During my career as a Crown Prosecutor, my goal has been to ascertain the truth as to what happened in any given incident because it is only upon discovering the truth that we can strive for justice, however illusive that goal may be to achieve in practice.  In search of the truth, it is of critical importance that all potentially relevant evidence be collected and evaluated, not just by the police but also by experts, where applicable, and by prosecutors.

In some cases, no expert analysis is required or even feasible as the evidence in the case is straightforward and can be fairly evaluated by counsel and the trier of fact without additional assistance.

In other cases, experts are needed to tell us what the facts actually are in a case, though not necessarily what they mean in terms of criminal liability.  Consider these examples:

• The police come upon the scene of a fatal motor vehicle collision where all parties have been killed and there are no eyewitnesses to the collision. A collision reconstruction expert will be required to evaluate the scene evidence and the vehicles involved in order to determine the true facts.  It is to the expert we will look to determine the vehicle dynamics, environmental and other factors that led to the collision.  The expert may not be able to tell us why, for example, one vehicle was on the wrong side of the road.  The collision may have been the result of criminal driving, distraction, fatigue, medical distress, animal avoidance or passenger intervention.  The expert cannot answer all questions about the collision but can at least determine vehicle positioning at the point of impact and prior to impact.
• A DNA expert may be able to determine whose DNA was found on or within the victim of an apparent sexual assault but not the circumstances of the incident itself. Specifically, the DNA expert cannot assist on the issue of how the DNA got to where it was found, the question of consent or the role, if any, of alcohol or drugs.
• A homicide scene is replete with thousands of bloodstains on the walls, ceiling, floor, windows and furniture. Coupled with DNA analysis to identify the contributor(s) of the stains, a bloodstain pattern analyst may be able to reconstruct much of what happened in the events leading up to and during the homicide, but the expert cannot tell us why it happened, whether self-defence was at play, whether drugs or alcohol were involved, or whether the suspect was criminally responsible.

In each of the above examples, the evidence of the expert is very important in understanding key aspects of the case but will not conclusively determine criminal liability, should it exist.  Eyewitness testimony, statements of involved parties, police observations and other evidence is required to reach those conclusions.

Finally, in some cases, expert evidence will provide the final, conclusive piece to the investigative puzzle that will determine the truth and allow for the assignment of criminal fault.  This was the situation in one of the most significant cases of my career.  In 1990, a female cab driver was found dead on the street in a mountain resort town in Alberta, Canada.  She had been stabbed over 30 times and her cab was missing.  A fellow cab driver, unable to reach his colleague on the radio, searched for her and saw her cab being driven by an unknown male.  After a brief chase, the unknown male abandoned the cab and ran into the woods. The murder weapon was found the next day on a nearby driveway.  There was blood in the driver’s area of the cab.  Serological testing showed that some of the blood did not belong to the victim. It was therefore evident that her killer had been injured in the struggle.  However, despite a very lengthy multi-national investigation, no suspect was developed.  Almost two years after the murder, someone looked at a police poster which showed, amongst other items, the murder weapon.  The witness commented that the knife looked like one a particular person had around the time of the murder and reported that observation to the police. Police determined that this person had been working in the resort town at the time of the murder and had left town hours after the murder.  After a lengthy undercover operation in another province, genetic samples were obtained from the suspect and a DNA profile was developed.  The suspect’s DNA profile was compared to the blood in the cab and a match was found to exist.  This was the first murder case in Alberta where the only evidence against the defendant was DNA evidence.  Expert evidence called by the Crown on this aspect of the case consisted of a DNA expert and two population geneticists.  The DNA evidence was the key to this case.  The expert witnesses were the ones who effectively solved the case.

In general, expert evidence tends to favor the prosecution, not because experts are predisposed to help the prosecution, but because generally the police are able to properly assess the case and identify the likely suspect and the expert evidence is confirmatory in nature.  Yet, there are cases where it is the expert evidence that shows that the event did not occur the way the police thought it did or that the defendant was not the responsible party, or both.  Expert evidence may present a radically different view of an event and result in the discontinuance of a prosecution and sometimes the identification of the real perpetrator.  Consider the many cases where DNA evidence has resulted in exonerations years after conviction.

There is no prosecution truth.  Nor is there defence truth.  There is only the truth.  The goal of investigators and prosecutors should be to ascertain the truth in all cases. Once we have that vital information, then important decisions can be made about whether anyone should be charged and if so, for what offences, or whether an existing prosecution should be resolved or discontinued.  Some prosecutors (mostly in the United States) keep statistics of their conviction/acquittal ratio.  There is no role for keeping score in the criminal justice system.  The goal should always be to search out the truth and if the truth is that the suspect or defendant did not do what he/she is accused of doing, or has an exonerating defence, then justice demands that the prosecution comes to an end.  If the truth is that the suspect or defendant did it and has no exonerating defence, then we are well equipped to commence or maintain a prosecution.  The pursuit of a just result is not a lofty goal; it is the mandate of a just society.