lab testing can either support a defendant's explanation or rule it out. Lab results that are consistent with an individual's explanation of suspicious circumstances support the idea that he or she was telling the truth.
Whether the defendants involved were actually telling the truth about what happened or just make a lucky guess is a different matter. Over the years the author has investigated several cases involving discharge of firearms in which lab results cleared defendants who were initially charged with a criminal offense. In one of these cases a biker was charged with shooting his female companion. According to the biker, they had pulled over and were taking a little break from their ride off the road in a rural area. The biker stated that he had a 380 semiautomatic pistol with him, and he had taken it out to shoot at a can. His companion, he said, was seated across from him on a rock. As he cocked the gun, he claimed it slipped out of his hand and fell to the ground, striking a rock and discharging the bullet that struck his companion.
When the pistol was brought into the crime laboratory, examiners tested it to determine whether it would discharge if dropped. Given the way the shooting allegedly took place, the gun would have had to have landed butt first against the rock. Testing revealed that the gun would accidentally fire when dropped butt first while cocked. Charges against the biker were then dropped, there being no other evidence against him.
In another case a man claimed that he and a second man were struggling over a sawed-off shotgun. The defendant said he was holding the shotgun with his right hand on the stock and his left hand around the back of the barrel, in front of the hammer, when the other man jerked forward. According to the defendant, this caused his left hand to slide back across the hammer, releasing the hammer and causing the gun to discharge, killing the second man.
Testing on the weapon confirmed that it would fire if the hammer was pulled back partially and released. Once again, the defendant was released because there was no further evidence to indicate that the shooting was not an accident, just as he had claimed.
Sometimes firearms examination assists in a criminal investigation even though no shots were fired. In one such case a man was arrested following an altercation with a police officer. The officer stated that the man had pointed the weapon at him and "may have pulled the trigger," although the gun did not fire. A check of the weapon by the investigating officer revealed a live round under the hammer with what appeared to be a light or partial firing pin impression. This finding seemed to support the idea that the hammer had been released (that is, the trigger had been pulled) and had struck the cartridge but failed to fire the bullet (that is, had misfired). Closer examination of the weapon by the firearms examiner, however, revealed that the firing pin protruded above the breech face plane slightly. This was just enough to produce a light firing pin impression on the primer of a round in the chamber but not enough to produce firing. Thus the firearms examiner was able to show that the light firing pin impression present on the cartridge case was not necessarily indicative of a misfire. By also firing several test rounds the firearms examiner was able to show the impact of the firing pin produced a significantly different mark. This was critical in showing the man was innocent.
In a similar case investigated by the author a man had reportedly pointed a revolver at two people and pulled the trigger, but the gun had not fired in either case. Police later arrested a suspect who had a revolver in his possession. There were two cartridges in the weapon with light marks on their primers. When tested in the crime lab, the weapon would occasionally misfire (fail to fire). This finding substantiated the allegations against the suspect.
Another longtime category of crime lab work is fingerprint examination. Historically, the question of whether a person handled a firearm required the presence of fingerprints that were identifiable to the person in question. But advances in DNA technology have overcome the need for identifiable fingerprints in many cases. When there is physical contact between an object and a person, there is a good chance that skin cells (epithelial cells) will be deposited. Since all nucleated cells, such as epithelial cells, have DNA present, it is possible to isolate and identify that DNA to the contributor. Thus, if a person merely handles a firearm, it is possible to show that person has been in contact with the firearm.
Ammunition and other objects involved in crimes are equally appropriate for determining whether DNA is present. For example, the crime lab can ascertain whether there is evidence that a person handled cartridges loaded into a weapon. Evidence like this can help prove or disprove a crime.
Finally, the trace evidence section of the crime lab often has the opportunity to play a vital role in criminal investigation. In a somewhat
unusual trace evidence case the author was involved in, a shot was fired at a robbery suspect, and the bullet was recovered. The bullet had passed through the suspect's jacket and wound up on the floor. The robber fled the scene. Upon examination of the bullet the crime lab observed that some coarse white fibers were embedded in the nose of the bullet. When a suspect was arrested several days later, police recovered a white jacket from his vehicle. A comparison of the jacket fabric to the fibers on the bullet showed them to be similar microscopically and chemically. The suspect was convicted largely on the basis of the fiber evidence.
Physical evidence has taken on an enhanced role in legal proceedings as a result of the increased public awareness of this evidence and their associated expectations as a result of the saturation of the airways with "CSI-related" drama. Juries not only expect to be presented with physical evidence but in some cases have failed to convict based on a perceived shortcoming on the part of the state in not pursuing some trivial bit of "evidence." The trial of actor Robert Blake in California in 2005 is a classic example of this "CSI effect." Jurors stated that they acquitted Blake due to the lab not having found gunshot residue on his hands. Clearly the state did a poor job in educating the jury regarding "Absence of evidence is not evidence of absence." Accordingly, crime laboratories must rigorously examine all evidence having any potential probative value as well as carefully explain the significance, or lack thereof, of not finding certain evidence.
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