A Philadelphia man was fatally shot at a Spotsylvania County gas station Friday as a state police trooper investigated a traffic accident across the street, launching a massive and frustrating search for thesniper who claimed his eighth victim in the Washington area over the past 10 days.

Law enforcement sources said Friday night that ballistic evidence conclusively links the shooting of Kenneth H. Bridges, 53, to thesniper who has killed seven others and wounded two in the Washington region -- each time slipping away undetected.

This time, a Virginia state trooper, who was only 50 yards away from Bridges at the time of the 9:30 a.m. attack, heard a gunshot and ran to help, but never saw the shooter, authorities said.

"With a uniformed trooper directly across the street, we're obviously dealing with an individual who is extremely violent and who doesn't care," said Maj. Howard Smith of the Spotsylvania sheriff's office.

Coming barely 36 hours after the sniper shot a Maryland man to death at a gas station north of Manassas, the shooting about 50 miles away bore enough similarities even before the ballistics match to set off a strategy stitched together by the task force investigating the string of shootings.

Witnesses reported seeing a white Chevy Astro van leaving the scene, prompting area law enforcement authorities to choke off major arteries between Fredericksburg and Washington in a frenzied, traffic-snarling dragnet that failed to snare any suspects.

Police funneled northbound traffic on Interstate 95 at the Springfield interchange to a single lane, while officers clad in flak jackets and cradling automatic weapons peered into each vehicle. Spotters watched from bridges and overpasses. Hundreds of white vans were pulled aside and searched.

But no one was arrested.

Col. W. Gerald Massengill, the Virginia State Police superintendent, said ruefully that the roadblocks had been put in place "obviously, not quick enough." Sounding frustrated, he added: "I don't want to send out signals that we're confident that this thing is not going to occur again. . . . I am confident that everything's being done that should be done and can be done."

It has been more than 10 days since the sniper attacks began and altered the way many residents go about chores that were once routine. Like a nightmare frozen in time, a forlorn tableau was shown on television throughout the day: the victim's silver Buick at the middle row of pumps with the nozzle still hanging in the tank.

Friday's shooting marked the 10th time a person was struck by a single shot fired from a distance by the sniper, the ninth shooting in a heavily trafficked shopping area, the fourth at a gas station and the fourth within a few hundred yards of a highway interchange. Two of the shootings were not fatal, and another shot was fired at a store, but no one was hit. They occurred in Montgomery, Prince George's, Prince William and Spotsylvania counties and the District.

Sources close to the investigation said that Bridges, a father of six driving a rented car, was shot once in the upper torso. They said the bullet remained in his body. Police were working to determine where the fatal shot came from but said there was little evidence at the scene.

Investigators say the shooter might be planning the attacks, visiting the scenes to map them out, plot quick escape routes and gauge the approximate distances of his shots. Several of the shootings took place just hundreds of feet from highways, and a number of them have been skillful -- what is known in the subculture of snipers as "one-shot, one-kill."

"It could be a hunter's mentality," one law enforcement source said. Judging distances, creating cover in a vehicle or on the ground and leaving little behind suggest some preparation, the source said.

Police continue to be stymied by the lack of evidence, which mostly includes bullet fragments, victims and vague descriptions of a potential suspect. Police said they have not developed a composite of the killer, but sources said officials are working to put together a sketch. Smith would not discuss whether there is an image of a possible suspect.

"I am not going to talk about any evidence in this case," Smith said.

But in this shooting, there were several witnesses.

Bruce Bingham, 46, said he remembers standing outside the Mobil station where he works chatting and smoking a cigarette when he heard a single shot across the street. A military veteran with 21 years' experience, Bingham looked across the intersection and said to a co-worker, "Somebody just got shot."

William Hodge, 64, thought the noise sounded more like somebody's tire blew out on Route 1, but Bingham turned to him again and said: "No, somebody just got shot."

Both men said they saw a white Chevrolet Astro turning the corner onto southbound Route 1 as the shot fired, moving in the direction of I-95. Bingham called police to report the vehicle.

