People sometimes mistake one kind of killer for another. You'll see a book written about a serial killer when the person was really a mass murderer.

During my research into the mind of the killer, I came up with three categories to define the killing of groups of people: serial murder, spree murder and mass murder.

Serial murder

Serial murder generally involves three or more victims. The main thing that sets this category apart from the others is that there's a cooling-off period between the murders. The hiatus could be days, months or years. In other words, the serial killer isn't killing with frequency.

Part of the reason for that is that the organized type of killer isn't generally a risk taker. He wants to be sure that if he decides to commit a crime, he's going to be in a win-win position.

Secondly, he doesn't have to kill often if he's taking mementos. He'll have some clothing or jewelry belonging to the victim, so he'll be able to relive the crime and extend the fantasy.

Goes after strangers

A serial killer usually goes after strangers, but the victims tend to share similarities such as gender, age or occupation. Though he prefers a certain look or background, it doesn't mean he won't substitute another victim if he can't find his intended target.

It's hard to estimate, but at any given time there are between 35 and 50 serial killers in this country -- and that's a conservative estimate. About a dozen serial killers are arrested each year.

Areas where you have prostitution, a drug culture, runaways, so-called throwaways, street people and children gravitating to bus depots are fertile grounds for serial killers.

Compounding the problem, there are more than 17,000 police agencies in this country, some with limited technology and the inability to share information. If there's an inability to link cases, agencies may not even know they have a serial killer on their hands. Throw into the mix the mobility of the offender -- within a state or across state lines -- and he can get away with murder.

Among the most notorious serial killers are Jeffrey Dahmer, Ted Bundy and John Wayne Gacy, who collectively killed at least 66 people.

Spree murder

The next category is spree murder. Spree killers tend to do their damage within a short time span. There's generally no cooling-off period. They're like killing machines up to the point that they're caught or they turn themselves in.

The killer often commits suicide or goes for what's known as suicide by cop -- putting himself in a position where police will have to kill him.

Spree killers usually select victims randomly, but go for those who will meet their personal needs at the time. In other words, they'll kill for money, sex or simply because they're hungry.

In cases involving spree killers, authorities usually know who they are looking for: They have the killer's identity. As a fugitive, he may go to an area where he feels comfortable, the way alleged railroad killer Angel Maturino Resendez, also known as Rafael Resendez-Ramirez, stuck to the tracks.

Andrew Cunanan, who killed four people before gunning down his last victim, fashion designer Gianni Versace, is another example of a spree killer.

Sometimes you hear about a spree serial killer, a sort of hybrid, where there's a shorter time span involved, perhaps days, and where the victims may not have a common thread.

Mass murder

The last category is mass murder. A mass murderer kills his victims -- three or more -- at one time and in one place. While it's one event, there may be multiple crime scenes. Someone may commit a murder inside a building and then kill more people outside or down the block.

These typically are the cases where there's violence in the workplace, like the shootings in post offices around the country. The rampage at Columbine High School and the more recent one in a Fort Worth, Texas, church involved mass murders.

Going back in history, Charles Whitman killed his wife and mother the night before climbing the tower at the University of Texas and gunning down 14 people in 1966.

While this is often a mission-oriented type of crime and the killer is going after a selected or symbolic group, he'll randomly kill other people who happen to be there at the time.