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Police: Criminal Investigations

Sources Of Information And Evidence In Criminal Investigations

As noted earlier, the major problem for the police in conducting criminal investigations is determining the utility of the information (evidence) collected. While much information may be discovered or otherwise available to the police, only a small portion of it may be accurate, complete, and relevant, and hence useful, in establishing the identity (and/or whereabouts) of the culprit. As discussed below, not all types of information are equal in this regard—some types of evidence are usually more useful than others.

Information from physical evidence. Physical evidence is evidence of a tangible nature relating directly to the crime. Physical evidence includes such items as fingerprints, blood, fibers, and crime tools (knife, gun, crowbar, etc.). Physical evidence is sometimes referred to as forensic or scientific evidence, implying that the evidence must be scientifically analyzed and the results interpreted in order to be useful.

Physical evidence can serve at least two important functions in the investigative or judicial process (Peterson et al.). First, physical evidence can help establish the elements of a crime. For example, pry marks left on a window (physical evidence) may help establish the occurrence of a burglary. Second, physical evidence can associate or link victims to crime scenes, offenders to crime scenes, victims to victims, instruments to crime scenes, offenders to instruments, and so on. For example, in a homicide case, a body of a young female was found along a rural road. Knotted around her neck was a black electrical cord (physical evidence). The cause of death was determined to be ligature strangulation via the electrical cord. Upon searching the area for evidence, an abandoned farmhouse was located and searched, and a piece of a similar electrical cord was found. This evidence led the investigators to believe that the farmhouse may have been where the murder actually occurred. Further examination of the scene revealed tire impressions from an automobile (more physical evidence). These tire impressions were subsequently linked to the suspect's vehicle.

Most forensic or physical evidence submitted for analysis is intended to establish associations. It is important to note that physical evidence is generally not very effective at identifying a culprit when one is not already known. Typically the identity of the culprit is developed in some other way and then physical evidence is used to help establish proof of guilt. Possible exceptions to this pattern are fingerprints (when analyzed through AFIS or Automated Fingerprint Identification System technology) and DNA banks. With AFIS technology, fingerprints recovered from a crime scene can be compared with thousands of other prints on file in the computer system at the law enforcement agency. Through a computerized matching process, the computer can select fingerprints that are close in characteristics. In this way a match may be made and a suspect's name produced.

DNA printing allows for the comparison of DNA obtained from human cells (most commonly blood and semen) in order to obtain a match between at least two samples. In order for traditional DNA analysis to be useful, a suspect must first be identified through some other means, so that a comparison of samples can be made. However, emerging technology involves the creation of DNA banks, similar to the computerized fingerprint systems, in order to compare and match DNA structures. There is little question, as technological capabilities advance, so too will the value of physical evidence.

Information from people. Beside physical evidence, another major source of information in a criminal investigation is people, namely witnesses and suspects. Witnesses can be classified as either primary or secondary. Primary witnesses are individuals who have direct knowledge of the crime because they overheard or observed its occurrence. This classification would include crime victims who observed or who were otherwise involved in the offense. Eyewitnesses would also be included here. Secondary witnesses possess information about related events before or after the crime. Informants (or street sources) and victims who did not observe the crime would be best classified as secondary witnesses.

A suspect can be defined as any individual within the scope of the investigation who may be responsible for the crime. Note that a witness may be initially considered a suspect by the police because information is not available to rule him or her out as the one responsible for the crime.

Besides the basic information about the particulars of the criminal event and possibly the actions of the perpetrator (to establish a modus operandi), another important type of information often provided by witnesses is eyewitness descriptions and identifications. Such information is quite powerful in establishing proof—for the police, prosecutor, judge, and jury—but the problem is that eyewitness identifications are often quite inaccurate and unreliable (Loftus et al.). Research has shown that many factors—such as environmental conditions, physical and emotional conditions of the observer, expectancies of the observer, perceived significance of the event, and knowledge of the item or person being described—can significantly influence the accuracy of eyewitness statements.

