FORENSIC INVESTIGATIONS (police)

Introduction

Prior to the twentieth century, the traditional investigative techniques used by law enforcement were the obtaining of confessions by interrogation, the testimony of eyewitnesses, the utilization of informants, and undercover operations. There were very few uses of forensic evidence beyond basic identifications. Today physical evidence plays a vital role in many types of criminal investigation. Perhaps high-profile cases and what has been dubbed the ”CSI effect” have generated enormous public interest in crime scene investigations and can partially explain this focus on forensic investigations. Advances in forensic sciences and the development of new crime scene techniques certainly offer new alternatives in finding the facts of a crime.

Utilization of Physical Evidence in Forensic Investigation

The following are some important uses of physical evidence in contributing to a forensic investigation:

1. Establishing a corpus delecti. Corpus delecti refers to those essential facts that show that a crime has taken place. It is the presence of elements of the law defining the occurrence of a criminal offense. Physical evidence and forensic testing results are often used to establish that a crime has, in fact, been committed. Establishing that a seized white powder substance is cocaine, that fire debris contains accelerant or gasoline residue, or that semen is present on a vaginal swab taken from a sexual assault complainant are all examples of utilizing physical evidence to establish corpus delecti.

2. Determining the modus operandi. Many criminals have a particular MO (modus operandi, or method of operation), which consists of their characteristic way of committing a crime. Physical evidence found at a crime scene often can help in establishing an MO. For example, in burglary cases, the means used to gain entry, tools that were used, types of items taken, and other telltale signs, such as urine left behind at the scene, are all important. In arson cases, the type of accelerant used and the way in which the fires are set constitute physical evidence that helps to establish the patterns, or ”signature,” of an arsonist.

3. Identifying people. The role of physical evidence in helping to identify a victim or a suspect is central to forensic investigation. Fingerprints have been used for more than a hundred years, but we have come to depend on DNA, bite marks, and a variety of other forensic techniques to identify a victim, a suspect, or a witness. Unambiguous identification is not restricted to homicide, or even to criminal cases. In accident or major disaster situations, victims have to be identified so that the remains can be returned to families.

In disputed parentage cases, the par-ent(s) of a particular child must be identified so that the law can assign proper responsibility for the child’s care and support.

4. Identifying substances. The positive identification of an item of physical evidence is one of the most important functions of forensic laboratories. Substances that require chemical or instrumental analysis to be identified must be subjected to these tests to prove that an offense occurred. For example, analysis can show that white powder seized by a detective from a defendant contains a particular controlled substance—heroin. Similarly, the scientific determination that a sample of blood taken from a driver contains more alcohol than the law allows for someone to operate a motor vehicle is critical to a driving while intoxicated (DWI) case.

5. Establishing linkages or exclusions. The most important application of physical evidence is developing linkages between a suspect and a crime. Linkage of victims, suspects, witnesses, and physical evidence can apply at many levels, from a mere possibility that two items could have a common source to complete individualizations. For example, paint chips found at a hit-and-run scene match the suspect’s vehicle, or DNA recovered from a vaginal swab of a sexual assault kit matches the suspect’s DNA profile. Linking a victim or suspect to a crime scene in some cases can be as important as a connection between the victim and suspect. A simple illustration is the situation in which a victim’s blood is found on a suspect’s clothing. Another example might occur when a bloody fingerprint found at a victim’s home matches a suspect’s right thumb.

6. Supporting or disproving statements. Another useful application of physical evidence in a forensic investigation is corroborating or disproving statements or testimony of a witness or suspect. Corroboration can be critical in finding the facts of a case. For example, a suspect denies he has been at a homicide scene, but the investigator found bloody shoeprints at the scene that are matched to the suspect’s shoes.

7. Providing investigative leads. Physical evidence can help investigators develop leads. For example, it can help in developing the perpetrator’s MO. Perpetrators of criminal acts follow behavioral patterns. In many situations the ability to connect two or three seemingly unrelated incidents provides critical information to help the investigators to progress in their investigation. With recent advances in forensic technology, data mining and artificial intelligence have provided an additional dimension in forensic investigation. AFIS, CODIS, and NIBIN are the major causes of that change. Each of these three computerized databases allows cases to be connected even if the source of the evidence is still not known. This can energize an investigation and give it direction.

Forensic Investigation Process

As forensic-based investigations became more prevalent, the scope of potential evidence and methods for examining that evidence expanded. Evidence available from a crime scene consists of more than just physical objects. It also includes a wide variety of pattern evidence, such as blood stains, imprints, gunshot residue, or the like, conditional evidence, which is created by an action or event, transient evidence, which is temporary in nature, and trace/ transfer or associative evidence, which provides a direct association between the victim and suspect.

