Vague and inconsistent eyewitness accounts make it impossible to draw a clear composite of the sniper who killed a woman in Fairfax County on Monday night, and optimism for an imminent breakthrough dimmed yesterday with no promising new leads.

Detectives and federal investigators still were hoping that witness accounts of the most recent shooting would ultimately lead to an arrest, but they said they still have no solid clue to the identity of thesniper who has killed nine and wounded two in the Washington region.

"Unfortunately, because of darkness and distance and perhaps excitement and adrenaline at the time, we are unable to come up with a composite," Montgomery County police Capt. Nancy Demme said at yesterday's only official news briefing on the attacks. "We don't have a refined description to go by. I know that's not what the public wants to hear."

Highlighting the often imprecise accounts given to police, one of the witnesses described the shooter as "not white, not black," according to a law enforcement source familiar with the case.

The killing Monday of Linda Franklin, 47, of Arlington outside a Home Depot store in the Seven Corners area of Fairfax County marked the first time since the attacks began Oct. 2 that witnesses have said they saw the firing of the shot. The shopping center was crowded, and investigators had more witnesses than in the previoussniper shootings.

Initially, there was optimism that the Seven Corners shooting would be different from the previous 10 connected to the sniper, in which witnesses had only a fleeting glimpse of the attacker. But police said yesterday that descriptions so far have not been of much help. Some described the shooter as a man with dark skin, others with olive complexion, of Middle Eastern appearance or Hispanic.

There was one generic consensus: "The only common denominator thus far is male," Demme said.

One law enforcement source said that investigators know more now than they did before Monday's shooting, but added, "To the extent we can't identify him is troublesome."

Other hopes from Monday night also dimmed yesterday. No shell casing has been found, and surveillance tapes from the shopping center are of no help at this point, a police source said. License plate numbers generated Monday night "were not necessarily a vehicle leaving the scene but driving in the vicinity," Fairfax police Lt. Amy Lubas said.

Investigators are not even certain how many suspects they are seeking.

"I don't think there's anything really hot anywhere," said one detective assigned to the regional task force investigating the shootings. "We're chasing people around all over the place. We're running the leads we have, and, hopefully, something will come up that's good. It just seems to me we need more."

A second consecutive day passed yesterday without a shooting, the first time the sniper has gone two weekdays without an attack since they began. Nine people have been killed and two have been seriously wounded while they went about ordinary tasks over the past two weeks. The shootings have occurred across the region -- in Montgomery, Prince George's, Spotsylvania, Prince William and Fairfax counties and in the District.

The 11 victims range in age from 13 to 72. There is no discernible connection among them -- they are male and female, different races and different ethnicities.

The sniper has struck morning, noon and night, but his most recent targets have been near highways. In the last two shootings, police threw up a massive dragnet that closed roads and snarled traffic for miles. But each time, the shooter was gone before authorities had choked off his escape route.

Reflecting the frustration with the reports they have so far received, Demme delivered suggestions for people in the vicinity of a shooting. Among them: Duck and seek cover at the sound of gunfire. Look in the direction of the sound. Make a mental note of the people or vehicles nearby -- write it down on paper or the palm of your hand, and don't "contaminate" your memory by comparing notes with other witnesses or reporters.

Differing, even completely erroneous witness reports are not unusual, but the sniper shootings are particularly problematic for eyewitnesses, according to nationally recognized experts in the field.

Gary L. Wells, a professor of psychology at Iowa State University in Ames and an expert on eyewitness accounts, said a sniper who shoots from a distance does not even draw the attention of many witnesses.

"In this case, your attention is drawn to the effect, which would be the victim, rather than to the cause, which is the shooter," Wells said. People in the vicinity "are oriented toward the result of the shot, and then it takes them a little while to realize that they should look around. And often, they don't really know where to look."

