How many unsolved homicides are due to serial killers?

Just how common is serial murder in America?

"Serial murder is a relatively rare event, estimated to comprise less than 1 percent of all murders committed in any given year," supervisors at the FBI's famed Behavioral Analysis Unit concluded in their published summary of a 2005 symposium the bureau sponsored in San Antonio, Texas.

That little-known but historically important gathering attracted 135 veteran homicide investigators, scholars and government crime experts.

The group reached an important agreement that serial murder should be defined as "the unlawful killing of two or more victims by the same offender in separate events." Until then, there was no consensus on how many deaths were required for a series of homicides to qualify.

Several participants at that meeting said the broad definition -- which they believe certainly encompasses more that 1 percent of all homicides -- was adopted out of concern that serial killings are often overlooked by police.

"We underestimate the prevalence of serial murder," concluded Jack Levin, co-director of Northeastern University's Brudnick Center on Violence and Conflict and a national expert on serial killing. "It is hard for police to admit they have a serial killer on the loose since it terrifies the public. After all, serial killers are the most successful kind of killer. They frequently stay on the loose for months, years or even decades."

The FBI reports it has 1,398 cases of known serial homicides among the more than 60,000 homicides reported since 1985 to its Violent Crime Apprehension Program in Quantico, Va. That figure represents more than 2 percent of the homicides on file with the bureau.

Veteran FBI agents who've participated in serial killer investigations agree there are many unsolved - and even undetected - serial killings among the nation's growing pile of cold-case homicides.

"We would be fooling ourselves to think that we've been able to identify every serial killer," said Bill Hagmaier, former chief of the FBI's National Center for the Analysis of Violent Crime. "We have to attribute a lot of the unsolved cases to successful predatory killers, some of whom are proficient serial killers. That's simply the way it is."

And there's agreement that even the official definition of serial killing is inadequate.

"You can have a contract killer who's killed five or six people or a gang enforcer, but those for us are not really serial murders. They do fit the definition. But they are motivated by devotion to a gang of love of money," said retired FBI profiler Mark Safarik.

"But we are talking about guys who are murdering people, mostly women, over long periods of time. There is a sexual piece to these crimes. That makes them different animals to guys who belong to a gang or who are involved in drug-related killings."

"The big question is how many serial murderers are active at any one time?" Safarik said. "It depends on who you ask. My best guess is that there are up to two dozen offenders who are active now."