Hypnosis and cognitive interviews are two investigative tools available in the interview setting for the purpose of enhancing memory recall, and thus enhancing the accuracy of eyewitness information. Hypnosis is typically viewed as an altered state of consciousness that is characterized by heightened suggestibility (Niehaus). For the police, hypnosis is used as a method of stimulating memory in an attempt to increase memory recall greater than that achieved otherwise. While the use of hypnosis has increased sharply in the 1990s many courts have refused to admit such testimony because of accuracy concerns, or have established strict procedures under which hypnotically elicited testimony must be obtained (e.g., interview must be videotaped, the hypnotist should know little or nothing about the particulars of the case, no other persons are to be present during the interview, etc.). Most problematic is that under hypnosis, one is more responsive to suggestions (by definition) and thus, the hypnotist (intentionally or not) may lead the subject and inaccurate information may result. Once again then, information is produced but it is unknown whether the information is accurate.

Another method used to enhance memory recall among witnesses involves the use of the cognitive interview (Niehaus). A cognitive interview is designed to fully immerse the subject in the situation once again, but through freedom of description not hypnosis. The subject is instructed to report everything he or she can think of no matter how trivial it may seem. The witness may be instructed to recount the incident in more than one order. The intent is to allow for a much deeper level of recollection than the traditional interview. Research has shown that the cognitive interview approach elicits significantly more accurate information than a standard police interview, which typically involves frequent interruptions of explanations and descriptions, includes many closed-ended and short answer questions, and involves the inappropriate or overly strict ordering of questions (Niehaus).

In contrast to interviews of witnesses, interrogations of suspects are often more accusatory in nature. Usually interrogations are more of a process of testing already developed information than of actually developing information. The ultimate objective in an interrogation is to obtain a confession (Zulawski and Wicklander).

For obvious reasons, offenders have great incentive to deceive investigators. Understanding this, there are several tools available to investigators who wish to separate truthful from deceptive information. First is the understanding of kinesic behavior, the use of body movement and posture to convey meaning (Walters). Although not admissible in court, information derived from an understanding and interpretation of body language can be quite useful in an investigation. The theory behind the study of nonverbal behavior is that lying is stressful and individuals try to cope with this stress through body positioning and movement.

Although no single behavior is always indicative of deception, there are patterns (Zulawski and Wicklander). For example, a deceptive subject will tend not to sit facing the interrogator with shoulders squared but will protect the abdominal region of the body (angled posture, crossed arms). Major body shifts are typical especially when asked incriminating questions; and use of manipulators (or created jobs) are also common among deceptive subjects (e.g., grooming gestures), as are particular eye movements (Zulawski and Wicklander).

Much like nonverbal behavior, verbal behavior can also provide information about the truthfulness of a suspect. For example, deceptive subjects tend to provide vague and confusing statements, talk very soft or mumble, provide premature explanations, focus on irrelevant (but truthful) points in an explanation, or may claim memory problems or have a selectively good memory. Of course, interpretations of nonverbal and verbal behaviors in terms of deception must consider individual, gender, and cultural differences in personal interaction.

The polygraph is a mechanical means of detecting deception. The polygraph is a machine that measures physiological responses to psychological phenomenon. The polygraph records blood pressure, pulse, breathing rate, and electro-dermal reactivity and changes in these factors when questioned. Interpretation of the resulting chart serves as the basis for a judgment about truthfulness. Once again, the theory is that a person experiences increased stress when providing deceptive information and the corresponding physiological responses can be detected, measured, and interpreted. While this general theory is well founded, the accuracy of the polygraph depends largely on the skill of the operator and the individual who interprets the results of the polygraph examination (Raskin). No one can be forced to take a polygraph and polygraph results are seldom admissible in court. Often investigators threaten suspects with a polygraph examination in order to judge the nature of their reaction to it, or to induce a confession (Raskin).

Additional topics

Law Library - American Law and Legal InformationCrime and Criminal LawPolice: Criminal Investigations - Criminal Investigation Defined, The Structure Of Criminal Investigations, Sources Of Information And Evidence In Criminal Investigations - Conclusion