RETRACING A TRAIL: THE SNIPER SUSPECTS; Serial Killing's Squarest Pegs: Not Solo, White, Psychosexual or Picky

The middle-aged man and the teenager were footloose traveling companions on a fathomless mission of horror. For three weeks, investigators say, they killed -- callously, wantonly, ceaselessly, driven by a logic known only to themselves -- and thus qualified themselves for inclusion in the macabre fraternity of the serial killer.

When the police captured a slumbering John Allen Muhammad, a 41-year-old Army veteran and expert marksman, and John Lee Malvo, a 17-year-old Jamaican citizen, at a highway rest stop early Thursday morning, the authorities declared an end to the sniper shootings that left 10 unconnected people dead and millions panic-stricken in the Washington suburbs.

If the men are convicted, they will add a highly peculiar chapter to the already saturated history of the multiple killer. If anything is clear in that roll call of malevolence, it is that all serial killers are their own story, with their own idiosyncrasies and twisting plot lines, their own tumble of complexities. Ted Bundy is not Jeffrey Dahmer is not John Wayne Gacy. The only true common denominator among them is skill at bringing about death.

But as criminologists and academicians try to find the proper context for the sniper suspects -- which of the notorious killers of yesteryear to align them with -- they have been struck by how unconventional the pair appear to be. In so many ways, based on the still sketchy information known about them, they seem to defy the broad connections that have been drawn among their criminal predecessors.

''This is certainly out of the realm of what I've seen in the past,'' said Peter Smerick, a former agent and criminal profiler with the Federal Bureau of Investigation now in private practice with the Academy Group, a forensic science consulting firm. ''Of all the thousands of cases I've analyzed, I haven't seen one exactly like this one.''

The Team Killer

The fact that there are two of them sets them apart. Serial killers are usually loners, who strike without accomplices or companions, propelled by their personal demons and objectives.

It is unclear whether both Mr. Muhammad and Mr. Malvo actually killed, but investigators say they traveled together in the three weeks of the shootings.

Several experts estimate that no more than 10 to 28 percent of serial killers are teams, although some of the pairs qualify as among the most infamous of all criminals. The Hillside Strangler, for instance, was actually two cousins, Angelo Buono Jr. and Kenneth Bianchi, who were convicted of kidnapping, raping, torturing and murdering young women in Los Angeles in the late 1970's.

Karla Homolka and Paul Bernardo, a Canadian husband and wife known to friends as Ken and Barbie, were accused of raping and murdering young girls. Mr. Bernardo was convicted of two murders, while Ms. Homolka pleaded guilty to manslaughter and testified against him.

Leonard Lake and Charles Ng acted as partners in turning the fortified bunker that Mr. Lake had built into his house near Wilseyville, Calif., into a grisly torture chamber where at least 25 people were thought to have died, their suffering recorded on videotapes made by the killers.

In team killings, according to students of serial killers, one member usually dominates.

''Typically, what you have is a dominant offender who is the driving force and the second individual is usually more subservient,'' said Gregg McCrary, who for 25 years worked in the F.B.I.'s behavioral sciences unit and now has his own company, Behavioral Criminology International. ''Rarely are they real peers.''

In his alliance with Mr. Ng, for example, Mr. Lake took the lead, said Dr. James Alan Fox, the Lipman Professor of Criminal Justice at Northeastern University in Boston, who studies serial killers. Mr. Lake, Dr. Fox said, ''was the one who sat in the easy chair and basically set out what they were going to do.''

In the Los Angeles killings, Mr. Buono appeared to have been the controlling force, though Mr. Bianchi was heavily involved in the crimes and committed additional murders after he left Los Angeles.

''Angelo, the older cousin, was admired and revered by Ken,'' Dr. Fox said. ''Angelo was the mastermind.''

The two men, Dr. Fox said, ''brought out the worst in each other,'' adding that had Mr. Buono and Mr. Bianchi never met, the killings might never have occurred.

Mr. Muhammad seemed to hold considerable sway over Mr. Malvo. Though apparently unrelated, Mr. Malvo called Mr. Muhammad father and is said to have adhered to a rigid diet of crackers, honey and vitamin supplements he insisted upon. Even among teams, though, the sniper suspects were unusual because of the 24-year disparity in their ages.

The Race Factor

He would be white. That was the consensus of many experts who furnished educated guesses on the sniper's identity before the arrests. Serial killing, they said, was a white man's game.

Both suspects are black.

There have been few studies of the race and ethnicity of serial killers, but the handful that have been done suggest that black serial killers occur in roughly equal -- or even slightly greater -- proportion to the number of blacks in the population. These studies estimate that between 13 and 22 percent of American serial killers are black.

