Falls Church, Va. ? Monday’s attack on an FBI terrorism analyst has yielded the most detailed clues yet, investigators say.

For the first time, witnesses were able to give information about license plates on vehicles seen fleeing the scene, including a light-colored Chevrolet Astro van with a burned-out rear taillight.

A law enforcement official, speaking on condition of anonymity, said another witness gave a description of a dark-skinned man, possibly Hispanic or Middle Eastern, in a white van.

“There was some additional information that we were able to get from last night’s case, and I am confident that that information is going to lead us to an arrest in the case,” Fairfax County Police Chief Tom Manger said.

At least one witness saw the sniper aim and fire on his victim, then flee in a van, The Washington Post reported on its Web site late Tuesday. The newspaper cited anonymous law enforcement sources.

Robert Young, a Washington construction worker, returned to the shopping center Tuesday to talk with police. He said he had heard a muffled gunshot and saw a white van the night before.

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 Chinese restaurant and out of sight.

Young described the driver as a short man of slight build who appeared to be Mideastern. He said, “I got a good look at the guy.”

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.”

Montgomery County Police Officer Joyce Utter, left, and Cpl. Robert Moroney of the Maryland State Police steady a composite of a Ford Econoline van being sought in possible connection with the sniper shootings. The sniper apparently likes a certain style of getaway vehicle, along the lines of the Econoline and the similar-looking Chevy Astro.

All the victims have been cut down by a single bullet fired from a distance with a high-powered rifle as the victims went about their everyday tasks. The sniper’s only apparent communication with investigators has been a tarot death card inscribed, “Dear Policeman, I am God.”

In a continuing appeal for the public’s help, Montgomery County (Md.) Police Chief Charles Moose, the head of the investigation, released composite images of a white van with roof racks that witnesses saw after Friday’s slaying of a man at a gas station near Fredericksburg, Va.

Moose said there appeared to be similarities between the van seen at Friday’s shooting and the van from Monday night’s attack. 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 sniper escaped a huge dragnet around Falls Church, 10 miles west of the nation’s capital. Traffic was backed up for miles as police surrounded and searched dozens of white vans.

Tod Burke, a former Maryland police officer who teaches criminal justice at Radford University in Virginia, said the killer is either escaping before the dragnet comes down or has some kind of hideout where he can watch the chaos that erupts.