The adjective ”forensic” is commonly used today to describe nearly every type of evidence related to a criminal case or in any way associated with a legal debate both in and out of a courtroom. Anything that has the potential to be evidence—aside from eyewitness evidence, hearsay, or speculation—can be considered forensic evidence. The word is typically associated with ”forensic science” and is related to forensic evidence examined in crime laboratories, now commonly called forensic laboratories. ”Material evidence” is synonymous with forensic evidence and is normally recovered from a crime scene. Physical items found at the crimes scene or on the victim are analyzed to determine what happened, who did it, how, and why.

Ideally, the thorough examination of the crime scene, the victim, and the associated items of evidence will result in clues for solving the case. However, no matter how thorough the investigation and the examination of the evidence, there is no guarantee a case will be solved. Some cases are solved immediately, while others may be solved years afterward when new techniques or new evidence develops; sadly, some are never solved.

A Criminal Act

The authority of forensic evidence is based on its probative value in court—that is, its ability to prove something. Forensic evidence may associate a person, place, or thing to another person, place, or thing or to a criminal act. However, sometimes valuable forensic evidence is destroyed or not located at all during the investigation. Legally, to have a crime there must be a person who commits an act, with a specific intent, and there must be a statute that makes the act illegal. For example, two people are killed in separate incidents as a result of being hit by an automobile; the first was deliberately run down by an enemy, while the second was a small boy whose parent accidentally ran over him while backing out of the driveway. Both deaths were the result of being hit by an automobile, but only the driver in the first case had criminal intent.

Intent is a key factor in determining whether or not a crime was committed, the severity of a crime, and whether a crime was a premeditated, first-degree crime or a lesser charge, such as first-degree murder as opposed to manslaughter. Intent is in the mind of the perpetrator. There is no forensic laboratory test for it—forensic scientists are not mind readers. However, the evidence may certainly give some indication of intent. A shotgun blast to the back of the head at the end of an abandoned road would likely lead an investigator to open a homicide case, because it appears that someone intended to kill the victim. An incident in which the victim was stabbed two hundred times and an incident in which the victim was stabbed once would each give a different impression of an accused killer to a jury, even though both victims died of stab wounds. Intent is determined by the jury, judge, or special panel only after hearing and studying all the facts and evidence associated with the case.

The Goal of Forensic Analysis

The goal of the analysis of forensic evidence is to find the truth. This is often accomplished by connecting the perpetrator to the criminal act itself through biological evidence (the exchange of blood and body fluids), physical evidence (such as chemicals, illegal substances, or tools or weapons used as implements of the act), and digital evidence (electronic evidence in a binary form found in computers, personal data assistants (PDAs), cell phones, digital answering machines, digital cameras, e-mails, etc.). Other biological evidence comes from identifying human features such as facial characteristics, scars, marks, and fingerprints, which may become forensic evidence if they can be used to connect the suspect to the act, the crime scene, or the victim. These same identifiers can also help identify the victim.

DNA-related forensic evidence is the most notable biological evidence. Prior to the use of DNA as evidence, blood types (O, A, B, and AB) were used to either include or exclude a suspect, but matching a blood type could not provide any individually identifying features.

The implements or tools used to commit an act, such as a car, a knife, a gun, or a pry bar, can also be associated to the crime scene and/or the victim through the development of forensic evidence. A car used to deliberately run down and kill a victim could have fibers or even large pieces of clothing attached to it that can be physically matched to the victim’s clothing. Fragments of the car can be found on the victim’s body as well. Additionally, the DNA in the blood on the car can be excellent forensic evidence to identify the victim hit by the car. A bullet removed from a shooting victim can be traced back to the gun used to fire the bullet. However, a bullet that fragments in the body may not have enough surface characteristics to trace the bullet fragments back to the weapon that fired it, even when investigators have the correct weapon for comparison. Tool marks from a pry bar used to forcibly enter a residence or business can be associated back to the tool. The association of the tool mark with a specific tool involves the comparison of the tool mark at the scene to a mark made in the laboratory by a tool taken as evidence from the suspect. Forensic science procedures and techniques make these associations possible. However, a forensic scientist may have the evidence from the scene, but it is not suitable for comparison to the known samples taken from the suspect due to degradation or destruction of the evidence or the known sample.

Characteristics of Evidence

Forensic evidence characteristics can be divided into two classifications: class characteristics and individual characteristics. For example, the largest percentage of the population has blood type O, whereas the smallest percentage of the population has blood type AB. Blood types are class characteristics. Suspects can be grouped by blood type and can either be set free or kept for further questioning. However, if the DNA of a suspect matched blood found under the victim’s fingernails, the DNA profile would be an individualizing characteristic that would narrow it down to one individual suspect.

An example of physical evidence with the same class characteristics could include a group of athletic shoes that were the same brand, the same color, the same size, the same style, and the same tread pattern. If a footwear impression was left at a crime scene, a tread pattern in blood, soil, dust, or mud, all of the footwear in this class of shoes could be suspected of having made the impression at the scene. The impression at the scene would be preserved by photography or a lift or casting, depending on the circumstances, and a search for individual characteristics on the impression would begin.

