After days of public anxiety, frantic police work and forensic research, the hunt for a lethal sniper appeared to enter a phase of old-fashioned, shoe-leather investigating yesterday, as police sifted through tips, knocked on doors and analyzed the assailant's attack patterns.

Four days after the first fatal shooting, Montgomery County Police Chief Charles A. Moose conceded, "Some of the more desirable smoking-gun leads just aren't there."

"Science is an asset, science helps, but talking to people, getting information from people, is our best ally," he said.

While forensic science has been a "great asset" during the initial days of the probe, Moose said, "we still want to hear from people. We are still not convinced that we have talked with the right people."

Ballistics tests have linked five of the seven shootings in Montgomery County, the District and Virginia to a single high-powered, .223-caliber rifle. Police said yesterday that they were attempting to discern whether there was any pattern to the attacks that might suggest where the gunman could strike again.

Ten years since the D.C. sniper shootings

"I can only hope that I can be as patient as we want the public to be," Moose said. "Every time my phone rings or someone sticks their head into my doorway . . . I'm hoping it is to tell me we have got somebody in custody."

Moose said at least 100 Montgomery County investigators were working the case, with about 50 agents from the federal Bureau of Alcohol, Tobacco and Firearms and the FBI. Moose said investigators have received an estimated 4,000 calls and 800 credible leads.

Moose said that in some instances investigators are handling tips over the telephone, while in other cases teams of detectives are dispatched to conduct interviews. The FBI has provided an advanced computer system into which all the information from tipsters is being entered and categorized by priority. A clerical staff keeps track of when tips are distributed to field investigators, what the investigations yield and whether the tips are closed out.

Police believe that in each of the shootings, a single shot was fired from a long distance at an unsuspecting victim by an assailant who then vanished unnoticed.

But authorities said they were not necessarily searching for a trained marksman. Gun experts said it does not take an award-winning sharpshooter to make the precision shots that characterized the recent attacks. Members of the military routinely train with targets set 300 to 500 yards away. But shooting from that distance is difficult with the type of .223-caliber bullets used, experts said, particularly if a scope was not used.

"If it's a scope rifle, you can shoot . . . 150 to 175 yards without too much difficulty," said a member of a shooting range who spoke on the condition that he not be identified. "But this guy's probably been practicing somewhere."

Local investigators have talked to FBI agents about three possible profiles for the shooter: a terrorist, a serial killer or a thrill killer. Moose said all three are being considered, but he publicly cast doubt on the first two. He contended that terrorists generally make their demands and intentions known to score political points and that serial killers often space their killings out over days, weeks or months.

In the annals of modern American crime, many similar cases have been solved by a tip from the public, a lucky hunch or a fortuitous turn of events.

From July 1976 to August 1977, New York City was roiled by the serial killer who came to be known as Son of Sam. In eight attacks during that 13-month stretch, David Berkowitz fatally shot five young women and a man and wounded seven other people. He attacked randomly and intermittently, largely targeting young couples in quiet public locations. A parking ticket issued to his car on the night he claimed his last victim eventually led police to his Yonkers apartment building, where he was arrested.

James E. Swann Jr., the "shotgun stalker" who terrorized two Northwest Washington neighborhoods in 1993, carried out his 14 attacks from Feb. 23 to April 19 of that year before he was arrested by an off-duty police officer who saw his car run red lights. Swann killed four people and wounded five others.

In the case of the Unabomber, Theodore J. Kaczynski, his campaign of attacks spanned 18 years, during which he struck 16 times using mail bombs that killed three people and injured 23 others. The Unabomber manhunt ended in 1996 when Kaczynski's brother, David, sent investigators to his Montana hovel after recognizing his writings in the Unabomber Manifesto jointly published by The Washington Post and the New York Times.

Former FBI profiler Gregg McCrary of Fredericksburg said he believes that the sniper in the recent shootings in Montgomery County, the District and Virginia is a remorseless, egocentric killer whose victims are simply targets "to be acquired."

"These are just victims of opportunity. . . . I think he's probably setting up in an area that he feels it's okay to shoot in, that he can get out of there, and then just waits for a victim of opportunity, someone to come out that he can just put those sights on, and then squeezes a round," said McCrary, who was a profiler from 1985 to 1995 and now runs a consulting business.

"With that, you take away all the other motives, robbery and sex assault and all those other things," McCrary added. "You're down to thrill of the kill, playing god. Having the power over these individuals. Life and death. That's real heady, a real rush. He's on a high now."

Another part of the thrill, he theorized, was that the killer, in most cases, acted in broad daylight in relatively busy locales.

"It's a risky thing. It's a calculated risk, and he's being successful with it," he said.

Another Virginia criminologist and former FBI profiler, Robert K. Ressler, suggested that the killings might be the work of two people. The initial witness report about the white box truck spotted at one of the shooting scenes described two occupants of the vehicle.

Ressler, who now heads Forensic Behavioral Services International, in Spotsylvania, portrayed two people, "one driving, one shooting," both probably white and with previous scrapes with the law.

"They're shocking society," he said. "They're on a roll, having a great time."

D.C. police who investigated the shotgun stalker case said it is virtually impossible to stop a serial shooter by catching him or her in the act.

Although a massive police presence on the street is important to make people feel safe and allow for a quick response to a shooting, they said catching the perpetrator will depend on investigative work and -- as in the Swann case -- considerable luck.

"It's basic investigation. You ask a lot of questions. You think about it. You theorize, and you put it out there," said J.T. McCann, a former D.C. homicide detective and supervisor.

McCann, who now works as a private investigator and has been hired by the family of Chandra Levy to investigate her killing, said police tried to cast as wide a net as possible when searching for the shotgun stalker. They researched sales of shotguns and ammunition in the area, ran down leads of vehicles that fit the one witnesses said was used in some of the attacks, developed a composite sketch of the shooter based on the recollections of a surviving victim and searched for connections among the victims or their locations that might provide a key to why they were targeted.

William O. Ritchie, former head of criminal investigations for the D.C. police, said the killer's apparently excellent marksmanship could lead Montgomery police to contact law enforcement agencies and the military in search of a disgruntled or recently discharged employee.

Staff writers Jo Becker, Phuong Ly and Debbi Wilgoren, research editor Margot Williams and researcher Donald S. Pohlman contributed to this report.