Introduction

We already know that two independent forensic audiologists have examined a recording of a 911 call by a woman who reported a struggle going on behind her residence. A long high-pitched scream for help that is audible in the background terminates abruptly with what sounds like a single shot.

Using different analytical methods the two experts have compared the scream to a police recording of George Zimmerman’s voice when he called a police non-emergency number approximately 10 to 15 minutes before the shooting to report a suspicious person in his neighborhood.

We now know that person was Trayvon Martin, who was walking back to his father’s girlfriend’s residence, whom he was visiting with his father, after walking to a nearby 711 store to purchase some Skittles and Arizona Iced Tea.

Both experts excluded George Zimmerman as the person screaming in the background to a reasonable scientific certainty. They have not compared the scream to a recording of Trayvon Martin’s voice, presumably because they do not have a recording of his voice.

Trayvon Martin’s mother has identified her son as the person screaming.

Since there is a witness who claims to have seen the struggle between Zimmerman and Martin with Martin on top and Zimmerman lying on his back in the grass yelling for help, there is an apparent conflict in the evidence between the eyewitness’s statement and the two expert opinions. I say “apparent conflict” because the witness did not observe the fatal shot. He locked his door, went upstairs, and when he looked out the window, he saw Martin lying face down in the grass and not moving.

Because the witness did not observe who initiated the physical confrontation or the fatal shot, he cannot tell us who was the aggressor or where Martin and Zimmerman were positioned and what Martin was doing when Zimmerman fired the fatal shot. This missing information is important because, under Florida law, Zimmerman’s claim of self-defense must be rejected, if he was the aggressor, or if he was not in danger of being killed or suffering serious bodily injury when he fired the fatal shot.

For example, during the time period while the witness was in transit between locking his door, going upstairs, and looking out the window, Zimmerman, who outweighed Martin by 40 pounds, according to Wikipedia, might have locked his arms and legs around Martin, rolled over on top straddling him, and then pulled his gun out of the holster and fired the fatal shot. He might even have separated from Martin and fired the fatal shot. Neither scenario would justify using deadly force in self-defense.

Let us now consider what other forensic evidence to see what it might tell us about the relative positions of Zimmerman and Martin when Zimmerman drew his gun and fired.

What Can Forensics Add To This Investigation?

I would want to review the autopsy report and interview the medical examiner who conducted the autopsy to find out whether he noticed any injuries other than the fatal gunshot. For example, did Martin have any abrasions on his hands and fingers.

I would have a lot of questions for the medical examiner regarding the nature of the gunshot wound and Martin’s clothing.

For example, the weapon was a black Kel Tek 9 mm PF9 semiautomatic. The bullet would have been discharged when Zimmerman pulled the trigger causing the hammer to strike the primer igniting the smokeless powder in the casing producing rapidly expanding gas (consisting of carbon monoxide, nitrogen dioxide, carbon dioxide and other gases) that ejects the bullet, burning and unburned gunpowder (the burn always is incomplete), and trace amounts of the primer that contains heavy metals, including lead, antimony, and bismuth. Depending on the nature of the wound and the presence of all, some, or none of these materials, a qualified forensic firearms expert can determine how far away the gun barrel was when the shot was fired.

The gases, including the heavy metals, and some smoke from unburned but gaseous carbon, are projected only a few inches. The effects of the gas produce what are called contact or near-contact wounds that are characterized by variable skin lacerations or tears from the expanding gases that rip the skin apart and stippling, which is blackening skin from the unburned smokeless gunpowder that is propelled out the barrel of the gun with the bullet by the rapidly expanding gas.

As the distance of the gun barrel to the skin increases, the effect of the gas diminishes and only the unburned powder and bullet are capable of penetrating the skin. Stippling is present when the gun barrel is 0.5 centimeter to 1 meter from the wound. The pattern gets larger as the distance increases. So-called distant wounds do not cause tearing or stippling.

Based on Zimmerman’s statement, I would expect Martin to have had a contact or near contact wound with skin laceration or tearing caused by the explosive gases entering the wound expanding and tearing the skin. I also would expect to see some stippling or unburned smokeless powder. If I did not see evidence of either in the wound or clothing fibers, I would conclude that the gun barrel was more than a meter away, which is at odds with Zimmerman’s statement in which he claimed that he shot Martin as Martin was on top and hitting him.

Given Zimmerman’s statement, I also would expect to see high velocity blood spatter (mist-like spray of blood drops about 1 mm in diameter) or blowback on the barrel of Zimmerman’s gun, his shooting hand, and the sleeve of his jacket.

Absence of skin tearing, stippling, and high velocity blowback or blood spatter would seriously undermine a claim of self-defense.

Cross posted from my law blog.