ROCKVILLE, Md. (Oct. 17) - A witness who described the Washington sniper, his cream-colored van and his assault weapon may have given a phony story, police said Thursday in a setback for investigators trying to catch the roving killer.

The announcement cast doubt on much of what the public thought it knew about the gunman who has terrorized the region for the past two weeks.

Fairfax County police Lt. Amy Lubas said the inaccurate account was exposed by checking it against that of several other witnesses.

Asked if the witness may have intentionally misled investigators, Montgomery County Police Chief Charles Moose, who is heading the investigation, at first said: ''I'm moving forward.'' When asked the question again, Moose simply said, ''Yes.''

Lubas said police and state prosecutors are investigating to determine whether the witness should be charged.

Investigators had showed a certain optimism after the latest attack seemed to yield the best details yet about the killer. The killing of an FBI cyberterrorism analyst Monday night in a crowded Virginia parking lot was the only slaying so far in which people actually saw the shooting.

But Moose said there was no credence to the chief witness' description of the cream-colored van. And while he did not give the witness' exact description of the shooter, he chastised reporters for running reports that variously described the gunman as dark-skinned, olive-skinned, Middle Eastern or Hispanic.

''When we have people from the media interviewing witnesses and publishing reports, we get confusion,'' Moose said. ''We get this noise ... out there that gives people tunnel vision and makes them focus in on things that are not appropriate. ... We would like to be able to do our job.''

Moose said the witness' emphatic description of the shooter's AK-74 is also bogus. But investigators cautioned that they still believe the sniper is using one of a family of more than 30 similar assault-type weapons capable of firing a .223-caliber bullet.

''The message we're trying to say is please keep an open mind,'' Moose said. ''People saw a description of a weapon over the last day and a half and we're convinced they eliminated people they know because they say, 'Their gun is not the weapon I saw in the paper.'''

Moose said the disclosure of the fake story, coming a day after investigators said they were unable to draw a composite sketch, was hardly a setback. He said investigators were still chasing leads and he stood by previous composite drawings of vehicles witnesses reported seeing leaving the attacks: a white box truck and a Chevrolet Astro van or Ford Econoline van.

Since Oct. 2, there have been 11 shootings in Maryland, Virginia and Washington, D.C., that have left nine people dead and two wounded. One of the wounded, a 13-year-old boy shot outside his school in Bowie, Md., was upgraded Thursday from critical to serious condition. The other wounded person, a woman in Virginia, was released from a hospital last week.

The victims were men and women of varying ages and ethnic backgrounds, each hit with a single bullet while going about everyday activities. A tarot death card left at one scene was inscribed: ''Dear Policeman, I am God.''

Law enforcement sources said the investigation by federal, state and local authorities has led to information about a number of people with high-powered guns, both legally and illegally owned.

A firearms safety instructor said the FBI has been asking registered owners of .223-caliber guns to bring them in for ballistic fingerprinting.

''They're looking for suspects,'' said Mike Heffernan, owner of Self Defense Technologies in Kensington. ''They're looking at people who have a background in firearms, possess .223 weapons, and have the capability of using them.''

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