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Crime Scene Fingerprints

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Fingerprints at the scene of the crime are of utmost importance in criminal investigative work because they may result in positive identification of the culprit, even though there are no eyewitnesses. Often a single-even fragmentary-impression forges an essential link in the chain of evidence connecting the criminal with the crime.

Fingerprints found at crime scenes are referred to as "latent" (hidden) impressions, because in most cases they are difficult or impossible to see and require "development" to make them visible for preservation as evidence, and for comparison with fingerprints of known persons.

Each ridge of the finger is dotted with a line of sweat pores, from which perspiration normally exudes, eventually accumulating in an amount sufficient to cover the ridges. When an object is touched, an outline of the ridges of the finger is left on it, in an almost invisible deposit of perspiration. Although there are no sebaceous glands in the ridges themselves, oily matter adhering to the fingers may also be contained in the latent print.

Latent prints may be left on any smooth surface, and those that are present on nonabsorbent surfaces are readily made visible by dusting them with fingerprint powder, which adheres to the moisture or oily matter present in the latent prints. The powders are finely ground and made in a variety of colors to contrast with the surface being examined. The types most commonly used are gray powder, composed generally of aluminum and chalk, and black powder, which contains lampblack. To dust a latent print the powder is applied with a soft-bristle brush.

Latent prints left on paper or cardboard dry rapidly and are best developed by the use of chemicals. Those prints that contain a large amount of oily matter will appear when they are exposed to iodine vapor in a small cabinet. Absorption of the vapor turns the ridges a yellowish brown color. Because the fumes evaporate rapidly, iodine prints must be photographed quickly, although they can be "fixed" to preserve them for a considerable time.

Many latent prints on paper and cardboard can be developed by dipping the specimens into a solution of silver nitrate. The prints, which become visible through the action of light in breaking down the silver compound formed in them, have a reddish brown appearance.

An organic chemical, ninhydrin, is used also as a latent print developer on paper surfaces, producing an impression of a maroon or purplish color. Through the use of these chemicals it is possible to develop latent prints many weeks, or even months, after they were made.

No other methods of developing latent prints have proved so efficient as those just mentioned. (A few novel methods include the application of magnetic powder to the print with a wandlike device, the use of powders which fluoresce under ultraviolet light, and the use of powders applied by pressure spraying.)

Latent prints are preserved by the criminal investigator for comparison, and as evidence, by "lifting" or by #photographing#. Powdered prints are "lifted" on tape having a tacky surface, for which both transparent and opaque tapes are commonly used. The prints are preferably photographed directly on the object. Either the photograph or the "lift" may be used in court as fingerprint evidence.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

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