The nature and mechanism of generation of such categories of physical evidence are critically important in forensic investigation. Identification and examination of physical evidence is only one part of forensic investigation. Knowing the mechanism that produced it in the precise location where it was found, observing changes at a crime scene, and determining the sequence of events are often the key in solving cases.

Utilization of forensic investigation in the justice system requires that a series of steps be taken in the proper sequence. This process may include any or all of the following major forensic activities:

1. Recognition. Recognition is the ability to separate important and potentially informative items in a case from the background and other unrelated materials. The evidence is selected on the basis of what is likely to help in distinguishing between the possibilities. The recognition process involves basic principles of forensic examination, pattern recognition, physical property observation, field testing, and information analysis.

2. Documentation. After recognition comes documentation. Proper and complete documentation is critical to fulfilling the requirements of chain of custody. Documentation of a scene is as important as documentation of individual items of physical evidence. Many types of pattern evidence and conditional evidence can be recorded only by proper documentation. The exact location where evidence was found can be very important in the reconstruction of an incident. A reconstruction requires the synthesis of all of the information available.

3. Collection and preservation. This is an important step to ensure that physical evidence was collected and preserved to meet the legal and scientific requirements. If a piece of physical evidence reaches the laboratory in a condition not suitable for laboratory analysis, it loses its legal and investigative value. Different types of physical evidence have somewhat different requirements in terms of packaging and marking. Investigators have to fellow the correct guidelines in handling physical evidence.

4. Identification. Identification is a process common to all of the sciences and, in fact, to everyday life. It may be regarded as a classification scheme, in which items are assigned to categories containing like items and given names. Different items within a given category all have the same generic name. In this way, botanists will identify plants by categorizing them and naming them. Likewise chemists identify chemical compounds. In forensic science, identification usually refers to the identification of items of physical evidence. Some types of physical evidence require that scientific tests be conducted to identify them. Drugs, arson accelerant, bloodstains, and seminal stains are examples. Objects are identified by comparing their class characteristics with those of known standards or previously established criteria.

5. Classification. Classification is done by comparing the class characteristics of the questioned evidence with those of known standards or control materials. If all of the measurable class characteristics are the same between the questioned sample and the known control, then these two samples could have come from the same source or origin. If there are significant differences in some of the class characteristic measurements, then the questioned sample can be absolutely excluded as having coming from the particular source. In other words, the exclusionary value of comparison in the forensic investigation is considered absolute.

6. Individualization. Individualization is unique to forensic investigation; it refers to the demonstration that a particular sample is unique, even among members of the same class. It may also refer to the demonstration that a questioned piece of physical evidence and a similar known sample have a common origin. Thus, in addition to class characteristics, objects and materials possess individual characteristics that can be used to distinguish members of the same class. The best examples are fingerprints and DNA matching.

7. Reconstruction. Reconstruction is based on the results of crime scene examination, laboratory analysis, and other independent sources of information to reconstruct case events. Reconstruction often involves the use of inductive and deductive logic, statistical data, information from the crime scene, pattern analysis, and laboratory analysis results on a variety of physical evidence. The developing fields of artificial intelligence and expert systems have opened up a new dimension in reconstruction. These systems allow the modeling and representation of laboratory analysis results, the reasoning and enacting of a crime scene, and the comparing and profiling of suspects.

8. Interpretation and testifying. Forensic scientists must be able not only to master analytical skills, but also to write clear and informative laboratory reports with proper interpretation of laboratory testing results. A final step in this multistep forensic investigation process is court testimony. Court testimony represents the capstone of a forensic scientist’s work. It is essential to treat every item and every case as if it will go to a major trial. The most important aspect of court testimony is to maintain the scientific objectivity and neutrality. Although the majority of forensic scientists put forth every effort to maintain objectivity, there is no doubt that they may disagree about the meaning or interpretation of results.

Conclusion

Forensic science is the application of scientific principles and techniques to matters of law. Because of this unique application, a forensic scientist not only must master scientific methods but also must know the limitations of science. A forensic scientist not only has to be familiar with the law but also has to appreciate the rules of evidence. As scientists, we have to possess the ability of recognition, the skill of documentation, the knowledge of examination, and the power of reconstruction. Also, we have to faithfully testify regarding laboratory findings in court in an objective manner.

Perhaps the most important issue in forensic investigation is establishing professional standards. There is a need for standards of practice in the collection, examination, and analysis of physical evidence, the interpretation of results, and the giving of testimony in court. Both forensic and legal professions must carefully examine their role in examining and presenting scientific evidence in the court of law. The professionalism and ethical standards of both groups in criminal and civil investigations must continue to develop to ensure that justice can be served.

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