ROCKVILLE, Md. -- People who saw the Washington-area sniper aim, open fire and flee in a white van were not able to give investigators enough details to create a composite sketch, police said Wednesday.

It was the latest setback in the hunt for the gunman who has killed nine in two weeks.

"Unfortunately, distance and darkness, and perhaps adrenaline have made them unable to give a clear composite that we can disseminate," Montgomery County Police Capt. Nancy Demme said. "I know that's not what the public wants to hear."

Investigators said Monday night's shooting of 47-year-old FBI analyst Linda Franklin outside a Falls Church, Va., Home Depot store was the first time witnesses saw an actual shooting rather than just shadowy figures fleeing.

More than one bystander reported seeing the shooter, but their descriptions were not consistent, investigators said. The gunman was variously described as dark-skinned, olive-skinned, Middle Eastern and Hispanic.

"The only common denominator thus far is male," Demme said. "We don't have a refined description to go by."

Demme said one witness told police the shooter used an AK-74 rifle. Police said the weapon can fire the .223-caliber round that has been the sniper's bullet of choice.

"The witness firmly believes this is the weapon," Demme said. "But we have to keep in mind that weapons are interchangeable, like vehicles. That may be what he thinks he saw."

So far police have released composite images of a white box truck and white vans -- either a Chevrolet Astro or Ford Econoline -- that have been seen at more than one shooting.

Demme gave a "how-to" list of tips for potential witnesses in case the sniper strikes again. Among them: Stare in the direction of the bullet noise, carry around a pen to take notes and, if necessary, write down details on your hand. She also warned witnesses not to "contaminate" their memories by comparing notes with other people or the media.