In 1936 the wife of an NBC executive was killed in their Manhattan brownstone. She had been strangled with her pajama top and left in the bathroom. All indications were that she had known her killer, and when there appeared to be few clues except some twine used to bind her, a chemist was brought in to examine the crime scene.
In the bedroom he found only one strand, half an inch long, of stiff white hair, which he soon identified through a microscope as horsehair. Since two furniture movers had delivered a horsehair couch that morning and it was those men who reported the body, the detective in charge speculated that one of them had paid an earlier call. He identified the likely culprit and then found a connection via the piece of twine, because it had sufficiently distinctive markings to be traced to a manufacturer and distributor. It turned out that the same twine had been sold to the furniture store. Using this evidence to put pressure on the suspect, the detective got the confession he needed for conviction. This was one of the early cases where technology was used on material fragments to solve a murder.
Every person who is physically involved in a crime leaves some minute trace of his or her presence, and often takes something away. This is Dr. Edmond Locard's principle of contact, proposed when he began his forensic laboratory in 1910. He closed a case two years later by examining what was under the fingernails of a female victim, and thereby showed how seemingly insignificant matter can make all the difference.
No matter how much someone tries to clean up a crime scene, something is generally left behind. It may not always be detected, but it's difficult to take any kind of violent action without shedding something. This principle became the motivating factor in the development of forensic science.
Trace evidence, though often insufficient on its own to make a case, may corroborate other evidence or even prompt a confession. Because trace evidence can be any number of things, from a paint chip to a piece of glass to plant debris, there are numerous different methods used for analysis. For some objects, there is a large database available for comparisons, while the science of others has not advanced that far. The main point is that some apparently foreign object or piece of material is present at a crime scene and tracing its origin can assist in an arrest and conviction. Similarly, finding some trace from the victim or crime scene on a suspect can have a strong impact on a case.
While there is no end to the types of trace evidence that can be found, most investigations center on fiber or hair, which is easier to see than pollen or dirt. Cases involving those substances will be covered at length, but some of the more unique types of trace evidence that have helped to close cases are included as well.
Let's look first at a controversial case involving fiber.