A 37-year-old man was shot last night outside a restaurant near Interstate 95, north of Richmond, in an attack that appeared similar to the sniper shootings that have spread fear across the Washington region.

Like 11 attacks attributed to the elusive gunman since Oct. 2, last night's shooting drew a massive police response in which officers combed the immediate area around the shooting site in Ashland, Va., and stopped and scrutinized vehicles on major roads from Richmond to Washington.

"We're not sure that it is and we're not sure that it isn't," Hanover County Sheriff V. Stuart Cook said when asked if the shooting was linked to the sniper. Explaining the massive police response, he said, "We cannot afford to take a chance."

D.C. police said that officers were stationed at all entry points from Virginia and Maryland into the District in search of a white van. Maryland officials were stopping traffic on the Capital Beltway at the American Legion Bridge. Virginia State Police officers were positioned at highway exit ramps.

The shooting victim was described as a man from outside Virginia who was traveling through the area and had stopped to get food and gasoline. Ashland Police Chief Frederic Pleasants Jr. said the man had left the restaurant about 8 p.m. with his wife, who heard the attacker's shot but at first did not realize what it was.

She "heard a crack from the tree line" of the wooded area behind the parking lot, Pleasants said. The man took three steps and collapsed. He was taken to the Virginia Commonwealth University's Medical College of Virginia Hospital, where he was reported to be in critical condition this morning after surgery.

A Virginia State Police dispatcher said officers were looking for a white van with a ladder rack. However, similar alerts were issued after several of the sniper shootings. Authorities in Virginia said last night that no witnesses had described a suspect or a vehicle.

Many details of the incident remained unclear last night.

"The shot came out of the darkness," Cook said. "We're still trying to determine where the shot came from," he said, asserting that despite initial accounts, authorities were not certain whether the shot was fired from the woods.

The restaurant where the shooting occurred is on Route 54 about a quarter-mile from both Route 1 and I-95, which was shut down as far south as Henrico County, just north of Richmond.

In addition to stopping cars on main roads in the shooting area, police threw up roadblocks and checked cars on highways in the immediate Washington metropolitan area, including Georgia Avenue NW and a checkpoint at Chevy Chase Circle. Late last night, traffic was slowed to a crawl there and on other principal Washington area arteries.

Most or all of the roads were reopened by early this morning.

The police chief in Ashland, which is about 80 miles south of Washington, had said that the highway shutdowns would continue "until each vehicle can be inspected and the occupants interviewed."

An employee at a restaurant near the Ponderosa said in a telephone interview last night that she was outside on a break and heard a shot. She ran to the parking lot and saw the victim on the pavement behind the Ponderosa. Bystanders were trying to stop bleeding from his abdomen. The employee said the shot sounded like a tire blowout.

If the shooting was carried out by the sniper, who has killed nine people and wounded two, it would be the first on a weekend and break a five-day lull in shootings attributed to the gunman.

Immediately after the shooting, investigators, including representatives of the multijurisdictional task force working on thesniper shootings, headed to the scene. One official said there was a contingent of 100 agents from the federal government alone, and the agents and police were combing the tree line behind the restaurant parking lot within minutes after the shooting as helicopters hovered overhead.

The shooting was similar to the other attacks in that it occurred outdoors, near a major road or highway. As in the sniper attacks, last night's shooting victim was hit once. The shooting was, however, more distant from Washington than any of the others.

In explaining why authorities believed last night's attack might be connected to the sniper, Pleasants said it was "very unusual" in his area for someone to be "shot for no apparent reason . . . from some kind of stealth position."

In a development earlier yesterday, law enforcement sources said the shell casing found in a white box truck seized by investigators in Virginia on Friday is of a larger caliber than the ammunition used in 12 attacks by the Washington area sniper.

That meant that the shell would not fit in the type of weapon authorities say the sniper has been firing.

The casing, found by a truck rental company on Friday, was for a 7.62mm bullet, which is the equivalent of about .30-caliber, the sources said. In 12 shooting incidents attributed to the sniper, in which nine people have been slain and two wounded since Oct. 2, .223-caliber bullets have been used, authorities have said. Those bullets are significantly smaller than .30-caliber.

In connection with the shell casing found Friday night, authorities noted that each size bullet -- .223-caliber and .30-caliber -- is used in a variety of firearms. But .30- and .223-caliber bullets require different chambers and barrels and cannot be fired from the same weapon.

A cleaning crew working for a truck rental agency near Dulles International Airport found the shell casing inside a white box truck after it was returned. The company notified police Friday afternoon, and authorities confiscated the casing and the truck. The casing was taken to the federal Bureau of Alcohol, Tobacco and Firearms laboratory in Rockville for testing.

