The Mechanics of Crime Laboratories

The American criminal justice system might best be compared to a track & field relay race where a baton is passed from runner to runner.  Similarly, evidence and information are passed from one law enforcement agency to the next until finally a case is ready to prosecute.  The amount of time and the number of agencies that participate in this chain depend on the complexity of the case.  The system works well when all of the agencies and individuals involved in an investigation perform competently and communicate effectively.  The system breaks down when they don't. 

Forensic science laboratories are but a single link in what is called a chain of custody.  Upon the discovery of a crime scene, evidence technicians photograph the scene and scour it for physical evidence likely to be helpful in solving the case.  This evidence is collected, packaged, and preserved for transport to a crime laboratory where it can be examined by scientists who have access to the appropriate facilities, supplies, and instruments. 

Even while in the crime laboratory, evidence may be passed from one scientist having a particular area of expertise to another scientist having a different area of expertise.  And as we will explain in section 2, the evidence must be sequenced properly to ensure that the first scientist to examine a piece of evidence does not contaminate or damage the evidence in a way that compromises the work performed by the next scientist.  Historically, latent fingerprint examiners were often the first to examine evidence so subsequent scientists would not have to worry about wiping away fingerprints.  But in the age of DNA, crime laboratories must frequently decide which evidence, fingerprints or DNA, is most important and likely to yield useful results.  As various analyses are completed and the necessary quality assurance checks are made, scientists issue written reports which are sent to the investigating agency to be added to the primary case file.

Once the crime laboratory's work is completed and the lab reports have been sent to the investigating police agency, the case is reviewed and prepared for the prosecutors who will then move forward with charges and eventually a trial or plea agreement.  In major cases, prosecutors may participate in the actual investigation to ensure that the work performed by police detectives is legally proper.  This often creates friction between police whose priority is to collect information and gather facts, and prosecutors who anticipate how these facts will be presented at a trial.  Ultimately, it comes down to personalities and the ability of people to work together towards a common goal.

Crime laboratories are very busy.  Many receive so many cases and so much evidence that they can't keep up.  This means it can take months for an analysis to be completed depending on what type of services are requested by the submitting police department.  Because the services of most public crime laboratories are free to police departments, there is nothing holding back eager investigators from calling on crime laboratories for help.

To get a better appreciation for how a crime laboratory works, think of a television repair shop where customers drop off broken televisions, explain what the problem is, and then come back to pick up the TV when the work is done.  If more TVs come in than what the current staff can handle, they develop a backlog that causes customers to wait longer than they'd probably like.  

While a crime laboratory is much more complex than a TV repair shop, the example is useful.  Evidence is dropped-off at a crime laboratory and then picked-up when the work is done.  The period of time measured from the day the evidence is submitted to the day the report is issued to the police or the courts is called turnaround time.  The actual number of cases or requests pending analysis is called backlog.

This raises an important question.  What is the difference between a case and a request?  Well, in a single case, there may be one or hundreds of evidence items submitted to the crime laboratory.   Furthermore, any one item of evidence may have one or several requests assigned to it.  For example, police detectives submitting a bloody knife to a crime laboratory may ask for both fingerprint and DNA analyses.  This would be two requests.  In such a case, the fingerprint examiners and DNA analysts may have to confer with each other to determine who gets the evidence first.  The fingerprint examiner runs the risk of contaminating the blood evidence, while the DNA analyst could wipe away any fingerprints that may be present.  This requires careful collaboration to ensure that the value of the knife is not compromised.  But as one might expect, the more requests that are assigned to a piece of evidence, the longer it will generally take the lab to process it.

This raises a second question.  Do different areas, or disciplines, in a crime laboratory have different backlogs and turnaround times?  The answer is yes.  In the knife example, the DNA unit may have a 15-case backlog with an average turnaround time of six weeks.  But if the fingerprint unit has a 300-case backlog with a four-month average turnaround time, then the forensic scientists may want to think carefully about delaying the DNA work due to the heavier backlog in the fingerprint unit.       

This may seem like a difficult decision to make, but realistically, violent crimes or cases with high public significance are prioritized ahead of other waiting cases.  Therefore, a homicide case will rarely be delayed by a burglary.  This point illustrates that raw backlog or turnaround-time statistics don't represent a complete picture of a crime laboratory's progress.  There are many qualitative considerations that come into play on a case by case basis.