But the cases so indelibly imprinted on the public consciousness by Hollywood and book publishing are generally white killers like David Berkowitz, the Son of Sam killer; and Jeffrey L. Dahmer, the Milwaukee killer of men and boys.

Most serial killers, black and white, kill within their race. This was true of Wayne Williams, who killed at least five black children in Atlanta in the 1980's, and Henry Louis Wallace, who killed nine young black women in Charlotte, N.C., between 1992 and 1994. Cleophus Prince Jr. was unusual in that he murdered six white women in San Diego in the 1990's.

The sniper suspects are particularly atypical in that the police believe they killed whites and blacks.

In one trait, however, they are unmistakably the usual suspects. They are men. As few as 5 percent of serial killers are thought to be women. Women who kill tend to choose family members and acquaintances as victims, and they usually use poison. There are exceptions, like Aileen Wuornos, a prostitute who shot truck drivers along Florida highways in 1989 and 1990.

The Nonspecialist

White. Black. Men. Women. Young. Old. The snipers killed them all. It did not seem to matter. To be a target, as one person suggested early on, all it took was a pulse.

Almost always, though, serial killers specialize, and by now the categories of choice are familiar: prostitutes, children, young women, gay men, hitchhikers.

John Wayne Gacy preyed on young men and boys. Ted Bundy trafficked in young college women. David Berkowitz selected couples necking in parked cars.

Random killers are rare. Thomas Dillon, sometimes referred to as the Outdoorsman Killer, shot five people to death in eastern Ohio between 1989 and 1992. Their only connection was that they were outside and near a road. Among the 13 victims of Herbert Mullin, who did his killing in California in 1972 and 1973, were a young married couple, a retired boxer, a Catholic priest in a confessional and some teenagers camping.

Investigators have been unable to establish any links between the sniper victims. Some investigators suspect that the snipers may have been killing by location rather than human being, that they intentionally chose to focus on the suburban areas outlying Washington to attract maximum attention.

''It wasn't the process of killing so much,'' said Dr. Eric Hickey, a criminologist at California State University at Fresno, in his speculation about the choice of victims. ''It was about eliciting a response from the community.''

The Speeded-Up Timeline

The timing was strange.

Most serial killers begin slowly, tentatively, almost testing the waters of death. With success, their confidence builds and they begin to speed up the death count. Generally, though, there are pauses between killings that can last days, weeks or years.

''Serial killers generally start slowly,'' Dr. Fox said. ''Their first kill may be something that is not really planned.''

For example, Dr. Fox said, a killer might pick up a prostitute and then, without premeditation, kill her. ''If they don't like what they've done, they won't do it again,'' he said. ''But sometimes they find they enjoy it.''

Mr. McCrary agreed that the serial killers he has studied ''start out more cautiously, trying it out, waiting to see what happens.''

''They wait and pull back and watch,'' he said. But as time goes on, he said, ''they get more confident. They act more quickly and you get an intensification of the frequency of the crime.''

The snipers, the authorities say, turned that protocol on its head. They began with a burst of violence, gunning down six people in just over 24 hours, and then followed that explosion with a series of single killings that slowed in frequency as time passed.

The Childhood

Are serial killers made or born?

Criminologists still know little about what makes these killers kill.

Some experts cite a so-called homicidal triad -- fire setting, bed-wetting beyond an appropriate age, and animal torturing -- that frequently shows up in the backgrounds of murderers. Other experts say physical or sexual abuse in childhood may also be a factor.

Many serial killers interviewed by researchers after they were convicted have described parents who were brutal, neglectful or, at the very least, difficult. Dr. Donald Lunde, a professor emeritus of psychiatry at Stanford University and the author of ''Murder and Madness,'' interviewed Edmund Kemper, who murdered his mother and a string of college women in Santa Cruz, Calif., in the 1970's. He said Mr. Kemper told him that after killing his mother he put her head on a bookshelf in his apartment and ''said all the things to her that he could never say before without her interrupting.''

But other specialists in violent crime maintain that the role of child abuse is exaggerated. Some killers appear to have had normal childhoods, while most people with histories of abuse do not kill.

''We've swallowed this abuse thing hook, line and sinker,'' Dr. Fox said. ''I'm not saying that child abuse or trauma or even adoption isn't important. But adolescent and adult experiences are just as important. If it were just childhood, why would it be that so many serial killers are in their 30's or older?''

What is clear, experts agree, is that few serial killers suffer from major mental disorders like schizophrenia. Herbert Mullin, though, insisted that he heard voices from, among others, his father and Albert Einstein. He claimed he needed to kill to prevent earthquakes and tidal waves. ''He is convinced to this day that he had a special insight that there had to be a certain number of deaths every year to avoid natural disasters,'' Dr. Lunde said.

Richard Chase, known as the Vampire of Sacramento, believed he had to consume human blood, and so he carried out the practice on his victims.

