Most mass killers relish watching their victims die. They are motivated by anger or revenge and kill people or categories of people they hold responsible for their problems.

But the sniper who has killed six people and wounded two others in the Washington area is different, experts on mass killings say. By firing a high-powered weapon at long range, he doesn't have the same connection with his victims as other killers, they say.

"He stops and shoots and doesn't hear the screams," said James Alan Fox, a professor of criminal justice at Northeastern University. "Others enjoy squeezing the last breath from their victim. It makes it easier for him psychologically to murder."

Keeping his distance and firing just one shot also has made him that much harder to catch. With no known motive and little physical evidence, experts and police officers who have chased mass killers agree that absent luck or an alert witness, the shootings will be difficult to solve.

The experts said the man who eludes police -- and experts believe thesniper is a man -- is not comparable to other mass or serial killers they know. The Washington area sniper doesn't appear to be shooting in an act of rage, Fox said. Most mass killers will open fire and continue until they run out of bullets or time or are shot by police, he said. This gunman has no category of victim -- women, for example -- and no exclusive geographic area for his killings.

But last night, after police sources said that the killer apparently had left a taunting message for police near the school where a 13-year-old boy was shot Monday, Fox said the sniper also wants authorities to know what he has done.

"He feels extremely proud of his ability to outsmart the police and he wants us to know it's him," Fox said. "He chose a school after police held a press conference saying schools are safe. Then he traveled outside Montgomery County after police said they were doing a geographic profile. At the school, he could've switched weapons but wants us to know it's him."

Fox said the message on the Tarot card is part of the thrill for the killer: "He's playing God. It is a game for him. That's his motive. It's all sport."

Robert K. Ressler, a former FBI profiler who directs Forensic Behavioral Services International in Spotsylvania County, said a killer will often leave clues by what he does to a body -- how it is positioned, whether there was a sexual assault, the kind of wounds.

Terrorism cannot be ruled out, but there is no evidence that it is the motivation, Fox said. Terrorists generally advance an agenda, and "we don't know what his agenda is," he said.

What will it take to catch the killer? 

"A lot of people say [serial killers] want to get caught," said Joseph Borrelli, a retired New York City detective who investigated a string of killings in 1976 and 1977 by David Berkowitz, who became known as the Son of Sam. "I don't believe it. If he's hiding out, he doesn't want to get caught. He's playing with them."

In the Son of Sam cases, Berkowitz wrote Borrelli a letter objecting to something he thought the detective had said. Berkowitz also received a parking ticket near one of the killings. Investigators in thesniper case need a similar break.

"He may think he's so smart he could talk to them," Borrelli said. "And that could be the first break in the case."

Peter Smerick, a former FBI profiler now with the Academy Group, a consulting firm, said a mistake by the killer may ultimately prove to be the break. "A great many resources are being directed toward this investigation, and the offender is aware of it -- meaning he's going to be a little paranoid," he said. "He may have already slipped up without realizing it."

Louis Graham, chief deputy of the DeKalb County, Ga., sheriff's department, who investigated a series of child killings in the Atlanta area in the 1980s, said the hundreds of tips pouring into the Montgomery County tip line may seem overwhelming, but they have to be investigated.

"You can't get flooded with too much information," said Graham, who was assistant police chief of Fulton County, Ga., during the Atlanta killings. "One of the things that we didn't do is we got information and didn't document it, and it really came back to haunt us."

Ultimately, fibers from a rug and bedspread and hairs from a family dog found on two bodies fished from a river led to the conviction of Wayne Williams in 1982 in two of the more than two dozen cases of missing or slain children.

But the most telling lessons may come closer to home.

A decade ago, a shotgun-wielding assailant terrorized two Northwest Washington neighborhoods with a series of attacks. The attacker, dubbed the shotgun stalker by police and residents, shot at 14 people in Columbia Heights and Mount Pleasant.

Once D.C. police concluded that a serial shooter was on the loose, they saturated the affected neighborhoods with round-the-clock foot and car patrols. But their presence did not stop the attacks.

Police who investigated that case say it is virtually impossible to catch a serial shooter in the act. "Prevention is something you have to do, but it's not how you stop it. Apprehension is how you stop it," said J.T. McCann, a former D.C. homicide detective and supervisor who works as a private investigator. "There are thousands of people on the street. There are thousands of street corners."

McCann said police tried to cast as wide a net as possible when searching for the shotgun stalker, researching sales of shotguns and ammunition, running down leads of vehicles that fit one witness's description and searching for connections among the victims or locations that might show why they were targeted.

The assailant was caught after he changed his pattern: Instead of shooting at night, he became brazen and fired during the day, allowing a witness to get a partial license plate number. The gunman's car eventually was spotted by an off-duty officer who saw it going through red lights.

William O. Ritchie, former head of criminal investigations for the D.C. police, said there are almost certainly clues in the neighborhoods the sniper has targeted. "Something has drawn the suspect or suspects to that area," he said.

But McCann said the best chance for a break may come from a person the sniper knows.

"People out there are probably aware of some very strange and bizarre behavior of a co-worker or a partner or a husband or something," McCann said.

Staff writers Petula Dvorak and Debbi Wilgoren contributed to this report.