An employee of a nearby Howard Johnson's motel said she also saw a white van shortly before the shooting. She said it was driven by a blonde woman with a man in the passenger seat. She did not see the shooting, however. The woman, who spoke on the condition that her name not be published, was questioned by police.

The sightings prompted a widespread search for white vans. Officer Julie Hersey, a Fairfax County police spokeswoman, said officers were looking for "any white van" and urged citizens observing white vans being driven erratically to report them to 911.

"I don't think anyone realized how many white vans there are out there until all of this happened," said Prince William Commonwealth's Attorney Paul B. Ebert. "They're all over the road."

Lucy Caldwell, a state police spokeswoman, said that scores of vehicles were stopped Friday but that no one was arrested.

Some are skeptical that the white van is more than a dead end. Some investigators said they don't doubt that witnesses to Friday's shooting saw a van, but noted that none knew whether the occupants -- some reported seeing one and others two -- had anything to do with the shooting.

Indeed, the focus on the van may be as much a reflection of the dearth of hard information as it is eyewitness reports.

"I wouldn't want the people of Virginia to focus just on white vans," Massengill said. "We don't want preconceived notions out there. But certainly . . . you have to play the hand that you're dealt. And the information that has come to us deals with white vans. So we're still interested in white vans."

Sources close to the investigation suggested that because the vans now command so much attention, people who once would have ignored them are more apt to notice them. They said the presence of a white van could be a coincidence.

A white minivan seen driving erratically at the site of the killing in Prince William County on Wednesday night turned out to be unrelated. Its driver came forward the next day and said he was just driving by.

Investigators were searching for other leads besides the white van.

For the first time, authorities noted the kind of suspect they're after. Montgomery County Police Chief Charles A. Moose said investigators want to hear from people who know someone who has missed a few days of work, has been arriving late to work or has not been keeping to a regular schedule, has been growing agitated and expressing anger toward police or who has been getting "some sort of satisfaction" from the shootings.

Police said Friday that they are interested in the fact that the three shootings in Virginia were so close to hotels. They are looking into whether the killer checked into a room to elude police.

Investigators have gathered lists of thousands of guests in motels near the Virginia shootings -- a Ramada Inn across from Friday's slaying; a Hampton Inn near the Sunoco in Prince William where Dean H. Myers was killed pumping gas Wednesday night; and a Best Western behind the Michaels craft store in Spotsylvania, where a woman was wounded Oct. 4 while loading her minivan. But the lists were so extensive that sources said it would be time-consuming to cull any leads.

Although Smith said investigators did not search any motel rooms Friday, witnesses said police with bloodhounds cordoned off Room 109 of the Ramada. Sources said they found nothing of interest.

Police briefly took a man they believed was linked to that room into custody Friday at the crime scene. Hobert "Dewey" Epps, 36, who lives in Georgia and is staying with his fiancee, said he was questioned.

Epps, one of several people detained briefly Friday, said FBI agents told him that he matched a tentative description of the shooter and that they were concerned about his presence at the scene. Epps said agents showed him a picture -- which he said appeared to have come from a video image -- that slightly resembled him. Epps is about 5-feet-9, of medium build with sandy brown hair and a mustache.

Epps said agents asked him to show his profile. He eventually was released. "They said I looked like the picture of someone at another shooting," he said.

Frustrations were mounting elsewhere in the investigation as well. Too many tips are overwhelming the system. In Prince William, police fielded 13,000 calls in just a day and a half.

As many as 1,000 calls an hour were pouring into the toll-free hotline. FBI Special Agent Gary Bald said he realized some callers have had trouble getting through, though the phone number leads to more than 60 phone lines staffed by agents. It may take days, or weeks, to get to some tips.

With the sniper loose, schools throughout the region remain locked down, and outdoor activities are becoming few. The region's congressional delegation has asked the Office of Personnel Management to encourage telecommuting for federal employees. And the U.S. Department of Health and Human Services has offered $250,000 grants for mental health services to Maryland, Virginia and the District.

Homecoming was canceled at Massaponax High School, a few miles from Friday's shooting. Cagny Lillard, a 15-year-old sophomore at the school, said the day reminded her of one a year ago.

"I never thought that I would be saying this, but this is like 9/11 all over again," she said.