How to explain away forensic DNA at a crime scene

Let’s assume that a lab has come back with a report that a person’s DNA was found at a crime scene, and let’s assume that the technician at the lab hasn’t committed some kind of fraud.  For instance, let’s assume that the technician hasn’t falsified the results.  What could explain a person’s DNA at a crime scene if that person was really and truly not the person who committed the crime.

Contamination.  If police suspect that a person is involved in a crime and want to test that person’s DNA against DNA found at the crime scene, contamination may occur which could seem to show the person was at the crime scene who he or she was no where near the crime.  Police will typically collect “knowns” from suspects either through that person’s consent or by getting a search warrant. They will use a buccal swab of the person’s inner cheek to collect DNA, which they will try to match against evidence collected from the crime scene.  If, however, in the process of doing this match, the lab technician inadvertently or mistakenly mixes the “buccal swab” with the unknown DNA on the evidence, the person’s DNA could end up on the evidence, having been mistakenly been put there by the lab technician. Then further testing would show that the person’s DNA was present on the evidence which would obviously be a result produced by contamination, and not accurate testing of the evidence.

Contact DNA.  DNA can be transferred from person to person.  Let’s assume that the real criminal knows the innocent person and that the two of them, for instance, shook hands.  Or maybe they’re close friends or family and share clothes.  It would be possible for DNA to be transferred from the innocent person, to the criminal, and then later left on evidence at the scene of the crime.  Or the innocent person could know one of the victims, and have gotten DNA, through a hug, or handshake, to the victim who was later victimized in the crime.

Residual DNA.  Let’s assume the person visited the home, business, car, or other place where the crime occurred.  Perhaps this visit happened months ago.  Maybe the person visited several times, or only once.  Whatever the case, it’s possible that the person left behind DNA from a purely innocent visit.  Later when the crime occurred and police conducted evidence collection, they collected items that have the person’s DNA on it even though the person was not involved in the crime at all.

Fake DNA. DNA could purposefully be planted.  For instance, a criminal, hoping to blame someone else for the crime, might intentionally collect DNA from someone who is entirely innocent of the crime.  The real criminal could commit the crime, and then leave DNA behind to try to get the innocent person accused and arrested of the crime.  This is obviously rare.

Mixed DNA.  Many DNA samples collected from a crime scene contain multiple DNA profiles from several people.  Some of those people may be victims, perpetrators, or even investigators or police officers.  It’s possible to distinguish a mixed DNA sample so that two partial profiles may be produced.  This can be done if one of the contributors to the DNA is identified.  For instance, if police obtain a swab from one of the victims and match it against DNA obtained from evidence, they will be able to get a partial profile for the other person (assuming only two DNA profiles are mixed on the evidence).  Mixed DNA samples make the mathematical computation of the final statistics somewhat less certain.  But even mixed DNA samples can yield results that would appear to be very conclusive to a potential jury.

These are some of the possible explanations for why an innocent person’s DNA might end up at a crime scene.

Basic overview of DNA and its use in Forensic Analysis

Deoxyribonucleic acid or DNA is a substance that contains the genetic instructions in all human beings (and nearly all life, with the exception of certain viruses).  Think of DNA has a ladder that spirals.  The two long spokes of the ladder are called nucleotides.  The rungs of the ladder contain the molecules, called bases, that form the actual genetic code.

There are four types of bases – Guanine, Cytosine, Adenine, and Thymine – which form pairs in the form of A-T and C-G.  These pairs form strings along the run of the ladder.  But because the bases appear in different orders – TTAAGGG or TAGAAG – and so forth, it’s the patterns that carry the instructions that enable the formation of cells that in turn form life, including human life.

DNA is contained as Chromosomes in human cells.  Humans typically have 46 chromosones (unless there’s a genetic abnormality).  Two of those chromosones indicate the sex of the human being – X-Y for male, and X-X for female.

There are roughly 3 billion base pairs – within those 46 chromosones.  More than 99 percent of the DNA in human beings is identical to other human beings.  What makes us different is determined by approximately 3 million base pairs, which indicate the different genetic characteristics – from hair color, to height, to other traits – that creates the differences between individual human beings.

No two human beings – with the exception of identical twins – share the same DNA.  We each get half of our DNA from our father, and half of our DNA from our mother.  But siblings – who are not identical twins – get different halves, and so brothers and sisters, while the share more DNA than strangers, do not have identical DNA patterns.

DNA was first identified in the 1950s.  But it wasn’t until the 1980s that scientists identified ways of easily and cheaply distinguishing between different DNA patterns.

The reason this is important is that people leave little bits of DNA behind whereever they go.  This is called “sloughing”.  Some people slough more than others, thereby leaving more DNA behind them. DNA might be carried in sweat, hair particles, skin particles, or saliva.

Unlike fingerprint analysis – which is always open to interpretation because it is relatively imprecise – DNA analysis is incredibly precise.  Assuming that the forensic analyst or technician has done his job properly, and assuming there has been no cross-contamination, the DNA profile generated by forensic analysts will reflect the DNA profile of people who left their DNA behind at a particular place.

And while there are more than 3 million differences between in the DNA between various human beings, by using statistical analysis, scientists can match profiles with very high degrees of precision based on just 15 commonly used loci (or spots) on a DNA strand.

