Crime scene technicians and investigators at the scene handle the collection of fingerprint evidence that can be used for identifying a suspect. Fingerprint collection can begin once the crime scene has been thoroughly documented and the locations of the evidence noted. There are several categories of fingerprint impressions found at a crime scene, and each one is collected and processed differently.
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Plastic fingerprints are left in material that will mould to the fingerprint pattern, such as wet paint, soft wax, blood or window putty. After photographing from a 90-degree angle, remove the section of material containing the print. Try to retain excess material around it to avoid distortion during removal. Glue the section removed to a piece of rigid cardboard and protect by taping a clean cup or jar over the cardboard. Label the cardboard with identifying information and package carefully in a secure container.
Patent prints are left by someone who had ink, paint, blood or other kind of transferable substance on his hands. This kind of print is clearly visible to the naked eye and needs no dusting or other enhancement. Patent prints are merely photographed, and sometimes the item on which they are found is collected.
Latent prints need enhancement from powders, chemicals or special lights to become visible to the naked eye. Once dusted with extremely fine fingerprint powder, they become visible for photographing. They are then transferred with adhesive lifters to a special rigid card in a contrasting colour (for instance, black powdered prints onto a white card). Magnetic powders are effective to bring out latent prints on cardboard or coated papers, and argon lasers are used to visualise prints on non-porous surfaces. Items can be processed on site or taken to the lab for processing. Collect the item with a clean tool and place it in a clean, properly labelled container in such a way as to prevent any rubbing or friction.
Fingerprints on Absorbent Materials
Fingerprints on paper or other porous materials can be fumed with iodine or cyanoacrylate (a fast-acting glue). The oils in the print collect the fumes, making them possible to photograph. Use latex gloves or forceps to insert items containing possible prints into a clean protector. Investigators or on-site technicians often have special envelopes for this purpose that are imprinted with blank forms.
Fingerprints on Skin
If a victim was grasped firmly on a clean and hairless part of the body such as inner arms, thighs or neck, it is possible to recover fingerprints. The affected skin area should not be washed or disturbed before prints are processed by means of powdering or fuming, and this must be done within three hours after the victim's contact with the assailant.
In April 2011, Dr Xanthe Spindler and colleagues at the University of Technology Sydney in Australia announced a technique that uses acid-binding antibodies attached to gold nanoparticles to reveal amino acids in fingerprints. The technique is expected to be able to detect faint, aged or dry fingerprints that are difficult to capture using traditional fingerprinting methods such as powdering or fuming with cyanoacrylate. It is also expected to yield further results in old cases and to improve the ability to enhance fingerprints on human skin.
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- Science Encyclopedia: Forensic Science - Fingerprints
- RSC Publishing: "Nanoparticles Help Reveal Hidden Fingerprints,"; Joanne Thomson; April 4, 2011
- University of Technology Sidney: "A Step Towards a Revolution in Law Enforcement"; Terry Clinton; June 2, 2011