Zimmerman: Absence of Evidence is not Evidence of Absence

September 6, 2012

As I reviewed comments to my article yesterday on crime scene investigation, I noticed some concern about the apparent failure of the police to determine whether there was any blood present on the sidewalk, or in the grass next to it that might confirm or deny Zimmerman’s claim that Martin repeatedly slammed the back of Zimmerman’s head into the sidewalk. I decided to comment on that concern. As so often happens, my answer grew in the telling and after an hour or so, I realized that I had written my next post.

How lawyers deal with missing evidence is the subject of this post. The short answer is absence of evidence is not evidence of absence.

Luminol is a liquid that reacts with blood to produce a bluish glow. Crime scene investigators spray it on areas that they suspect might have been cleaned up to get rid of any blood that might have been present. The luminol spray is extremely sensitive and will react with invisible trace amounts of blood residue to produce a visible glow in the dark that can be photographed with the right equipment.

Although luminol is more often used in indoor environments to confirm or deny a suspicion that a particular area is a cleaned-up crime scene, it also can be used in outdoor areas where rain might have washed away all of the visible blood.

I have not seen any indication in the discovery that the crime scene investigators used or considered using luminol to detect trace amounts of blood on the sidewalk or in the grass next to it. It’s possible that they might have considered, but rejected, using luminol for some reason and did not mention it in a report.

Lawyers characterize this type of situation as a missing evidence problem. Although not relevant here, other known causes of missing evidence are intentional destruction of evidence and unexplained loss of evidence.

Depending on the materiality or importance of the missing evidence to proving its case, missing evidence can make it more difficult, if not impossible, for a prosecutor to prove a case. When the missing evidence would have been favorable to the defense, it can cause an innocent person to be convicted.

Missing evidence problems are not uncommon. I have most often encountered them when I reviewed police reports while preparing for a hearing or trial. They arise during police and crime scene investigations because the people involved did not have adequate training or a set of rules or procedures or even a checklist to follow. In this case, for example, the investigators might not have known about luminol, or, assuming they did, they might have forgotten to try it.

I most often spot potential problems, like the one suggested in this case by the apparent failure to use or consider using luminol, because I know enough about forensics, crime scene investigations, and police procedures to spot issues and ask the right questions. I know what I know because I made it my business to study and learn it. Given the ever increasing importance of forensics in criminal trials, I firmly believe that all lawyers involved in that work should do the same. I used to educate lawyers about forensics at continuing
legal education conferences in the western states and I developed and taught an upper level course emphasizing forensics in law school on the causes of wrongful convictions of innocent people.

In the Zimmerman case, I believe the defense should consult with an expert to determine whether the use of luminol is a standard practice by crime scene investigators in this type of situation. If the answer is “yes,” the defense should endorse that expert and add him or her to the defense witness list. It also should ask the expert to produce a written report explaining why the use of luminol is a standard practice in a situation like this.

The defense would send a copy of the report to the prosecutors to comply with its discovery obligations.

The defense would call this witness to testify during the defense case. If I were to make the closing argument, I would handle the missing evidence issue by emphasizing the following points. As you read this, notice that the factual foundation for the argument would have been established by the testimony of the defense expert and the argument itself incorporates the self-defense instruction while emphasizing that the prosecution has the burden of proof.

See if you can spot the problem with this argument.

(1) The prosecution has the burden of proving beyond a reasonable doubt that the defendant’s use of deadly force was unreasonable under the circumstances;

(2) The defendant testified and told you that he believed he was going to die or suffer serious injury, unless he used his gun to defend himself.

(3) The Court’s jury instructions set out the legal definition of self-defense. The instruction tells you that a person may use deadly force when he reasonably believes he is in imminent danger of being killed or suffering serious bodily injury.

(4) The defendant told you what happened. He described the circumstances that he faced that night and those circumstances satisfy the legal definition of self-defense set forth in the jury instruction.

(5) Unfortunately, the police failed to preserve important evidence in this case when they failed to confirm the presence of the defendant’s blood on the sidewalk and in the grass next to it by spraying the area with luminol and recording the bluish glow for y’all to see in a photograph.

(6) Their failure to do that not only would have supported his claim that he believed he was in imminent danger of being killed or suffering serious bodily injury, it would have conclusively confirmed that his belief was reasonable.

(7) We would not be here today, if the police had done their job.

(8) Since the prosecution has the burden of proof and its investigators were uniquely able to confirm the presence of blood, but failed to do so, you should hold the prosecution responsible for depriving you and the defendant of the evidence that would have confirmed his testimony and established his innocence.

The problem with this argument is that it assumes that detectable trace amounts of the defendant’s blood were present on the sidewalk and grass next to it and would have been detected, if only the police had used luminol.

A good prosecutor would have spotted this problem and responded with a statement like this:

Absence of evidence is not evidence of absence. If there were no blood on the sidewalk or in the grass beside it, the use of luminol would not have produced a visible result.

If you carefully examine these close-up photographs of the defendant’s head taken at the police department after the shooting, you will see that his wounds are minor and the blood coagulated in the form of of a small river that flowed down curling around the back and lower part of his ear. From there it flowed along his chin line and collected in his goatee.

Ladies and Gentlemen: Even if you believe the defendant’s claim that Martin bashed his head repeatedly into the sidewalk, here is no reason to believe that a single drop of blood fell on the sidewalk.

Take another look at those little cuts. The absence of significant abrasions and bruising tells you that the defendant’s head did not repeatedly come in contact with the sidewalk.


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