Criminal case investigations often start with Locard’s “exchange principle” – “It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of this presence.”  As otherwise described, “[t]his theory states that, whenever two objects come into contact, an exchange of materials occurs between them.” Mistek et al,  , Toward Locard’s Exchange Principle: Recent Developments in Forensic Trace Evidence Analysis, (last visited July 17, 2023).

That principle, or a variant thereof, is at the heart of toolmark comparison testimony – the tool “leaves traces of its presence” on an item at the crime scene:  the pliers or screwdriver mark the lock or window frame; the firearm marks the bullet and cartridge case.  But what has plagued trials is the claim that this tool – the one recovered from or linked to the accused – is the source of that impression.  Two recent decisions and one study bear on this issue.

The first comes from a news report on tool mark ‘analysis.’  In a 1991 prosecution in Colorado, where the charge involved the creating and detonating pip bombs, a tool mark analyst made a number of bold claims, all pertaining to needle nose pliers found in the suspect’s home:

·        all tools possess unique identifiers at a microscopic level, and these unique characteristics imprint a signature mark on other substances, such as wire, that come into contact with the tool.

·        an examiner can microscopically determine whether a “suspect tool” was responsible for the striations made on a wire that are caused by the unique manufacturing marks left on the tool.

·        he had never encountered a situation in which the mark left by a tool was not unique.

·        he had identified three tools seized from Genrich’s room — to the exclusion of any other tool — as the tools used in the creation of one or more of the bombs.

People v. Genrich, 2019 COA 132M, P20-P23.  One might pause and ask

  • how is that knowable, that no two pliers in the entire world leave the same marks?

  • even if that is knowable, what degree of magnification is required to get to the level of detail where it can be certain that there are no other pliers with the same markings?

  • Finally, how proficient is this expert at making such visual comparisons and not missing or misinterpreting markings or their absence?

Through litigation spearheaded by the Innocence Project, a new trial was recently ordered.  As reported, the Judge’s OPINION concluded that “the testimony of the expert was unreliable because there are no scientific principles underlying or supporting Agent (John) O’Neil’s opinions that would render such testimony reliable and thus admissible.” (last visited July 18, 2023).  The OPINION of the post-conviction court was unequivocal – “the individualization testimony of the people’s expert was inappropriate and would be inadmissible at a new trial. ..[T]here are no scientific principles underlying or supporting  Agent O’Neill’s opinions that would render such testimony reliable and admissible…”  People v. Genrich, ORDER, July 7, 2023.

The same concern with the limits of scientific foundation let to the Maryland Supreme Court ordering a new trial after similar “individualization” testimony, this time involving firearms.  Here is what the Court analyzed and concluded:

At the trial of the petitioner, Kobina Ebo Abruquah, the Circuit Court for Prince George’s County permitted a firearms examiner to testify, without qualification, that bullets left at a murder scene were fired from a gun that Mr. Abruquah had acknowledged was his. Based on reports, studies, and testimony calling into question the reliability of firearms identification analysis, Mr. Abruquah contends that the circuit court abused its discretion in permitting the firearms examiner’s testimony.  The State, relying on different studies and testimony, contends that the examiner’s opinion was properly admitted.

…[T]he examiner should not have been permitted to offer an unqualified opinion that the crime scene bullets were fired from Mr. Abruquah’s gun. The reports, studies, and testimony presented to the circuit court demonstrate that the firearms identification methodology employed in this case can support reliable conclusions that patterns and markings on bullets are consistent or inconsistent with those on bullets fired from a particular firearm. Those reports, studies, and testimony do not, however, demonstrate that that methodology can reliably support an unqualified conclusion that such bullets were fired from a particular firearm.

Abruquah v. State, 2023 Md. LEXIS 297, *2-3.

There are studies showing proficiency for trained examiners in matching fired cartridge cases to the weapons that fired the projectiles.  Guyll et al, VALIDITY OF FORENSIC CARTRIDGE-CASE COMPARISONS, PNAS, Vol. 120, No. 20 (May 8, 2023). (last visited July 18, 2023).  Yet even there one can find limitations – “restricting evaluation to only the conclusive decisions of identification and elimination yielded true-positive and true-negative rates exceeding 99%, but incorporating inconclusives caused these values to drop to 93.4% and 63.5%, respectively.” Id.

The study has other limitations – there is no showing that the condition of the examined cartridge cases was similar to that of cases recovered from crime scenes where environmental conditions may affect the clarity of the impressions; and of course the ability to match cases to a limited universe of source firearms is different from the issue of whether any other firearm in the world could have left similar [identical] markings.

Read together, what are the lessons?  Toolmark examiners may have much to offer in a particular case, but the data and research are not supportive of source attribution.  More research is needed; and it is targeted, well-financed, and well-resourced litigation that is required to expose forensic examiner overstatements  for what they are – not science but beliefs.  And beliefs should not be the basis for a criminal prosecution.



Leave a Comment