A four-day lull in the random sniper shootings that have terrorized the Washington area left police grasping for new clues yesterday to catch the killer and anxious residents wondering when they could return to such everyday activities as shopping or pumping gas without worry.

After two weeks in which the gunman had not let more than two days pass without striking, officials and residents had no way to know yesterday whether the sniper had abruptly stopped his terror campaign or was lying low to avoid detection.

Police officials remained optimistic, and last night Montgomery County police said authorities were examining a shell casing found inside a white box truck at a car rental agency near Dulles International Airport in Virginia, according to the Associated Press.

Montgomery County police said it was unclear whether the truck has anything to do with the sniper investigation. They said the rental agency contacted police last night.

Earlier yesterday, police charged a Virginia man with intentionally misleading investigators after his detailed account of the most recent shooting outside a Home Depot in Falls Church, Va., proved impossible - he was inside the store when the shooting happened.

Earlier, police said that while several witnesses provided important information in Monday's shooting, they still did not have enough to create a composite sketch of the killer.

Montgomery County Police Chief Charles A. Moose refused to describe yesterday what progress police have made, but he said they had learned some lessons.

At a news briefing, Moose said the boundaries of the crime scene and road closures in Fairfax County after Monday's killing at the Home Depot should have been wider, and he said police were adding patrols to ensure a faster, broader response if there is another shooting.

"We are certainly making these adjustments, talking to first responders about that kind of thing," Moose said. "We're all learning as we go."

In Fairfax County late yesterday, police charged Matthew M. Dowdy, 37, of Falls Church, with making a false statement to investigators. The charge is a misdemeanor that can carry a penalty of six months in jail.

Dowdy had offered police a detailed description of a gunman wielding a Soviet-style AK-74 assault rifle and escaping in a cream-colored van after Monday night's killing of a 47-year-old Arlington woman outside the Home Depot.

But investigators said Dowdy's story unraveled over the next two days.

Dowdy was inside the Home Depot at the time of the shooting in the parking lot outside, although he told police at the scene Monday night that he was a witness, according to a source close to the investigation.

Investigators began to doubt his story when it didn't jibe with accounts from other witnesses or with evidence at the scene.

Dowdy maintained his account of the shooting until Wednesday night, when he broke down during questioning and admitted he had not seen the shooting, the source said.

"That was two days that could have been spent working other aspects of the case," the source said. "Everybody involved was angry that somebody would mislead police, given that lives are at stake."

Leads run short

With few promising leads and no apparent motive, police faced the possibility that the shooter, who has killed nine people and wounded two in Maryland, Virginia and Washington, D.C., since Oct. 2, could remain free unless there is another shooting that would yield new clues or expose the killer to capture.

"I really doubt the police will just roll up on this guy or knock on his door; they'll probably catch him when he tries to kill again," said Mike Rustigan, a San Francisco State University criminology professor who teaches courses on serial murder to police departments.

He and other criminologists say the shooter is likely to strike again. They say he might be keeping a low profile because of the intense, and widely publicized, police surveillance.

"If he stopped now completely, for good, we might never catch him," Rustigan said. "But the nature of most spree killers - if that's what he is - is to strike again."

The uncertainty of what the killer might do next left many residents in the Washington area uneasy as they tried to go about their everyday fall activities, planning for Halloween parties and high school football games, raking leaves or simply filling their gas tanks.

Schools in the region continued to operate on "lockdown" restrictions, with outdoor sports and other activities canceled.

Montgomery County school officials planned a meeting next week to figure out how to get fall sports teams back on the field, possibly using out-of-county sites or centralized facilities that would offer "the best security and accessibility," Superintendent Jerry D. Weast said in a letter to parents.

Shock subsides

For some parents, though, the shock that followed the initial killings is slowly subsiding.

Dianne Whitaker took her son and daughter to a movie theater in downtown Rockville yesterday for an afternoon showing of the new film Tuck Everlasting.

"We feel a little less anxious than we did a few days ago," Whitaker, a Rockville librarian, said as she stood in the sunny parking lot with 9-year-old Victoria and 11-year-old Connor. "The most recent killings have been in Virginia, and that doesn't feel as close."

Whitaker said she was willing for the first time since the shootings began to let her children play. But she still planned to drive them to school each day, rather than send them outside to wait for the school bus.

Dolores Jacome of Burtonsville let her 7-year-old daughter and a friend roam fearlessly through the grounds of the Rockville courthouse yesterday. But she still won't pump her own gas.

"My boyfriend does that," said Jacome, who does not believe the shooter is finished. If anything, the four days of relative quiet have made her more nervous.

"We're just waiting to see what happens next," she said. "I don't think he's going to stop until he's apprehended."

So far, the only arrest is the case of the discredited witness.

The Montgomery County police chief said yesterday that police concerns about Dowdy's false account should not discourage other witnesses from coming forward, but he added: "If we have a person who violated a law, then very clearly I'm in favor of that being taken care of."

For the most part, police continued yesterday to keep virtually all details of the case under wraps.

Police have not released any information about the FBI's psychological profile of the shooter or released a copy of the only known correspondence with the killer - a handwritten tarot death card left at the scene of the school shooting in Bowie that read: "I am God."

As the nationwide search for the person responsible for the deadly anthrax mailings unfolded last fall, investigators made public a profile of the likely suspect as well as copies of the notes he had mailed - hoping that information about his motivations and examples of his handwriting could prompt new leads.

As little as possible

Moose would not explain why the task force on the sniper case has not taken similar steps.

He said his inclination was to give the media as little information as possible.

"Until we have the person or persons involved in this in custody, it would behoove us to keep almost any investigative information close to our chest," Moose said. He told the assembled press, "My gut as a police officer doesn't want you to have anything."

The Associated Press contributed to this article.