So much fear, so few clues.

Ten days, eight dead, two wounded in a region under siege by a ghost gunman, a stealth assailant who is predictable, it seems, only in his unpredictability.

So many theories, so many tips, so many dead ends.

And so many questions.

What is known: 

The single-shot sniper -- apparently a unique figure in the annals of serial murder for his terrifying combination of elusiveness, proficiency and brazen ambition -- is an indiscriminate assassin, squeezing a rifle trigger without regard to the race, sex or age of the distant victims in his cross hairs.

He -- if it is a he -- has fired in daylight and darkness, in sunshine and rain. He has shot along crowded thoroughfares in Montgomery County, at a busy intersection in Northwest Washington and more recently in quieter places, in Bowie and in Prince William and Spotsylvania counties. He appears to favor gas stations -- though not always -- and somehow he gets away clean, maybe in a white vehicle, or maybe not.

He took last weekend off, and yesterday was quiet.

"Everybody, society at large, is vulnerable," said Greg Cooper, a former FBI profiler and ex-police chief of Provo, Utah. "It's not any particular class or group. . . . None of the victims are doing anything to put themselves at risk. They are just living."

If there is a theme to the sniper's attacks, it is obscure to the public and perhaps also to the legions of investigators working desperately to decipher his motivation, to understand him, to find him and to stop him.

For now, there are just pieces of a puzzle.

Eleven shots have been attributed to the sniper, only one of them harmless.

What to make of the timing? 

The seven initial incidents -- in which six people were slain and a bullet crashed through a store window without injuring anyone -- came in a blood-soaked span of 28 hours, beginning at 5:20 p.m. Oct. 2. Five victims were slain in about a 2 1/2-mile radius in Montgomery County, and the sixth went down not far away, just over the District line. The four most recent attacks, which killed two people and wounded two others, were spread over seven days in Bowie, Prince William and Spotsylvania.

Of the 11 gunshots, four occurred in the evening, six in the morning and one in the afternoon.

"It could be viewed that the sniper has slowed down somewhat and is moving from being a spree killer to a serial killer who murders multiple people, with cooling-off periods in between attacks," said Todd W. Burke, a former Maryland police officer who teaches criminal justice at Radford University in Virginia.

But Burke, who has studied serial killers, said: "I do not believe that there have been cooling-off periods. I suspect that he is just being more selective about his locations. . . . The police have blocked him off and forced him to change."

The first incident, at a Michaels craft store in Aspen Hill where no one was injured, happened at 5:20 p.m. Oct. 2. A fatal shooting followed 44 minutes later in the parking lot of a Shoppers Food Warehouse in Wheaton.

Thirteen hours 37 minutes passed. At 7:41 a.m. Oct. 3, the next victim was hit, while mowing grass along Rockville Pike in White Flint. A cabdriver was cut down at an Aspen Hill gas station 31 minutes later, and 25 minutes after that, a woman was shot while sitting on a bench in Silver Spring. At 9:58 a.m., another woman, at a gas station in Silver Spring, fell dead.

There was a lull of 11 hours 22 minutes until the sniper struck again, killing a man as he crossed a street in Northwest Washington.

Then the attacks spread out. Seventeen hours 10 minutes after the District shooting -- on Friday, Oct. 4 -- a woman was wounded outside the Spotsylvania Mall, before the sniper ceased fire for the weekend. Monday morning brought another rifle shot -- and a 13-year-old boy wounded outside a Bowie middle school. Sixty hours 12 minutes later, the sniper struck at another gas station, killing a man Wednesday night in Prince William County. The most recent victim died two days later, at a Spotsylvania gas station.

Kenneth V. Lanning, a former FBI profiler, said that based on the accounts of the shootings, he sees an apparent change in thesniper's pattern.

"What is different and intriguing about this guy is he appears to be a combination of a spree killer and a serial killer," Lanning said. "From Spotsylvania on, he is acting more like a short-interval serial killer, taking longer pauses between attacks, as compared to the spree killings."

As investigators try to discern logic in the sniper's movements, some patterns are clear. In each incident, he fired a single bullet.

"One shot, one victim," said former FBI profiler Clinton Van Zandt.

Another common thread: The sniper apparently plans his escapes carefully, choosing perches with quick getaway routes, at long distances from his targets yet not so far away that his accuracy has been seriously impaired, said Van Zandt and other law enforcement experts. They said that the random victims, going about mundane tasks of daily life, were hit only because they had the misfortune of appearing in the sniper's sights.

"He is unique in that the attacks have been done in public and at a distance," Cooper said. "Generally, these kinds of crimes are done in secluded areas and are done up close and personal. Oftentimes, [killers] like the personal contact. They like to see the victim, and they like to be seen by the victims. It is an element of power."

In the congested areas of Montgomery and the District, the snipermay have used traffic to camouflage his escapes. The four most recent attacks were in less densely populated areas -- in Bowie, Spotsylvania and Prince William -- but occurred within several hundred feet of major highways, making it easy for him to come and go despite one of the biggest manhunts in the region's history.

Forensic pathologist Michael M. Baden, who is not involved in the case, said autopsies could provide some clues on where the sniperwas positioned during each attack. If police know the direction a victim was facing at the time of the shooting, a medical examiner can analyze the wound and "draw back a trajectory" that would point toward the sniper's perch, Baden said.

That analysis should be possible in the case of Sarah Ramos, 34, who was on a bench near the Leisure World retirement community in Silver Spring when she was shot. But it could be harder to make such a determination for a victim who was moving, Baden said, unless a witness can describe the victim's position when the shot was fired.

