Saturday, January 15, 2011

Locard's Exchange Principle

Try as they may, perpetrators are learning that it is indeed impossible to commit the perfect crime.
The same high-tech, forensic tools giving today's investigators an upper hand in their quest for the truth can help solve crimes of years gone by. Cases may grow cold, but they are never closed.
A motivating factor in the development of forensic science has been Locard's Exchange Principle, which dictates that "with contact between two items, there will be an exchange." It’s the basis of trace evidence collection at a crime scene.
Trace evidence is material found at a crime scene that can prove a suspect had been there.  No matter how much a person tries to sanitize a crime scene, something is left behind.
As a medical examiner in World War I, Edmond Locard analyzed stains or dirt on fallen soldiers' uniforms to help the French Secret Service determine where and how the men had died. He opened the world's first crime investigation lab in 1910 at Lyons, France.
Locard wrote, "It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of his presence."
Perpetrators bring something to — and take something away from — crime scenes. Things brought to the scene could be finger prints, foot prints, vehicle tire patterns, particles of hair or skin, bodily fluids such as saliva, semen or blood and much more. Contact with the victim makes the killer a magnet for things he takes with him — like blood spatter, hair, dirt, clothing fibers and other evidence common to the scene and the victim.
Investigators' cases are developed and strengthened if they can trace the origin of items found at crime scenes or find traces of the victim on a suspect.
"Physical evidence cannot be wrong; it cannot perjure itself; it cannot be wholly absent," Locard wrote. "Only human failure to find it, study and understand it, can diminish its value."
Knowledge of Locard's Exchange Principal may encourage people to assist local police after reading or seeing reports of a crime. An abandoned vehicle in the neighborhood could be a clue, as could seeing a neighbor or someone you know uncharacteristically cleaning his vehicle late at night or at a car wash when he usually cleans it at home. You may see someone throwing items out of a car window or displaying visible signs of blood or evidence that he or she had been in a struggle. An obvious change in a person's mood or unexplained restlessness could be revealing.
The slightest bit of information you offer to investigators could make the difference between a cold case and a conviction.