saying why it's circling over Fairfax City every night.

Residents in the Halemhurst area of Fairfax City say that for the last three weeks or so a single-engine plane, possibly operated by a government agency on a surveillance mission, has been flying a tight circle about 1,000 feet above their neighborhood.

"It's like a mosquito buzzing in your ear all night long," said Harry Fitzwater. "I don't know what the hell is going on up there."

Fitzwater said when his annoyance reached a peak he called the control tower at Dulles International Airport to inquire about the mysterious plane. He said he was told the plane belonged to a government agency -- and that no more information could be released.

Numerous agencies contacted yesterday added to the mystery with a chorus of no comments.

"All I can say is . . . I can't say anything about it," said Federal Aviation Administration spokesman Fred Farrar.

The FBI: "My comment is 'no comment,' " said special agent James E. Mull.

The Drug Enforcement Administration: "I doubt it's us. We're better than that -- we wouldn't be seen," said special agent Robert O'Leary.

Art Harrison, air traffic manager at the Dulles Control Tower said, "It probably will be there for a while." So he knows about the plane? "Perhaps."

Fairfax City Police Department: "We are aware of it, but it is a matter I could not discuss," said Capt. John J. Skinner.

Fitzwater, who lives at 4124 Orchard Dr., says he knows more about the plane than he cares to. He notices it mostly between 8 p.m. and midnight.

But not everyone is complaining. Fitzwater's neighbor, Lois Rusche, said her family has been more intrigued than annoyed.

She said they first noticed the plane circling low one evening a few weeks ago when they were swimming in their pool. "We have lights on the pool, and I said, 'Gosh, I hope this guy doesn't think it's an airstrip.' "