Baden, the former chief of forensic investigations for the New York State Police, said an autopsy will not reveal how far away the shooter was from the victim. But police officers familiar with sharpshooting said that if investigators determine the type of weapon used, they can estimate the shooter's distance by factoring the gun's effective range and the velocity of its ammunition into the medical examiner's findings.

Monday morning's shooting at Benjamin Tasker Middle School in Bowie provided investigators with perhaps the most tangible evidence to date: an apparent sniper's perch in woods about 150 yards from the school, where grass was matted, indicating that the assailant had waited, flat on the ground. There, investigators discovered a spent shell casing by a fallen tree and a tarot card bearing a taunting, handwritten message to police.

Outside experts said the wooded area, the casing and the card could yield, for example, clothing fibers, fingerprints, DNA, footprints and other evidence.

Prince George's County Police Chief Gerald M. Wilson, whose officers were the first on the scene of the Bowie shooting, acknowledged that the discovery of the intact brass casing could be important. "The significance of that piece of evidence," he said, "is that, when compiled with the other pieces of evidence we have and hope to get, it will help our case in court."

Wilson and other police officials have declined to say whether a fingerprint was lifted from the casing or whether casings were found after any of the other Maryland shootings. Authorities in Virginia have said that no shell casings related to the sniper attacks have been found at the Prince William or Spotsylvania scenes.

So, except in Bowie, has the sniper sighted on his victims from inside a vehicle? Authorities, if they know, will not say.

Based on ballistics analyses, authorities have said that at least eight of the 10 victims were shot with the same .223-caliber rifle. Bullet fragments from the other two victims could not be accurately tested because they were too badly damaged, officials said.

Experts also have tested bullet fragments removed from a man wounded outside a Silver Spring liquor store Sept. 14. They said the fragments could not be accurately tested, but police have not ruled out a connection to the sniper attacks that started Oct. 2.

According to firearms experts and law enforcement officials, bullets and shell casings provide investigators with what is often the best physical evidence in crimes involving guns. Each weapon puts a different marking on bullets and shell casings, and those markings are used to link crimes, as well as suspects to crimes.

A recovered bullet or fragment can be linked to a weapon by unique marks left by the inside of gun's barrel as the bullet passes through. A shell casing can be matched to a specific firearm by unique marks left on the casing by the gun's shell-extractor mechanism.

But experts said none of the markings is guaranteed. Sometimes, markings on bullets or shell casings fired from older or poorly manufactured weapons are unidentifiable. Bullets also may be too badly deformed upon impact with a target.

The possibility that a white box truck was involved in the snipershootings was initially suggested to police Oct. 3 by a Spanish-speaking worker near Leisure World, police said. The witness gave his account to a Spanish-speaking police employee called to the scene to interpret. What emerged was a sketchy account of a white box truck, which authorities said yesterday was seen by witnesses at other shooting sites in Montgomery that day.

Montgomery officers arriving at the Leisure World shooting scene promptly started gathering witnesses. One of the people investigators interviewed was a man doing landscaping or cleaning at the shopping center.

The man spoke only Spanish. A crime analyst assigned to the Wheaton police station about a mile away was the closest Spanish speaker and was called to the scene within five minutes.

Police said that the witness was cooperative and that he has been interviewed by investigators more than once. But police have since said that there were difficulties in obtaining specific details from the man because of the language barrier and the need to carefully translate everything he could recall seeing around the time of the attack.

After investigators, desperate for leads, felt they had obtained enough information from the witness, a rough description of a white box truck seen leaving the area after the attack was broadcast on the police radio -- just before the assailant struck again in Kensington.

At a Mobil station at Aspen Hill Road and Connecticut Avenue -- where a shooting had taken place before the attack close to Leisure World -- Montgomery County Police Chief Charles A. Moose and his command staff were briefed about the white box truck description.

Deputy Police Chief Bill O'Toole said police realized that the information was crude, but they quickly made a decision to go public with it.

"It was the only information we had to work on. We had to put that information out. Really, that's all we had," O'Toole said. "We had four murders that morning with nothing to go on."

"We still need a good vehicle," Van Zandt said. "We are chasing white elephants and shadows."

Witnesses in Bowie reported seeing a white vehicle. Following Wednesday night's fatal shooting at a service station near Manassas, police discounted early reports about a white Dodge Caravan sighted there. Authorities said the people in that van had come forward and offered a "reasonable explanation" of their actions at the scene.

After Friday morning's deadly sniper attack at the service station in Spotsylvania County, witnesses reported seeing a white Chevy Astro van leaving the area.

The multiple reports of a white van or truck led to a widespread hunt for such vehicles. Officer Julie Hersey, a Fairfax County police spokeswoman, said officers were looking for "any white van" and urged residents who might see white vans being driven erratically to report the sightings to 911.

So far, no vehicle linked to the attacks has been found.

So today, the hunt for the sniper goes on.

Former FBI profiler Gregg McCrary said that at this stage, authorities are evaluating the large numbers of tips they are receiving, in conjunction with computer-assisted analysis.

"They are doing more and more data mining: checking purchases of firearms, DMV records, hunting licenses, parole and probation records, and cross-referencing all those databases looking for a hit," McCrary said. "And then you are filtering that through the geographic profile as well as a behavioral profile they are developing."

He added: "They are certainly not out of leads, and many of those leads still have yet to be pursued. Where investigations go stale is where leads run out. But we certainly are not at that point.

"On the contrary, there are still a lot of fresh things to look at. This keeps the morale up and the investigators focused."