The Beltway Sniper remains at large, and police on Wednesday urged terrified residents of the Washington metropolitan area to stay alert and not have any preconceived notions of what the killer might look like.

Because of distance, darkness and other factors, witnesses to the last two attacks have provided conflicting descriptions of the murderous marksman, and police have been unable to compile a reasonable composite sketch.

"There is no composite available," Capt. Nancy Demme of the Montgomery County Police said Wednesday, adding that people should not be looking for any particular type of person.

"Don't let your view become contaminated with what you have heard earlier," Demme said. "We're telling people not to narrow down their focus."

"The only common denominator right now is male," Demme said. "We don't have a refined description to go by. I know that's not what the public wants to hear."

She said one witness told police the shooter used an AK-74 rifle to kill FBI analyst Linda Franklin Monday night in the parking lot outside a Home Depot store in Falls Church, Va. Police said the weapon can fire the .223-caliber round recovered from some of the shooting scenes.

"The witness firmly believes this is the weapon," Demme said. "But we have to keep in mind that weapons are interchangeable, like vehicles."

With police unable to release more specific information, Demme gave a "how-to" list of tips for potential witnesses in case the sniper strikes again. Among them: commit to memory what you see, carry around a pen to make notes and, if necessary, write down descriptions and details on your hand.

She also warned witnesses not to "contaminate" their remembrances by talking to other people or reporters.

Investigators said Monday night's shooting has yielded the most detailed clues yet in the search for the sniper, who has murdered nine people and wounded two others in his two-week killing spree. Those clues include license plate information and a description of a man seen leaving the scene in a white Chevy Astro van with a burned-out or broken left taillight.

The numbers have not been released. Montgomery County Police Chief Charles Moose said the numbers were partials, "and we don't put out partials."

But the witness reports of the shooter -- some of which described the suspect as olive-skinned and Middle Eastern or Hispanic -- were not consistent.

Robert Young, a Washington construction worker, was among the witnesses to Monday night's shooting who returned to the shopping center Tuesday to talk with police. He said he heard a muffled gunshot and saw a white van.

Young said as he backed his truck out of his parking spot, a white Astro van with two men inside tried to turn into his lane. He said the driver appeared very agitated to find his way blocked and instead drove by a neighboring restaurant and out of sight.

Young described the driver as a short man of slight build who appeared to be Middle Eastern. "I got a good look at the guy," he said.

The driver "seemed to be excessively irritated because he couldn't pull into my lane," he said. "I thought this fool was going to want to get out of the van and duke or something. But he didn't. He kept on going."

Law enforcement sources said there was no indication the sniper targeted Franklin, 47, because of her job with the FBI's Cyber-Crimes Division, created last year to focus on computer crimes as well as intellectual property cases.

In Maryland, Montgomery County Police Chief Charles Moose, the head of the investigation, emphasized that Franklin was not working on the sniper case.

Defense Secretary Donald H. Rumsfeld agreed Tuesday evening to provide military surveillance aircraft in the hunt for the killer, a Pentagon spokesman said. Sources said federal agents on the plane will relay any information they collect to authorities on the ground.

The Army also is searching its records for people with sniper training.

Homeland Security Director Tom Ridge said investigators are hesitant to rule out the possibility that the slayings are the work of a terrorist because there is no hard evidence about motive.

Each victim was cut down with a single bullet fired from a distance by a high-powered rifle. All were going about everyday tasks.

Last week, police found a tarot death card at a crime scene inscribed, "Dear Policeman, I am God."

In a continuing appeal for the public's help, Moose released a composite image of a white Astro with a ladder rack that witnesses saw after Friday's slaying of a man at a gas station near Fredericksburg, Va. He also released a similar image of a Ford Econoline van.

Moose said there appeared to be similarities between the van seen at Friday's shooting and the van from Monday night's attack. Fairfax County Police Chief Tom Manger would not say whether witnesses to the latest attack were able to give complete license plate numbers to investigators.

"Each shooting has revealed more to this investigation. We're encouraged every day," said Michael Bouchard, an agent with the Bureau of Alcohol, Tobacco and Firearms.

The Associated Press contributed to this report.