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Locard’s Principle of Exchange has been an absolute fundamental in criminal forensics for a century. The concept that the perpetrator will always take traces of the victim and the scene with them, while leaving traces of themselves in exchange, is the basis of all modern investigation.
However, the principle has gotten a little more complex with how sensitive DNA tests have become in recent years. Secondary transfer of human DNA has been demonstrated through handshakes. Now, a study has found that fingerprint brushes used at crime scenes to find latent prints could actually be picking up and then dropping genetic material in different locations.
The DNA was found in low-copy number techniques, according to the Journal of Forensic Sciences study, authored by forensic scientists at Florida International University.
“The dusting of latent prints may dislodge cellular debris from the latent print or substrate. That debris then adheres to the brush,” they write. “This brush is then used on another item of evidence, or at another crime scene, where it is subject to the same mechanical maneuvering and where it can dislodge cellular debris, leaving traces of biological evidence not pertinent to the evidence being handled.”
The more-exacting polymerase chain reaction process of amplification led to detection of DNA transfer: in 5 of the 12 samples in the 28-cycle process, and a startling 10 of 12 tests using a post-PCR cleanup process.
But the risk of false associations based on the contaminated DNA was only “moderate,” considering their laboratory conditions and analytic procedures, they conclude.
Since the possibility exists, however, standard protocols to handling latent prints before DNA testing needs to be established to eliminate the possibility of false results.
“Under LCN conditions, it may be possible to obtain DNA results that are not relevant to the case due to a secondary transfer by fingerprint brush contamination,” they conclude. “Comparisons to these results may lead to matches or inclusions thereby potentially producing false associations between the evidence and crime scene.
“Improper procedures may lead to false exclusions or false association between evidence and crime scene,” they add.
Bruce McCord, the lead author of the study, and his team at Florida International University were the recipients of the most National Institute of Justice awards during 2015, totaling $1.5 million – partly for their DNA analysis work, and also for studies into forensic chemistry and other topics.
McCord told a university publication that he was working on a DNA analysis method for on-scene results within six minutes.