Rockville, Md. ? A witness who says he saw the Washington-area sniper fire with an assault rifle and flee in a cream-colored van gave a phony story, investigators said Thursday in a setback that cast doubt on much of what the public thought it knew about the roving killer.

Prosecutors are investigating the witness, whose name wasn’t released, to determine whether he should be charged with filing a false statement.

Fairfax County Police Lt. Amy Lubas said the inaccurate account didn’t match that of other witnesses to Monday night’s killing of an FBI cyberterrorism analyst in a crowded Virginia parking lot outside a Home Depot. It was the only shooting so far that people actually saw.

Asked if the witness intentionally misled investigators with his description of a cream-colored van and a burned-out rear taillight, Montgomery County Police Chief Charles Moose, who is heading the investigation, said simply, “Yes.”

Investigators were optimistic after the latest attack seemed to yield the best details yet about the killer. But that gave way to anger Thursday.

While Moose did not give the witness’ exact description of the shooter, he chastised the media 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 would like to be able to do our job.”

Authorities continue to seek leads in the sniper shootings in the Washington, D.C., area. On Thursday, Fairfax County police re-examined a parking lot across the street from the Home Depot store in Falls Church, Va., where the ninth victim was shot Monday night.

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

“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.

Moose said the phony account, 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.

After two massive police dragnets failed to catch the killer, Virginia police doubled their patrols outside Washington, police spokeswoman Lucy Caldwell said.

Since Oct. 2, there have been 11 shootings in Maryland, Virginia and Washington, D.C., that have left nine people dead and two wounded.