Today the whole process is so automated and standardized, that forensic analysis of DNA can be accomplished relatively cheaply by easily-trained forensic analysts.  These analysts are not scientists.  Rather they are people who know how to use several machines and computers that help extract, quantify, amplify, and then analyze DNA strands.  They then use this information to “exclude” possible contributors to the DNA.  If, however, they cannot exclude a contributor, then they will sometimes conclude that the there is a 1 in 1,000,000 or 10,000,000 or even higher chance that the person’s DNA is not present.  The higher the number, the more likely it is that the person was a contributor.

SBI Abuses Impact Criminal Trials

Recent revelations that the analysts at the North Carolina State Bureau of Investigations misreported results, lied about lab tests, failed to report results that were inconsistent with what prosecutors wanted to hear has created shock waves in North Carolina.

The News and Observer reports that distrust is building in courtrooms throughout the state:

“All this information came from the SBI, and we all know what they say doesn’t matter anymore. They have no credibility now,” [Criminal Defense lawyer] Reece said in an interview Monday.

It is a scene expected to play out in courtrooms across the state in the coming months as attacks on the SBI’s credibility reverberate. Judges anticipate a slew of motions from prisoners seeking a review of their cases, citing problems over SBI analysis. Prosecutors are trying to figure out how to defend SBI agents and analysts, often their star witnesses in front of juries.

Forensic Science is Sometimes Junk Science

Great article in Newsweek about the unreliability, corruption, and plain error in forensic “science.” Many jurors – including jurors deciding criminal case in Raleigh and Wake County – believe forensic science, and are unable to determine whether forensic science is junk science or not. If you need a criminal lawyer Raleigh, call (919) 352-9411.

From the article:

There is no “may” about it. An Idaho man was convicted of rape and murder based on shoe prints; he spent 18 years on death row. A Louisiana man was convicted of rape when a bite-mark analyst testified that he “is the person who bit this lady.” Both were exonerated by DNA.

Confrontation Clause and DWI Cases – Melendez-Diaz v. Massachusetts

For years North Carolina, like many states, permitted the State to introduce the results of a DWI defendant’s breath test. That meant that the Defendant’s attorney would have no opportunity to question the breath analyst on the stand about the procedure used, whether the breath analyst was certified at the time the test was given, and whether all the rules and regulations were followed.

In June of 2009, the United States Supreme Court handed down the landmark Melendez-Diaz v. Massachusetts case, which turns on the Confrontation Clause of the Sixth Amendment which permits defendants to question all accusers at trial.

The Supreme Court ruled that it’s insufficient for the state merely to provide an affidavit. The state, in order to introduce evidence such as the results of lab tests, must provide the actual analyst so that the Defendant can have an opportunity to cross-examine the analyst.

Lie Detectors: Silly, stupid, nonsense…

I had a debate with a friend at lunch today about lie detectors.  He argued that lie detectors, even if they’re mostly nonsense, are at least valuable because they “sometimes” detect a lie.  I said lie detectors are not just nonsense, but actually really powerful tools in the hands of police departments.  It’s not because lie detectors can tell if you’re lying or not.  They can’t.  It’s because most people think that lie detectors can tell whether they are lying, and so they will tell police things!

Most people who swear up and down that polygraph machines are accurate are themselves polygraph analysts.  Of course an analyst will say that the polygraph machine is accurate, just as a witchdoctor will say that voodoo medicine actually cures people.

But most scientists agree that polygraph machines are not at all effective at detecting lies or truth.  In fact there are numerous reasons to doubt that polygraph machines can tell the truth, most of them are described on

The important thing to remember is that the results of polygraph exams are not admissible in court because polygraph exams are not reliable.

So why do police agencies use polygraph machines?  For two reasons: First, the average citizen believes that the polygraph is accurate.  Consequently, it’s a well known fact that people will admit to things during a polygraph examination, believing that if they lie, the detector will reveal the lie.  As a result, even though the polygraph machine is a piece of junk, it’s effective because it creates the illusion that it can detect a liar. So it gets suspects talking, and a police detective would much rather have a talking suspect, than a silent suspect who requests a lawyer.

Second, polygraph machines are used because they offer another opportunity for the police to question the suspect.  Police can ask all kinds of questions, the suspect can say that he’s innocent, and the police officer can say: “Well, how about you take a lie detector test,” holding out hope to the suspect that if the suspect “passes” the test, he’ll be able to go home free.

The problem is that in many cases, the suspect never “passes.” That’s because the polygraph exam is designed so that the suspect will “fail” even if the suspect has told the absolute truth.  At the end of the exam, the detective can come in to the suspect, say that the suspect has failed the exam, and say “Well, if you’re telling the truth, why did you fail the exam.”  And the police can point to squiggly lines on the “exam,” which may indicate lying, but also may indicate a nervous suspect.  And who isn’t nervous talking to police?  Everyone is nervous being questioned by police, even the Pope himself.

The person then will panic and try to think of some reason why the lie detector said he was lying, and make additional statements that sound incriminating, even when they aren’t.  I have worked with individuals who, in their desperation to explain why the lie detector exam found that they lied, have written out “explanations” that read like confessions.

At that point, even though the lie detector test is a bogus test, it has achieved its purpose for the police: it has gotten the suspect to “confess.”

The truth of the matter is that lie detectors are junk, and just as someone under investigation should refuse to talk to police unless an attorney is present, someone under investigation should refuse to take any polygraph exam unless an attorney is present.

Refusing to take a polygraph exam is NOT an admission of guilt.  It is your right to refuse to talk to police without a lawyer present.  It is also your right to refuse to take a polygraph exam.  And you should say, “Thanks, but no thanks. I’d like a lawyer, please.”



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