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Getting a Clue

The latest evidence collection tools and techniques allow investigators to piece together the story of the crime before the scene is cold.

May 01, 2002  |  by David Spraggs


Evidence collection is one of the most important components of criminal investigation and prosecution. Evidence tells a story, allowing investigators to develop sound theories based on the totality of circumstances of the available information. Physical evidence can positively link a suspect to a crime, ensuring a conviction, or it can prove innocence.

Technological advances in instrumentation and equipment let law enforcement continually rethink what is possible when collecting forensic evidence. Utilizing new products and technologies often allows more evidence to be located, identified, and collected. More physical evidence leads to an increase in crime clearance rates and successful criminal prosecutions. However, it's important to remember that technology is merely a tool; new equipment will never replace well-trained personnel.

Crime scene processing and evidence collection is an involved process. Items that may or may not have evidentiary value need to be located, identified, collected, packaged, analyzed, and stored under less than ideal circumstances, and often under scrutiny from the media and the public. Evidence varies in every way imaginable, and there are many extraordinary crime scene products and tools available to help us locate evidence that may have previously gone uncollected.

Short Tandem Repeat

In order for a DNA sample to be entered into the CODIS (Combined DNA Index System) database, which is administered by the FBI, it must be analyzed using short tandem repeat (STR) technology. STR analysis supports the 13 loci (genetic markers) chosen for human identity testing. Companies like Applied Biosystems, which produces industry standard instruments and chemicals, support forensic human identification testing and research and have been instrumental in ensuring that STR technology is accepted in court.

The most significant benefit of this new technology as it relates to evidence collection is that the sample size required for successful DNA testing is dramatically reduced. Previously, a fairly large amount of product was necessary for DNA testing, especially considering that half of the sample must be retained for the defense. Now, DNA technology has progressed to the point where an incredibly small sample is all that is needed for DNA testing.

This means that investigators at a crime scene can be creative in identifying pieces of evidence that potentially have evidentiary DNA value. Instead of focusing on the visible blood at a scene, we can look in other areas. For example, cigarette butts, cans, glass bottles, glasses, dirty silverware, and even licked envelopes or used stamps all can contain critical DNA evidence.

A few years ago, the Boulder (Colo.) Police Department (the author's agency) had a home invasion sexual assault where the suspect wore a condom and had a shaved pubic area so that when he penetrated the victim he wouldn't leave any biological evidence. However, during the assault he licked the victim's breast.

The victim's breast was swabbed with a sterile cotton swab soaked in distilled water. The Colorado Bureau of Investigation analyzed the swab, and the suspect's full DNA profile was obtained from this miniscule sample of saliva. His DNA profile was entered into CODIS, and it was discovered that the suspect, who is still unidentified, committed sexual assaults in Texas and another jurisdiction in Colorado. Colorado now has a "John Doe" arrest warrant based on the perpetrator's DNA profile.

Few agencies are more advanced in the use of trace DNA technology than the Royal Canadian Mounted Police and the Vancouver (British Columbia) Police Department. Approximately two years ago, Vancouver PD investigated a kidnapping where the suspect accosted a young girl in front of her school. The kidnapper grabbed the victim's bicycle and forced her into his van. Police collected the victim's bicycle. After photographing the fingerprint ridge detail, crime scene technicians swabbed the bicycle frame where the suspect touched it and were able to develop a DNA profile. While this is an extraordinary example, it shows how DNA technology can allow investigators and technicians to think in new ways about evidence collection.

Tricks of the Light

Alternate light source (ALS) is a generic term used to describe instruments that output specific bands or wavelengths of light that are useful in locating physical evidence.

An ALS system consists of a powerful light (usually a xenon gas bulb or an argon laser) that can be tuned to output narrow wavelengths of visible light, between about 415 nanometers (nm) (violet light) and 700 nm (red light), depending upon the model. Most ALS systems also have long wave ultraviolet (365-nm) capabilities, and some have infrared (IR) capabilities.

Each of these bands of light is used for locating specific types of evidence, including biological fluids, hairs, and fibers. For example, many biological fluids-semen, urine, saliva, and vaginal fluid-fluoresce around 450 nm. Note: an ALS is not a presumptive test for any biological substance.

At 450 nm the instrument operator must wear orange-colored barrier filter goggles to block the blue light, while allowing light from the previously unseen evidence, which is visible as a brightly glowing area, to pass through the goggles. The evidence can be photographed with either a film or digital camera that is fitted with an appropriate filter.

The most basic ALS is a 365-nm ultraviolet (UV) light source. Many hairs and fibers, as well as certain biological fluids, are visible under this wavelength. A 365-nm UV light source is also used in conjunction with certain fingerprint processes, including fluorescent fingerprint powder and cyanoacrylate stains such as Ardrox.

Sirchie Finger Print Laboratories and many other companies sell this type of light source, with prices beginning around $50. Advantages include low cost and high portability. The major disadvantage is that these sources lack the flexibility to allow the operator to change to other wavelengths of light, thus fewer types of evidence can be located.

CONTINUED: Getting a Clue «   Page 1 of 2   »

Tags: Forensics, Chain of Evidence, DNA, Fingerprint Identification

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