Wells said that reports of a white box truck or white van seen fleeing the area of several of the shootings essentially become a self-fulfilling prophesy as witnesses to a shooting basically look for a vehicle that is white -- one of the most common colors.

"What we need from witnesses is more objective and less expectancy-based observations because the white vehicles may just be a distraction from something else more meaningful," he said.

Kathy Pezdek, a professor of psychology at Claremont Graduate University near Los Angeles who specializes in eyewitness memory, said people have the erroneous impression that memory works like a video camera that can be replayed at will.

"If information is not preserved or laid down in one's memory, it is not available later," she said.

Witnesses to a sniper shooting may notice vehicles leaving an area before knowing a victim has been shot.

"The sequence of events here is critical to eyewitness memory because there is no motivation to remember what the vehicle looked like until after they learn that someone has been shot," Pezdek said. "If they never saw more than general details of the car or the shooter, all that will be there will be sketchy details."

Demme said one witness told police that the shooter used an AK-74 rifle to kill Franklin, an FBI analyst. Police said the weapon can fire the .223-caliber bullets recovered from some of the shooting sites. But they are skeptical.

"The witness firmly believes this is the weapon that he saw," Demme said. "Keep in mind that, just like the vehicle, each witness firmly believes what they do see. It's not to discredit the witness in the least. That may be what he thinks he saw, and we have to keep in mind that weapons are interchangeable, as are vehicles. Please don't narrow your focus to just one weapon."

It remains unknown how the suspect escaped from Seven Corners even as police surrounded the area. "If we knew the answer to that, we could stop him," Lubas said.

On Monday night, surveillance teams were watching several "persons of interest" who had had been identified from tips to the FBI and local police phone lines. One law enforcement source said there were fewer than a half-dozen people being watched at the time of the shooting.

The source said that those people have been eliminated as suspects and are no longer being watched. But a new group of people is now under police surveillance, the source said.

Authorities said there is no central list of suspects, but police in every jurisdiction involved in the investigation continue to compile lists of potential people to watch based on tips and other information gleaned from the investigation. But so far, the lists have not panned out.

"Right now we're hunting for a needle in a haystack," a task force member said. "But we're hunting. If we grab onto the right piece of hay, maybe we'll find it."

Investigators still are focusing on a white Chevy Astro van with a roof ladder rack and a malfunctioning left taillight that was seen at Monday night's shooting and the one immediately before that on Friday. Police released a composite graphic of the van from Friday's shooting and are hopeful that they can do the same from Monday's.

Police will soon be getting more high-tech assistance. The Pentagon is contributing its RC-7 Airborne Reconnaissance Low plane and at least one other type of aircraft with surveillance capabilities. Last week, the U.S. Customs Service offered three A-star helicopters and one Black Hawk helicopter to fly patrols over the area looking for anything suspicious, said Customs spokesman Dean Boyd. The helicopters have infrared sensors that can detect heat from a body or car.

Also, the American Trucking Associations offered the assistance of its "Highway Watch" program. Drivers will be watching and using their cell phones to call police if they see a suspect vehicle.

There are those who believe the best clues come not from the images of surveillance planes. The answer to the mystery of the sniper's motive may be at the places where he started his string of shootings, said William O. Ritchie, a retired D.C. police deputy chief who oversaw the 1993 investigation of a shotgun stalker in the District.

Ritchie, who now works for a security company that contracts with D.C. Public Schools, said investigators should focus on the area of eastern Montgomery County where the shootings began.

He recalled that the shotgun stalker focused his attacks in Mount Pleasant and Columbia Heights because it was a part of the city he had visited with a co-worker who had made fun of him and angered him. The stalker's first target was a barbershop he had visited with that co-worker, Ritchie said. But police did not learn that context until after the shooter had been arrested by an alert off-duty officer who saw him go through a red light.

"I just think the key is still in the first area where it started," Ritchie said. "This person started in that area for a reason. And now, there's nothing -- nothing -- to make me believe that this person is going to stop."