Police in the sniper case have been searching for a white box truck since witnesses reported seeing one near some of the shooting scenes on Oct. 3 in Montgomery County, where four people were slain in just over two hours.

Of the eleven shooting cases in which people were killed or wounded, ballistics tests found that the same .223-caliber gun was used in nine, authorities said. Bullet fragments from the other two shootings, they said, were too damaged to be accurately tested. The fragments from a 12th incident in which a shot was fired but no one was injured also were too damaged.

Montgomery County Police Chief Charles A. Moose said at a noon news conference yesterday that forensic tests on the truck and shell casing are continuing and that results would not be released before tomorrow. Asked specifically last night about the 7.62mm casing, Montgomery police spokesman Derek Baliles would not confirm what size shell had been found in the truck.

"We're not making any other press announcements until the final tests are done," probably tomorrow, Baliles said. "In the meantime, we don't know whether this truck is related, so we're still asking people to call in information about box trucks."

He added: "We don't mean to convey any optimism or pessimism. We're just trying to be patient while these tests are performed."

The information police are seeking from tipsters relates to vehicles described by witnesses: a white box truck with black lettering and damage to its rear that was reportedly seen in Montgomery and a Chevrolet Astro van or Ford Econoline van with a ladder rack, which was reportedly seen Oct. 11 near a shooting in Spotsylvania County.

"Until we can finish this work on this truck, yes, we are still interested in white box trucks and two composite [images of] vans that were issued from the Spotsylvania sheriff's office from the shooting there," said Moose, speaking at the Rockville headquarters of a multijurisdictional task force hunting for the sniper.

As for the shell casing, ATF supervisor Michael Bouchard said at the news conference, "We're not going to talk about anything that was found in the truck, where it was found, how many, until we're perfectly confident about what we're going to talk about."

After 12 shooting incidents, none more than three days apart, yesterday began as the fifth day without a shot attributed to thesniper. The daylight hours passed without any indication of a new attack, but fear remained.

Across the region, high school football games were canceled or moved to locations that were not disclosed publicly, and businesses suffered as shoppers stayed away. The sniper has fired on people in Montgomery, Prince George's, Prince William, Fairfax and Spotsylvania counties and the District.

Two of the nine people killed were laid to rest yesterday: Dean Harold Meyers, 53, a Vietnam veteran and civil engineer who was slain Oct. 9 in Prince William County, and Pascal Charlot, 72, a carpenter who emigrated from Haiti almost four decades ago and was gunned down Oct. 3 in the District.

Moose praised the rental agency that called a tip hot line to report finding the shell casing after the truck was returned. He declined to identify the company or the person who rented the truck.

"This is a good example of hearing things, people seeing things, and then notifying local authorities," Moose said. "We appreciate that, so we still encourage people to call our tip line."

Moose said residents of the Washington area must make their own decisions about whether they feel comfortable going about their outdoor errands and activities while the sniper remains on the loose. But Bouchard tried to reassure nervous residents that police were working hard.

"The best people in this country are working on this case from all the different agencies," he said. "I'm confident that everything possible from the law enforcement perspective is being done."

As the manhunt continues, police activity once considered routine has taken on a new urgency.

Yesterday, for example, a Stafford County sheriff's deputy stopped a vehicle about 3:30 a.m. The driver was suspected of selling stolen goods from his vehicle outside a convenience store. Sheriff Charles Jett said the driver, who was wanted by police in Maryland on a weapons charge, sped away and eventually abandoned his car to flee on foot. Helicopters joined the search, but the man remained at large.

Such incidents often prompt questions from reporters at the next Rockville news conference on the sniper case, and Moose's answers usually are less than definitive.

Asked yesterday about the Stafford incident, Moose said: "That case is still developing. It's too early to determine whether it's related or not" to the sniper case.

Meanwhile, with no arrests being made in the case, many people have been keeping their excursions to a minimum, and some residents are going to extremes in risk avoidance.

Scott Kerman, president of Personal Attention, a Bethesda-based errand business, said that for the first time since the company opened 16 years ago, clients are asking his workers to take their cars to the gas station.

Since the latest confirmed sniper shooting Monday night, the errand service, which charges $25 an hour with a two-hour minimum, has filled vehicle gas tanks for 14 customers, Kerman said. He said requests for other simple tasks also have gone up.

"It feels like everybody is paralyzed," Kerman said. "People are so freaked out they don't want to do anything. They'll have a dry cleaner right across the street and have us go for them."

Staff writers Allan Lengel, Carol Morello, Maria Glod, Christian Davenport, Hamil R. Harris, Neely Tucker, Clarence Williams, Abhi Raghunathan, Jamie Stockwell, Craig Timberg, Lyndsey Layton, Peter Whoriskey and Josh White and Metro researcher Bobbye Pratt contributed to this report.