Washington ? There are few like this in the annals of mob hits, drive-by shootings, drug murders, family feuds, racial killings and crimes of passion, opportunity and blind rage.

The roaming sniper believed responsible for killing six people and wounding two in the Washington area is an odd fit in America’s vast and varied experience with murder, making him hard to understand and therefore hard to catch.

Americans have seen witless rampages by the dozen. But a lethal undertaking against any group of strangers, regardless of their race, sex, age or occupation, and carried out with such exacting precision, is rare.

Authorities aren’t used to a sniper aimless in every way except in the aim of his gun.

“Most have a firm fixation,” Robert K. Ressler, a specialist in criminal psychology and a former FBI behavioral analyst, said of serial killers. “This guy is very indiscriminate.”

To be sure, people have gone off on a purely random shooting spree before, but they tended to be lousy shots, not the marksman on the loose now. In 1985, a sniper enveloped in fog opened up for nearly three hours near a San Jose, Calif., freeway, managing to miss every time before slipping away.

Experts in serial murder say such killers usually leave tracks of some sort apart from physical evidence hints to others about what they intend to do or have done, signs of their dangerous disaffection, some twisted logic that can eventually be discerned as a useful pattern by investigators.

“Most spree murderers enjoy close contact with victims … seeing fear in victims’ eyes,” said W. Scott Thornsley, a criminal justice scholar at Mansfield University in Pennsylvania. The Washington-area sniper is shooting from a distance, he noted. “This is a very unusual case.”

Thornsley said the shootings remind him of the case of Thomas Dillon, who shot and killed five southern Ohio hunters and fishermen from 1989 to 1992.

When police went public asking for clues, a former friend of Dillon came forward to say the two of them used to drive around rural areas shooting at animals and discussing how to get away with random murders, Thornsley said.

The FBI began following him and saw him visit a victim’s grave, held him on an unrelated weapons charge and matched the slugs on the rifle Dillon sold after the last killing. He’s serving life in prison. “I have major problems,” Dillon once explained. “I’m crazy.”

Benjamin Tasker Middle School students file off a bus into the front entrance of the school guarded by Prince George's County police officers in Bowie, Md. On Monday, a 13-year-old boy was shot in front of the school by a sniper's bullet. The boy was the eighth victim linked since last week to the sniper, and is one of only two people to survive the shootings.

“Police may have to wait for the killer to make a mistake,” Thornsley said of the Washington-area shootings.

He said police should ask people: “Has anyone ever spoken to you about what it would be like to shoot someone, like a sniper, in a suburban area, and get away with it?”

No good hints were found in New York City in the early 1980s when a sniper randomly shot seven people over 10 months in and around Pennsylvania Station, killing one. Police did not even realize the shootings were linked until the last one had taken place. So little could be gleaned about the assailant that a psychological profile could not be drawn. The shooter got away.

Some investigators say another case that loosely resembles the latest attacks happened in 1994 on Long Island, N.Y. Like the Washington-area sniper, Peter Sylvester shot one round at a time, over several days.

He killed a man inside a diner, blasted the glass at a gas station and wounded a woman inside a Burger King from sniper perches outside.

As random as the shootings were, Sylvester offered one motive after his arrest that he was shooting strangers to provide cover for the murder of one person he really wanted to kill before he was thwarted.

Sylvester was caught when police found the person who sold him the rifle and matched bullets to the gun.