They account for less than 1 percent of all killings in the U.S. in a given year, but serial killers attract the most attention. They fascinate us. They terrify us. And we wonder if they walk among us.

It’s difficult to know exactly which murders are serial killings. The FBI’s Behavior Analysis Unit – the so-called “profilers” - define serial murder as: “The unlawful killing of two or more victims by the same offender(s) in separate events.”

By that definition, many gang killings or organized crime hits could be considered serial killings.

And while it is difficult to rule those sources out, there are hundreds of unsolved murders across the country that are being investigated as possible serial killings.

Here are just a few:

--Atlantic City, N.J. – In November 2006, police discovered the bodies of four women in a ditch on the outskirts of town. Investigators believe they were dumped there at different times, but all were barefoot and had their heads facing east, toward the casinos of Atlantic City. Police have no leads at this point, but some experts believe the person responsible may have either died, or is in prison on an unrelated charge.

--Daytona Beach, Fla. – From December 2005 to February 2006, police discovered the bodies of four women, each of which had been shot execution-style. Three were found inside the city limits – the other was in a wooded area near the edge of town. While there is no way to know for sure, investigators are wondering if the murders may be connected to a string of killings across the I-4 corridor between Daytona Beach and Tampa. In all, 19 women have been killed.

Highways figure prominently in serial killings. The FBI is investigating a series of murders along the I-40 corridor from Mississippi west to Texas. Profilers believe they may be the work of long haul truckers who pick up victims at truck stops, sexually assault them, kill them and dump them just off the interstate. So far two men who may have been working together have been arrested in connection with several of those murders.

When it comes to interstate killings, the numbers are staggering. In 2009, the FBI launched its Highway Serial Killing Initiative, identifying 500 murders across the nation that may have been committed by long haul truckers. The victims are predominantly women, prostitutes, drug abusers or other women at risk. The FBI has developed 200 possible suspects. Police have so far arrested 10 men who they believe may be responsible for as many as 30 murders.

Many serial murders go unsolved. Others take decades to unravel. In Marin County, Calif., police may have finally cracked the serial murder of four women in Marin, Yuba and Contra Costa counties that go back to 1977. Joseph Naso, a resident of Reno, Nev. was arraigned on Tuesday of this week in connection with the murders, which occurred over a span of 16 years.

Naso, now 77 years old, was a photographer who traveled the country. In the 1960s, he lived in Rochester, N.Y., where police are now looking into a possible connection between Naso and the murder of three Catholic school girls. Those murders became known as the “double initial” killings, because the victims, Carmen Colon, Wanda Walkowitz and Michelle Maenza all had first and last names that began with the same letter. Carmen Colon was also the name of one of Naso’s alleged California victims, who was killed near the town of Port Costa in 1978.

According to the FBI, there is no set profile of a serial killer.

They span all racial groups – white, black, Hispanic and Asian.

They are motivated by different things: sex, anger, thrill-seeking or attention-seeking are some common themes.

They tend to operate within a comfort zone with an anchor point – their home, place of work, or a relative’s home.

They display a number of different personality disorders – psychopathy or anti-social behavior are common, but the FBI says most are not insane.

And contrary to popular belief, most are not reclusive or social misfits who live alone. They often have families, homes, jobs and appear quite normal.

Consider what one of this nation’s most notorious serial killers, Ted Bundy once said:

“We serial killers are your sons, we are your husbands, we are everywhere.”