Other multiple murderers seem to have a capacity for love, though they dehumanize those they kill.

But experts contend that the majority of serial killers are best understood as psychopaths, aware of the difference between right and wrong but unremorseful.

There are researchers who say that psychopaths suffer abnormalities in their brains that may predispose them to violent crime. In a 2000 study, Dr. Adrian Raine, a professor of psychology at the University of Southern California, reported finding an 11 percent reduction in the gray matter of the brain's frontal lobe in men diagnosed with antisocial personality disorder, a category that includes psychopathic killers.

Other investigators, however, argue that it is unclear how such findings relate to criminal behavior, and they doubt that brain abnormalities will prove important in explaining the actions of serial killers.

Little is yet known about the upbringings and inner dynamics of either of the sniper suspects.

The Motives

Why kill? Why kill again and again?

The motivation of serial killers is one of the murkier areas of inquiry, and even their own interpretations often prove unsatisfying. Ted Bundy said pornography made him kill, but did it?

Various profilers and researchers try to divide motivations into broad classifications. Dr. Fox settles on five: power and control; profit; revenge; terror; and loyalty.

But he and others find that the most common drive is power and control, usually expressed in sexual fixation. The majority of serial killers, experts said, use their crimes to act out elaborate sexual fantasies, sometimes involving rape or torture. The very process of killing -- seeing that look of terror in a victim's eyes, hearing a victim beg for mercy -- often elicits a sexual thrill. The murderer typically enjoys the intimate physical contact of the crime.

''Traditionally, that's what it's all about, having a victim at your disposal, holding life and death authority over that person, enjoying inflicting pain and suffering,'' said Robert K. Ressler, a former F.B.I. agent and expert on serial killers who runs a consulting company called Forensic Behavioral Services International.

Dr. Lunde said that Edmund Kemper ''described in great detail the excitement of killing up close, so close that their bodies were touching.'' Mr. Kemper, Dr. Lunde noted, killed by strangulation, ''and you can't get any closer than that.''

Jerry Brudos, who preyed on young women in Oregon in the late 1960's, had a foot fetish and a fascination with women's shoes. According to Dr. Eric Hickey, a criminologist at California State University in Fresno who studied 399 serial killers, Mr. Brudos would hack up his victims and store their body parts in a freezer.

It is unclear what motivated the snipers, or in fact whether there was only one motivation. The authorities have avoided speculating, though there have been certain indications that Mr. Muhammad harbored anti-American sentiments. And there was a written demand for $10 million, an unusual request for a serial killer.

Experts say the manner in which the crimes were carried out suggests that the snipers were not sexually motivated. They delivered death from a distance and are not known to have actually touched any of the victims. The suffering of the victims appears to have held little interest for them, since they apparently left the scenes quickly.

Mr. McCrary said it was not unheard of ''to have that kind of disconnect between killer and victim, but it's a less common scenario than we usually see.'' Human behavior and psychological motivation being complicated, he said that a sexual motive could not be ruled out.

Others suggested that the sniper murders more mimicked revenge killings than sexual homicides. These crimes often take the form of mass murders, like the massacre of fellow office workers by a disgruntled employee.

Revenge or anger does occasionally disclose itself in serial killings, and some serial murderers embark on their crimes after experiencing life failures -- the dissolution of a marriage, a financial crisis, the loss of a job.

Such killers, Mr. Ressler said, are usually devoid of hope and have set upon an irreversible course of self-destruction. ''These guys are not thinking, 'I'm going to do this for a couple of weeks and then go back to my family and we'll go to Disneyland,' '' he said. ''A person doing this is heading for death. They know there is nowhere to go but to hell.''

Mr. Muhammad is known to have gone through two divorces and to have suffered financial setbacks. The two men were believed to be close to destitute. Such circumstances alone, of course, do not bring on murder.

The Catch-Me Killer

Most kill in silence.

As a rule, experts said, serial killers do not want to be lured into a cat-and-mouse game with the police that they may lose. They are often arrogant and persuaded of their own superiority. But they want to kill, and while they may relish the idea of outsmarting the police, it is a fringe benefit rather than a goal.

So they deliberately avoid communicating with the rest of the world. For every Keith Hunter Jesperson, the Happy Face Killer who wrote letters to newspapers about his killing women in the Northwest, signing them with a smiling face, or Theodore J. Kaczynski, the Unabomber, who sent letters and offered up his bitter anti-technology manifesto, there are scores upon scores of killers who remain mum about their acts.

The snipers felt the need to talk. They left letters at their sniper perches. They called the police. In one conversation, they even offered up the clue about having committed a murder-robbery in Montgomery, Ala., that set off the chain of events that the police say solved the case.

They behaved as serial killers generally do not behave. They could not shut up. In the end, it was their mouths that got two suspects caught.