Individual characteristics caused by wear and unique damage to a shoe sole are identified by the forensic scientist by shape and type. These features are mapped onto a photograph, much like the identifying features of a fingerprint are mapped onto an enlarged chart to show that they match the known fingerprint minutia of the suspect. Comparisons are made with impressions from the actual suspected footwear to see if the individual characteristics are unique and can be matched to the footwear impression found at the scene. The forensic scientist is able to match the shoe impression from the scene, which was submitted to the laboratory by the investigator, to the known shoe the investigator removed from a closet in the suspect’s apartment during a legal search. On the witness stand the forensic scientist would testify only to the work performed personally, and the findings might read as in the following fictitious case:

Footwear impressions and photographs, marked as exhibits Q1, Q2, and Q3 from the crime scene of case number 12346ABC, were hand carried in a sealed package to the forensic laboratory on June 2, 2005, by Investigator John Smith. We were requested to examine the footwear sole pattern for investigative leads, such as associating the tread pattern of the footwear impression to a particular brand and type of footwear.

On June 10,2005, a pair of athletic-type footwear was submitted to the laboratory by Investigator Smith. The athletic footwear was described in the letter of submission as exhibits K1 and K2 and was taken from the closet of John Doe, on June 9, 2005, during the execution of a search warrant.

Visual examinations in the laboratory of K1 and K2 revealed that the sole and heel footwear patterns were consistent with the herringbone tread design of an athletic footwear impression at the crime scene. Extensive microscopic examination of the wear patterns to the sole and analysis of numerous individual characteristics revealed that exhibits K1 and K2 match the multiple individual characteristics on footwear impressions at the crime scene. Exhibit K2 was identified as having made the impression in photograph Q3.

The questioned exhibits Q1, Q2, and Q3 were returned to Investigator Smith by registered mail No. 345,876,213 on July 15, 2005. The known exhibits K1 and K2 were returned to Investigator Smith by registered mail No. 345,876,214 on July 15, 2005.

The official report would bear the signature of the forensic scientist who worked the case, and in the case file would be the initials of the case reviewer. This is a quality assurance measure. The reviewer would be able to testify to the findings if something happened to the original examiner.

Chain of Custody

Forensic evidence has no value in court unless it is accompanied by a rigorous legal ”chain of custody” and is protected from degradation. Proper packaging is required to protect the evidence and documentation attached to the item or associated with the item. This documentation is critical and must describe the place from which it was seized and the date, time, and initials or notation of the person who collected the evidence (noted on an evidence tag attached to the evidence or its container).

Additionally, every time an item of evidence changes hands, the evidence must be signed for and properly documented by the person receiving the evidence. The person who had custody of it would have had to sign and date documentation when it was received, and the same process would occur when it was turned over to yet another person. It would not be accepted as evidence in the court without these procedures. The questioned and known exhibits should be packaged separately so that cross contamination does not occur. Forensic examiners must also ”mark the evidence for identification” so that they can identify items on the witness stand as those examined in the laboratory. The ”mark” is usually the initials of the forensic scientist and the date of examination, made in a manner not easily removed or destroyed.

Digital Evidence

One of the newer forms of forensic evidence is in binary format, found on computers, cell phones, digital voice recorders, and digital pictures and movies and referred to as digital evidence. Digital evidence was defined in 1998 by the Scientific Working Group on Digital Evidence (SWGDE) as ”information of probative value that is stored or transmitted in a binary form.”

In the past when a homicide of a woman in her bedroom was discovered, investigators secured the area so that they could look for physical evidence. Not just the bedroom but the entire house, garage, and yard are the crime scene. It was critical to photograph or videotape the scene as it was when the first responders arrived. The medical examiner would determine the official cause of death, but the investigators looked for a potential cause of death and evidence of a forced entry. This would help them decide if the perpetrator knew the victim. They would look for the object(s) that might have caused the victim’s death. Was there anything unusual about the scene or how the victim was killed? Then they would look for items such as an address book, a diary, notes on the bedside table, numbers scribbled near the telephone, letters, bills, and other documents that might lead them to a suspect.

Today most of the addresses are not on paper but can be found in a PDA; letters, bills, and banking can be on a personal computer or the PDA. All of the same investigative information is now on computers or stored in a digital format on answering machines, faxes, cell phones, or the like.

Rules of Evidence

Related to giving testimony in court about forensic evidence, under the Federal Rules of Evidence, Rule 702 deals with the admissibility of expert testimony as follows:

If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education may testify hereto in the form of an opinion or otherwise, if (1) the testimony is based on sufficient facts or data, (2) the testimony is the product of reliable principles and methods, (3) the witness has applied the principles and methods reliably to the facts in the case.


Vital forensic evidence can be the smallest, most unlikely particle at the crime scene. It is only by the hard work and professionalism of crime scene processors that the potentially suspect items are collected, sealed and preserved in containers, labeled as to where they were located in the crime scene and with the evidence tags signed and dated by the collectors, and logged into the evidence log. It does not matter how large or small the crime scene; the same care and attention to detail must be maintained. The effort, professionalism, and dedication of forensic scientists give families of victims the rights afforded to them in a democracy and will continue to bring criminals to justice.

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