A Mathematical Method to Locate Killers

ByAmanda Onion

Oct. 8, 2002 -- If the shootings of eight people this past week in Maryland and nearby areas are the work of a single suspect, then mapping and analyzing the locations of each crime can help investigators locate the killer's base.

One way to do this is using a recently developed software program called geographic profiling.

"The coordinates for each crime location is fed into the system," explains Kim Rossmo, director of research at the Police Foundation and creator of the geographic profiling system. "Then the resulting geometric pattern is interpreted by the algorithm. The outcome shows the probability of the residence of the perpetrator."

The program is based on a complex algorithm that Rossmo formulated using a collection of general rules applying to serial offenders. A series of crimes such as the recent shootings may seem random, but as Rossmo argues, they usually follow a pattern.

"There are stranger-to-stranger crimes — but they're not random crimes in terms of where a person chooses to hunt or commit the crimes."

The Nearness Principle

A key rule, for example, is the "nearness principle." A large body of forensic research has shown that most individuals tend to commit crimes fairly close to home.

"Criminals don't function that differently from ordinary citizens when they do things such as shopping," says Rossmo. "They usually remain within a limited range. They also don't function that differently from animals, who tend to forage within a limited range from their base."

The extent of an offender's range depends on his or her preferred mode of transportation. A sniper operating from a vehicle would have more range, for example, than one using a bicycle or one who travels on foot. The geographic profile system incorporates all methods of transportation available to an offender, including buses, subways and cars when calculating a likely base.

There are some exceptions to the nearness principle. Rossmo says that older offenders often travel farther than younger ones and whites appear willing to travel further than blacks to commit crimes. But as a general rule, Rossmo says, people "don't go any farther than we have to to accomplish our goals."

Offenders also usually maintain a buffer zone — a minimal amount of distance around their base, usually their home.

"Killers want to operate in a comfort zone — their own neighborhoods — but they also want to operate in a place where they have some anonymity," Rossmo says.

A Sifting System

By studying the two or three-dimensional maps produced by the geographic profile system, investigators can sometimes see crime locations surrounding, but always a key distance from a particular region.

Police then use this map to determine where to send police for surveillance or which police-record systems to search for clues. Sometimes the information is used in mail-outs, when police send fliers to residents in a particular region asking for specific clues.

If there is a general description of an offender's vehicle, investigators can use a geographic profile to do a focused search for all cars resembling that description.

"Any pattern is important to any investigation and geographic profiling is one that can help investigators navigate a flood of information," Rossmo says.

It was geographic profiling that helped investigators eventually find the "South Side Rapist," who raped more than a dozen women in and near St. Louis over a period of 11 years. Using victims' recollections and DNA matches, police linked a series of rape crimes to one suspect. Geographic profiling revealed his probable location and police found their suspect in the nearly exact center of the area where the rapes took place.

In the investigation of the apparent serial shootings in Maryland, Washington and Virginia, clues and witnesses remain scarce, so a geographic profile could prove critical for honing in on the killer's trail. Rossmo is quick to point out, however, that geographic profiling can't locate an offender on its own.

"It doesn't give you an 'X' that marks the spot," he explained recently to a conference of the National Criminal Intelligence Service, "but it does allow you to focus investigative efforts."

Can it help investigators determine a likely location of the next crime?

Unfortunately, Rossmo said today on ABCNEWS' Good Morning America, "That is a much harder challenge."

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