Locard's exchange principle

In Forensic science, Locard's exchange principle (sometimes simply Locard's principle) holds that the perpetrator of a crime will bring something into the crime scene and leave with something from it, and that both can be used as forensic evidence. Dr. Edmond Locard (13 December 1877 – 4 May 1966) was a pioneer in forensic science who became known as the Sherlock Holmes of France. He formulated the basic principle of forensic science as: "Every contact leaves a trace". Paul L. Kirk expressed the principle as follows:

Fragmentary or trace evidence is any type of material left at (or taken from) a crime scene, or the result of contact between two surfaces, such as shoes and the floor covering or soil, or fibers from where someone sat on an upholstered chair.

When a crime is committed, fragmentary (or trace) evidence needs to be collected from the scene. A team of specialized police technicians go to the scene of the crime and seal it off. They both record video and take photographs of the crime scene, victim (if there is one) and items of evidence. If necessary, they undertake a firearms and ballistics examination. They check for shoe and tire mark impressions, examine any vehicles and check for fingerprints.

The case studies below show how prevalent Locard's Exchange Principle is in each and every crime and . The examples using Locard's Principle show not only how the transfer of trace evidence can tell the tale of what happened, but also how much care is required when collecting and evaluating trace evidence.

Karola and Melanie Weimar, aged 5 and 7, lived with their parents, Reinhard and Monika, in Germany. They were reported missing on 4 August 1986. Their bodies were found on 7 August. They had been murdered.

Monika first said the children had breakfast, then went to a playground. Three weeks later she said they were already dead when she returned home the previous night: Reinhard was sitting on the edge of Karola's bed, weeping and confused; he then disposed of the bodies.

Both parents were suspected, but Monika was having an affair, and was seen where Melanie's body was later found. She was convicted, but after serving her sentence, was released in 2006.

Investigators determined what clothes Monika was wearing on 3 and 4 August, but not Reinhard's clothes, so only fibers from her clothing were identified on the children's bodies, yet they were also constantly in contact with him.

...
Wikipedia