Friday, May 28, 2010

The Ballistics Expert in Criminal Defense

Ballistics has been on my mind these past few weeks. I'm defending a criminal murder case in Brazos County which relies heavily upon the prosecutor's ability to link my client to the alleged murder weapon with the expert testimony of a firearms examiner, or ballistics expert.

The criminal defense attorney defending such prosecutions needs a fundamental understanding of the firearms examiner's role in presenting evidence concerning the identification of firearms and ammunition. This is especially true in cases where such identifications are the lynchpin of the prosecution or defense case. Understanding how bullets and cartridge cases can be identified as having come from a specific weapon depends on some knowledge of how firearms are manufactured, particularly pistols and rifle barrels.

First, a hole is bored through a cylindrical bar of steel of the desired diameter. That diameter determines the caliber of the weapon. Next, after the hole is bored, twisting grooves (or rifling) are created inside the barrel. This process causes a fired bullet to spin as it leaves the barrel, giving it more rotational velocity and, consequently, more stability with better accuracy. Regardless of the manufacturing process, each barrel acquires minute marks, called striations (or striae) through minor accidental occurrences in the rifling process. These striations are supposedly not the same for any two barrels and are the basis for the "individuality" of each rifle barrel. Any bullet fired through the barrel will be effected by the unique rifling of that particular weapon.

If a bullet (or bullet fragment) is obtained during the police investigation, often the police want to determine whether a particular weapon fired it. Assuming the police also possess the suspected weapon, the process of identification is straight forward. First, the examiner fires a series of test bullets from the suspected weapon and uses them to compare with the "unknown" bullet obtained during the investigation. The examiner uses a binocular comparison microscope, which is an instrument consisting of two separate microscopes mounted side-by-side. The unknown bullet is placed under one microscope and the test bullet under the other. The examiner then scrutinizes both bullets and attempts to locate similar striations that "match" both the test and unknown bullets. In theory, a careful study of all the detail on both bullets permits the examiner to conclude whether both bullets were, or were not, fired through the same barrel.

In a later post, we'll discuss the identification of a cartridge case and whether is was fired from a particular weapon. Although different from bullet identification, the same principles of probability apply.

Saturday, May 8, 2010

Computer Forensic Experts

The spread of crime using computers was inevitable, even in the sleepy towns of Bryan|College Station, Texas. The ease with which people can access literally billion of documents and images over the internet has made computer "crimes" a hot area in law enforcement in recent years. Our question is how to defend against such charges by the government? One answer is using computer forensics, that is, utilizing a defense expert to preserve, analyze, and produce data from computer media storage.

When conducting an analysis in computer forensics, the “expert” uses tools (i.e., software) to examine and extract information pertaining to the alleged crime. However, a problem area is whether one can be considered an expert solely based on their ability to use a tool or software package to analyze the computer data, without the ability to clearly define how the tool works or reviewing the source code. The majority of the tools and software used by computer forensics experts is proprietary and copyrighted. This eliminates the ability to access the source code. Currently, this inability of the expert to test the code and understand how it works has not hindered the admissibility of an expert’s testimony. In Texas, criminal courts have found that an expert does not need to know the code of the software package nor the background processes. (see Williford v. Texas 127 S.W.3d 309) Questions arise concerning whether an expert who cannot attest to area three of Daubert qualifies as an expert.

The third criteria of Daubert states specific factors such as peer review, error rates, and acceptability in the relevant scientific community are important elements to consider when determining the reliability of a scientific test, including proprietary software programs used to analyze a computer hard drive. However, it's difficult to meet the third criteria due to a lack of error rates for most of the software used by the forensic experts. Additionally, there are no standards in the field or peer reviews of methods. The courts have found that an inanimate object (e.g. a software package) cannot be considered an expert. This does not mean the object (or results from that object) cannot be used for scientific testimony. The individual using the software package simply needs to testify regarding the procedures used.

A possible argument to be made in criminal court regarding the third criteria of Daubert is that the computer forensic community has accepted certain industry standard tools such as EnCase, a common program used by experts to analyze computers. The question, then, becomes whether it's justified to say that the "relevant scientific community" has accepted certain software packages? Currently, experts must qualify their educational background, which includes courses taken by corporate or federal agencies on how to operate software packages (like EnCase).

The bottom line is that computer forensic experts can be invaluable to the computer crime defense attorney. For court-appointed cases, the defense lawyer can use ex parte motions for expert assistance to obtain funds to hire such experts.