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When Crime Labs Fail To Follow The Evidence

Mistakes at crime labs sometimes lead to the guilty going free or to the innocent going to prison.
microscope in a laboratory
Crime labs do excellent, essential work, but they’re not perfect.
Photo: Chokniti Khongchum

As any fan of the various incarnations of the hit television series CSI can attest, to catch the bad guys you need three things: a properly equipped crime lab, highly trained and dedicated staff, and forensic scientists with the determination to follow the evidence wherever it leads.

If only real life was as ideal as television.

In the real world, crime labs are rarely furnished with state-of-the-art equipment; there just isn’t enough money. The staff, while no doubt diligent and dedicated, are merely human, prone to error. And the forensic scientists overseeing the work suffer from the same bias as the rest of us, a desire to capture the bad guys whatever the cost.

These flaws sometimes lead to the guilty going free or to the innocent going to prison.

A Crushing Workload

In a typical CSI episode, the intrepid investigators collect most of their evidence in the first few minutes of the story. Then, after a witness lets some incriminating pearl of information slip during an interview, they find one more piece of evidence that closes the case. And this usually happens twice in an episode, because the investigators rarely handle more than two or three cases each week.

The forensic scientists working in real state crime labs are rarely that idle. They may be called upon to examine tens-of-thousands of individual samples in thousands of cases every year.

Contaminated Evidence

At one US state crime lab, tests were conducted on the panties of a 10-year old girl, the victim of a violent rape. The girl’s uncle, who had a prior sex-crime conviction, had already confessed to the assault, but the prosecution needed hard physical evidence.

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The forensic scientist examining the panties found the evidence the prosecution needed, and more: genetic material that matched neither the accused nor the victim. Faced with this incongruous evidence that undermined their case, prosecutors allowed the accused to plead to a much reduced charge and cut a decade off his 26-year sentence.

Only later was it discovered that the mystery DNA was the result of a lab mistake. The scientist testing the evidence had accidentally contaminated it with DNA from another unrelated case.

Other cases have been compromised when lab technicians and scientists skipped critical steps or ran samples in the wrong order, accidentally threw out evidence swabs, misplaced notes, or used instruments and latex gloves contaminated with their own DNA and DNA from other cases.

It’s not difficult to understand how samples could be unintentionally contaminated.

In the early years of DNA fingerprinting, forensic scientists needed a blood or semen sample the size of a US quarter to produce reliable results, and tests took about six weeks to complete.

Today, using polymerase chain-reaction (PCR) tests, results can be obtained in a few days from a tissue specimen of only about 40 cells, a sample too small to be seen with the naked eye.

These minuscule samples are thus much more susceptible to contamination. Someone standing on the other side of the room could contaminate the sample with an action as otherwise innocuous as a sneeze. This could occur at the crime scene before the sample is collected, at the lab when the sample is being analyzed, or at any time in-between.

Unintentional Bias

Ideally, crime lab workers are unbiased and impartial professionals. Their sole goal is delivering accurate and impartial test reports, results based solely on scientific principles, without regard for which party — the prosecution or the defense — may benefit from their findings.

But who pays the salary for state crime lab workers? The same taxpayer’s purse that pays for police, prosecuting attorneys, and judges. And who delivers the evidence to the crime labs? Police detectives, who may already have a suspect in mind and who may — unintentionally or otherwise — communicate this to the scientists examining the evidence. (Remember, this is the real world, not the idealized CSI environment on television where the crime scene investigators not only perform lab tests, but also collect the evidence, interrogate witnesses, and track down other clues.)

A case at another crime lab reveals the inevitable result of this conflict of interest.

A forensic scientist reported a DNA match implicating a suspect in a brutal rape and attempted murder. Only later did she realize she had misinterpreted the results; the evidence actually excluded the suspect. The false-positive match, caused by the scientist rushing to complete the work and satisfy the police who assured her they had a strong suspect, nearly sent an innocent man to prison.

Intentional Fraud

Sometimes there is no mistake; errors are intentional. The state-of-the-art FBI crime lab in Quantico, Virginia was the site of a scandal when it was discovered that a DNA analyst had intentionally falsified numerous reports over a two-year period. Some of her flawed reports may have been included in CODIS, the FBI’s Combined DNA Index System, a national database used to track the genetic profile from convicted offenders and from unsolved crimes.

Other cases, often only discovered after innocent people have been convicted, included forensic scientists called as expert witnesses exaggerating their credentials, testifying about tests never conducted, hiding exculpatory evidence that may have exonerated the accused, and exaggerating statistics to support their conclusions.

Fixing The Problem

Efforts are being made to correct these problems. Possible measures include ensuring that evidence is always preserved so it may be retested, enforcing requirements for full disclosure, setting higher standards for the education and certification of crime lab staff, and frequent blind proficiency tests of both crime labs and individual workers.

Of course, all of this costs money.

In the US, Congress passed the 2004 Justice For All Act which requires states seeking federal funding for crime labs to carry out external and independent investigations when allegations of misconduct arise.

Gil Grissom, played by actor William Petersen on the original CSI: Las Vegas says, “There is always a clue.” For the sake of the victims of crime, for those innocent of those crimes, and for all of us who live with the consequences of crime, we have to do everything we can to make sure every clue is found and properly handled.

Jonathan Mycroft is a freelance writer and author. His work includes suspense-thrillers, sci-fi/fantasy, humor, and stage plays. His latest novel, Trial by Fear, follows a serial sex killer on the lose after another man is wrongfully arrested for the crimes. You can read an excerpt from Trial by Fear at www.jonathanmycroft.com