Police investigating the deadly sniper attacks around Washington, D.C., are hoping for a breakthrough, but there's a chance it may never come.
Not all serial killer cases are solved quickly, and some are never solved at all.
"Overall, of the 1,600 or 1,700 [serial slayings] I've personally cataloged, about 20 percent are unresolved," says Michael Newton, the author of the Encyclopedia of Serial Killers.
These cases have frustrated police ever since Jack the Ripper stalked the streets of London in 1888. The killer was tied to the deaths of at least five prostitutes, but was never brought to justice.
More recently, several high-profile serial killings remain unsolved.
Zodiac, Southside Slayer Mysteries Linger
There are many theories regarding the Zodiac Killer, believed to have claimed 37 lives in San Francisco area in the late 1960s and early 1970s, but his identity has never been determined conclusively. No one was ever prosecuted for the crimes.
Some investigators believe the killer was Arthur Leigh Allen, a convicted child molester from Vallejo, Calif., who died in 1992 at age 58. Others are unconvinced, however.
A killer known as the Southside Slayer took the lives of perhaps as many as 20 prostitutes in the Los Angeles-area in the mid-1980s. No one was ever charged in the crimes, which mysteriously stopped in 1987.
Some cold cases date back 70 years, such as the so-called Cleveland Torso Killer. The mysterious attacker killed at least 16 people in the 1930s, but was never caught, despite the efforts of famed federal agent Elliot Ness.
Other open cases are terrifyingly current.
In Baton Rouge, La., police believe a man murdered at least three women over the past year. Police released a profile of the killer in August, but there have been no breakthroughs in the case.
There are dozens more such cases scattered around the country, says Jack Levin, the director of the Brudnick Center on Violence at Northeastern University. There are open serial killer investigations in Kansas City, Pittsburgh, and Flint, Mich., for example.
"Almost every major city has one case," he says.
Common estimates of active serial killers range from 20 to 50 at any given time. But serial killers who change their targets, methods, or location may never be detected. Killers who target prostitutes — women who lead relatively anonymous lives and are not quickly reported missing — likewise may receive little police or media attention.
In the case with Jeffrey Dahmer, for example, police did not know a serial killer was loose in Milwaukee until one of his victims escaped. Dahmer admitted killing and dismembering 17 young men and was convicted in 1992.
In unsolved cases, experts can only speculate on what might have happened to the killers.
Some may have committed suicide or died of natural causes, says Newton. They may have been convicted of other crimes, or may be in mental institutions. Others might have moved and continued their crimes elsewhere, or they might have simply stopped killing.
"It is very possible to get away with these kind of crimes," Newton says.
Waiting for a Mistake
But often a lucky break or improved technology will help resolve long-standing cases.
"I think the myth of the methodical hyperintelligent serial killer — it's not really true," says Antonio Mendoza, the author of Killers on the Loose: Unsolved Cases of Serial Murder.
The Green River Killer — who is believed to have killed at least 49 women killed in Seattle between 1982 and 1984 — baffled police for almost two decades, but a suspect finally was arrested last year.
Last December, Gary Leon Ridgway pleaded not guilty to killing four women. Prosecutors expect to make the other Green River deaths an issue in the trial, which is not expected to begin for another year.
In the Green River case and other longstanding serial killer investigations, DNA evidence proved to be the key.
In a few rare instances, serial killers have simply turned themselves in.
In 1998, Wayne Adam Ford walked into the Humboldt County Sheriff's Department in Northern California and told deputies he'd killed four women. He then took out a plastic bag with a human body part inside.
But in many cases, it was a simply good luck or a key mistake that led police to the attacker.
Charles Ng was convicted 1999 of 11 murders in Northern California. The killings took place in 1984 and 1985, but investigators found Ng only after his partner in crime, Leonard Lake, was caught shoplifting and poisoned himself to death in police custody.
"It's often just a matter of luck. It really is," says Harold Schechter, a literature professor at Queens College in New York, who has written extensively about serial killers.