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Connecting the Dots: The Science of CSI

Trace

Edmond Locard

Edmond Locard was a forensic scientist in Lyon, France, from 1910 until his death in 1966. He is remembered today for his theory that “every contact leaves a trace.” Known by forensic scientists as “Locard’s Exchange Principle,” it means that when an individual commits a crime they will leave a trace of themselves at the scene and take something from the scene when they leave. Forensic scientists classify this phenomenon as trace evidence that encompasses everything from hairs, fibers, paint, fingerprints, and glass to soil, plant material, bodily fluids, bloodstain pattern analysis, and many other objects.

Photo of Edmond Locard sitting at a desk in front of a wall of books

Edmond Locard at his desk. Image source: Yount, Lisa. Forensic Science: From Fibers to Fingerprints. Chelsea House, 2007. View Address

“Matching” is an important part of working with trace evidence. It can be as varied as matching shoe prints found at a crime scene (top) or cuts made with a knife on tree branches (bottom).

Image source: Kirk, Paul. Crime Investigation: Physical Evidence and the Police Laboratory. Interscience, 1953. View Source

Bloodstain Pattern Analysis

Paul Kirk, a professor of biochemistry, set up the first U.S. academic criminalistics program at the University of California, Berkley, in 1929. Dr. Kirk gained national attention when he served as an expert witness in the case of Air Force doctor Sam Sheppard who had been found guilty of murdering his wife. In 1966, defense attorney F. Lee Bailey hired Kirk during Sheppard’s retrial. Among other findings, Dr. Kirk was able to establish the relative positions of the attacker and victim based on blood pattern analysis, concluding that the killer was left-handed (Sam Sheppard was right-handed). Based in large part on Dr. Kirk’s testimony, the jury acquitted Sheppard.

The effect of angle on blood splatter. The image shows blood dropped from a height of three feet at A-90 degrees, B-80, C-70, D-60, E-50, F-40, G-30, H-20, and I-10. Image source: Kirk, Paul. Crime Investigation: Physical Evidence and the Police Laboratory. Interscience, 1953. View Source
Image showing the effects of velocity on bloodstain patterns

The effects of velocity on bloodstain patterns. This image shows blood dropped from a 5 degree angle at: A-3 inches, B-6 inches, C-9 inches, D-1 foot, E-1.5 feet, F-2 feet, G-3 feet, H-4 feet, I-4.5 feet, J-5 feet, and K-6 feet. Interpretation of bloodstain patterns, wrote Paul Kirk, is “often the greatest significance in determining not only the course of [criminal] events, but frequently their cause as well.” Image source: Kirk, Paul. Crime Investigation: Physical Evidence and the Police Laboratory. Interscience, 1953. View Source