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Why a Criminalistics Expert Can Be Helpful to Criminal Defense Investigations

by Deborah Stonebarger

I am a licensed private investigator and a criminalistics expert with 16 years of experience as a criminalist (a forensic scientist) in an accredited crime laboratory—achieving certification in Comprehensive Criminalistics by the American Board of Criminalistics. The criminalistics expert in me is quite thorough and mindful when it comes to reviewing discoveries containing anything related to evidence such as, proper crime scene processing and evidence collection, proper evidence packaging and storage, analyses in the laboratory, appropriate reporting, and proper testimony in court. However, the human being in me understands that the copious amounts of analytical notes, data, and lab reports can seem overwhelming and probably boring to many people reviewing discovery for a case. Perceptions and myths about forensic science can complicate the matter further. The private investigator in me wants to share information about forensic science to help my fellow investigators out there working in criminal defense cases.

People have unrealistic expectations of what information forensic science can provide, what capabilities the laboratories have, and how significant the analytical findings are. Jurors have been known to give more weight to the testimony of forensic scientists than other witnesses. Forensic evidence, such as DNA, can be exceptionally powerful to the justice system, but that does not apply to all cases or all evidence—jurors sometimes don’t understand that fact.

Testimony from forensic scientists that overstates evidence significance or fails to discuss the limitations of the interpretation of analysis, combined with the fact that jurors give extra weight to the testimony of forensic scientists, can lead to wrongful convictions. Some jurors have mentioned in post-trial polls that they decided to vote guilty as soon as they heard “DNA testimony.”

The articles The Impact of False or Misleading Forensic Evidence on Wrongful Convictions published by the National Institute of Justice in November of 2023 and Bad Science and Bad Statistics in the Courtroom Convict Innocent People published in Scientific American in December of 2023, further discuss the part that forensic evidence has contributed to wrongful convictions. Almost every discipline of forensic science, including footwear impressions, blood spatter, serology, firearms identification, latent prints, trace evidence, and yes, even DNA analysis all had errors that contributed to wrongful convictions.

Types of errors included incorrect identifications and exclusions, errors in lab reports, testimony errors that mischaracterized the weight or statistical significance of the findings, mishandling of evidence at either the law enforcement or laboratory level, and straight-up fraud. A criminalistics expert can help explain the “science parts” to clients and attorneys by thoroughly reviewing a case, and possibly offer an expert opinion that can help to mitigate potentially damaging evidence.

Poor laboratory communication, improper training of staff, and improper laboratory resources were cited in the mentioned articles above as some reasons for error. I would like to emphasize, though, that laboratory staff are human beings, and as such, mistakes will be made that don’t always have to do with improper training or resources. There are many procedures in place to catch as many of these mistakes prior to releasing the evidence analysis report. However, mistakes will occasionally slip through the cracks.

It is worth noting that most agencies don’t have very effective procedures concerning evidence testimony analysis— this can lead to forensic scientists testifying in manners that may over-inflate the value of the analytical results, fail to mention the limitations to the analytical results, or even start to testify in related areas for which they haven’t been properly trained. Using a criminalistics expert in post-conviction review cases or cases that initially led to mistrials can be vital to recognizing improper testimony, which can help impeach the original witness.

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I would like to illustrate issues that can arise out of one type of forensic evidence that people sometimes think is inscrutable or provides what some people think of as insurmountable evidence: DNA. Although it is true that some quantity of DNA can be detected almost anywhere, the significance of the DNA can and should be questioned. Trace DNA, sometimes referred to as “touch” DNA, is present in very small or “trace” quantities, can sometimes be in a degraded or lower quality state, and is often part of a DNA mixture. The presence of trace quantities of an individual’s DNA on the surface of an item does not necessarily mean that the person had direct contact with the item. There are documented examples of secondary and tertiary transfer of DNA to items.

One example is Individual A coughing or sneezing on an item such as a pen, thus leaving some of their DNA on the pen. That pen gets picked up and handled by Individual B, who now has some of Individual A’s DNA on their hand. Then, Individual B grabs a hammer and transfers their individual DNA along with small quantities of Individual A’s DNA to the hammer. It may be possible to obtain a DNA profile or partial profile from Individual A on swabs of the hammer even though they never touched the hammer.

Some people are known to shed cells more frequently than others and those people have a higher chance of leaving detectable quantities of their DNA on a surface, or because they are depositing more DNA, their profile may be detectable for a longer period of time than other people. This, too, plays a part in DNA transfer.

Spermatozoa provide DNA evidence in sexual assault cases. Did you know that depending on laundering conditions, some spermatozoa can survive being laundered and transferred to other items of clothing during the process? Finding a few sperm cells in the crotch area of someone else’s underwear that resides in the same house would bear further investigation, but finding a few sperm cells on some other piece of clothing in some random location might just be indicative of transfer in laundry and not that sexual activity occurred. If the whole context is not provided in a situation like this during testimony, jurors might hear “sperm cells” and “DNA” and assume the guilt of a sexual crime.

Trace DNA from commonly touched surfaces, such as a door handle on a public building, may lead to detecting a complex DNA mixture. In a case with a mixture of only two profiles, there is usually a major contributor and a minor contributor, allowing for a reliable interpretation of each profile. In cases with a mixture of three people, it may become more complicated to reliably interpret all three profiles. When there are cases with a DNA mixture of four or more people, the statistical significance of being able to detect a person’s DNA profile or partial profile becomes less significant.

As the DNA mixture becomes more complicated, it may be impossible to indicate the number of contributors. Some lab policies direct an analyst to refrain from interpreting complex mixtures. Even though some lab policies have threshold limits for reporting and interpreting DNA profile information, there may be some labs that have exceptions to that policy and are allowed to lower that threshold limit if the case meets certain criteria. Whatever the criteria, DNA reports should clearly state the limits of the significance of the findings; however, sometimes that gets overlooked.

Errors can happen in any forensic discipline in the laboratory, not just DNA. I chose to use DNA as an example in this article merely to emphasize the point that even the “gold standard” of evidence analysis has areas in which investigators should be cognizant while reviewing discovery in their criminal cases.

Each forensic discipline has its own procedures and criteria for interpreting results as well as suggested reporting language that should include applicable limitations, if any. Having a criminalist review the case can help with all types of forensic evidence.

As stated, errors affecting forensic evidence can happen not only in the lab but also at the crime scene and in the courtroom when it comes to testimony. The point of this article is to raise awareness of these potential errors. Due to some of the myths and misconceptions about forensic science and evidence analysis, your clients and attorneys may not be aware of the potential for finding mitigating or even potentially exonerating information within the crime scene and laboratory reports. Every defendant has the right to the best defense possible, and a criminalistics expert may be able to contribute to that.

About the Author
Deborah Stonebarger is the owner of Analytic Investigations in Arizona. She is a licensed private investigator and criminalistics subject matter expert. She is a former Senior Criminalist with the California Department of Justice. Ms. Stonebarger was certified in Comprehensive Criminalistics by the American Board of Criminalistics and is experienced in the analysis of crime scenes and evidence collection, biological evidence and DNA, controlled substances, forensic alcohol analysis, and forensic firearms analysis.

We’re always listening. Send your story submission/idea to the Editor: kendra@orep.org.

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