Laser Technology Cracks
14-Year-Old Cold Case in Virginia
Derek Hardy and Bill Holtkamp
On July 21st, 2001, a female victim reported a burglary, assault and attempted rape that had oc- curred at her residence in Fairfax County, Va. With no actual rape, there was no semen or other fluids, from which to extract DNA for database profiling. However, there were several objects handled by the perpetrator at the crime scene, including a pair of sunglasses belonging to the suspect. These
were processed in the standard manner for latent prints at that time, including superglue (cyanoacrylate)
and black powder, followed by tape lifting. Unfortunately, this resulted in some ridge detail, but nothing
approaching the quality needed for AFIS matching purposes. Several
other items were sent to the forensic lab and swabbed for DNA but
without success. With no print, no DNA, and no other trace evidence
to go on, the case inevitably went cold. In 2014, a Fairfax County cold
case detective, Jeremy Hinson, revisited the case, and on July 9th,
decided to see if new latent print imaging modalities could reveal any
usable print details.
Laser on Sunglasses
During August 2014, we examined three pieces of evidence from this
cold case that had previously been processed for trace evidence: a black
nylon wallet, a black cordless phone, and a pair of black-framed sunglasses.
Examination of these clearly showed that prior testing had occurred. The
wallet had been processed using ninhydrin, and the
phone had been processed using superglue and black
powder. Neither of these objects showed any promise or
potential for further processing. However, the sunglasses
still showed a ridged area, and we decided to focus all
further efforts on this area. The Fairfax County Police
Department had acquired a forensic laser in 2010, and
our first course of action was therefore to treat this area
with rhodamine and photograph the area with this green
laser at a wavelength equal to 532 nanometers.
Laser illumination is now widely accepted as the
best approach for capturing fluorescent images of prints
and other organic trace evidence—either treated with
rhodamine or even from native (incipient) fluorescence.
The principles of fluorescent imaging are schematically
illustrated in figure 2. The object or surface under test
is illuminated by light in particular wavelength band,
usually green. The natural fluorescence or dye-aided fluo-
rescence of trace organics occurs at a longer wavelength
in the yellow and orange part of the visible spectrum.
Figure 1—Original lift im-
age of ridge details found on
the suspect’s sunglasses.
8 Forensic Magazine | www.forensicmag.com FEBRUARY/MARCH 2016
Figure 2—Forensic light sources, including lasers,
are used to excited fluorescence in trace evidence.
Optimum images can be obtained by excitation with
blue/green light with a wavelength in the 488-535 nm
range. Emission is then observed through an orange
filter somewhere around the 580nm wavelength band,
which blocks the intense scattered light in the illumination (green) band.