Many criminal trials in Pennsylvania and other states, especially those involving violent crimes such as murder, assault, or robbery, often turn on testimony from scientists who claim to possess special expertise in examining evidence from the crime scene that can be used to prove that the defendant was on the crime scene or touched the victim. One of the most common forms of such evidence is the comparison of hair samples from the crime scene or the victim's body with samples taken from the defendant. In a development that stunned many people in the criminal legal system, the Federal Bureau of Investigation last week admitted that its forensic specialists in the hair laboratory had greatly exaggerated the evidentiary weight of such evidence.
In response to an investigation by a Washington newspaper, the FBI acknowledged that testimony by experts from its microscopic hair comparison unit during the two decades prior to 2000 greatly overstated the likelihood that matching hair samples proved the defendant's location at a specific location or other involvement in the crime. In a study conducted by the National Association of Criminal Defense Lawyers and the Innocence Project, 26 of 28 FBI examiners overstated the evidentiary weight of hair fiber comparisons. The experts appear to have slanted their testimony by relying on statistical evidence from past cases that were likewise slanted in favor of confirming the defendant's identity. In reality, no acceptable scientific research has ever existed showing a visual comparison of hair fibers led to a reliable identification.
The FBI stressed that in many cases where hair comparisons were used, other evidence of guilt was presented to the jury. Nevertheless, the FBI and Justice Department are working with the prosecutors in these cases to determine if grounds still exist for an appeal of the verdict.
The FBI's admission shows that forensic evidence may not be as reliable as the prosecution asserts. Any person facing a criminal charge in which forensic evidence is likely to be offered by the prosecution may benefit by consulting an experienced criminal defense lawyer who is familiar with challenges to forensic evidence. A successful challenge to such evidence, either by excluding it entirely or by vigorous cross-examination of the expert offering the evidence, may mean the difference between conviction and acquittal.
Source: New York Times, "Report: DOJ, FBI Acknowledge Flawed Testimony From Unit," Associated Press, April 18, 2015