Police: Criminal Investigations

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This entry provides an overview of the criminal investigation process and investigative methods. The focus of the discussion is on definitional issues along with the identification and evaluation of the types and sources of information often used in criminal investigations.

Criminal investigation defined

An investigation refers to the process of collecting information in order to reach some goal; for example, collecting information about the reliability and performance of a vehicle prior to purchase in order to enhance the likelihood of buying a good car. Applied to the criminal realm, a criminal investigation refers to the process of collecting information (or evidence) about a crime in order to: (1) determine if a crime has been committed; (2) identify the perpetrator; (3) apprehend the perpetrator; and (4) provide evidence to support a conviction in court. If the first three objectives are successfully attained, then the crime can be said to be solved. Several other outcomes such as recovering stolen property, deterring individuals from engaging in criminal behaviors, and satisfying crime victims have also been associated with the process.

A useful perspective on the criminal investigation process is provided by information theory (Willmer). According to information theory, the criminal investigation process resembles a battle between the police and the perpetrator over crime-related information. In committing the crime, the offender emits "signals," or leaves behind information of various sorts (fingerprints, eyewitness descriptions, murder weapon, etc.), which the police attempt to collect through investigative activities. If the perpetrator is able to minimize the amount of information available for the police to collect, or if the police are unable to recognize the information left behind, then the perpetrator will not be apprehended and therefore, the perpetrator will win the battle. If the police are able to collect a significant number of signals from the perpetrator, then the perpetrator will be identified and apprehended, and the police win. This perspective clearly underscores the importance of information in a criminal investigation.

The major problem for the police in conducting a criminal investigation is that not only is there potentially massive amounts of information available, but the relevance of the information is often unknown, the information is often incomplete, and the information is often inaccurate. Further, to be useful in proving guilt in court (where beyond a reasonable doubt is the standard), the evidence must have certain other qualities, and certain rules and procedures must be followed in collecting the evidence.

The structure of criminal investigations

Criminal investigations can be either reactive, where the police respond to a crime that has already occurred, or proactive, where the investigation may go on before and during the commission of the offense.

The reactive criminal investigation process can be organized into several stages. The first stage is initial discovery and response. Of course, before the criminal investigation process can begin, the police must discover that a crime occurred or the victim (or witness) must realize that a crime occurred and notify the police. In the vast majority of cases it is the victim that first realizes a crime occurred and notifies the police. Then, most often, a patrol officer is dispatched to the crime scene or the location of the victim.

The second stage, the initial investigation, consists of the immediate post-crime activities of the patrol officer who arrives at the crime scene. The tasks of the patrol officer during the initial investigation are to arrest the culprit (if known and present), locate and interview witnesses, and collect and preserve other evidence.

If the perpetrator is not arrested during the initial investigation, then the case may be selected for a follow-up investigation, the third stage of the reactive investigation process. The follow-up investigation consists of additional investigative activities performed on a case, and these activities are usually performed by a detective. The process of deciding which cases should receive additional investigative effort is referred to as case screening. This decision is most often made by a detective supervisor and is most often guided by consideration of the seriousness of the crime (e.g., the amount of property loss or injury to the victim) and solvability factors (key pieces of crime-related information that, if present, enhance the probability of an arrest being made) (Brandl; Brandl and Frank).

Finally, at any time in the process the case may be closed and investigative activities terminated (e.g., victim cancels the investigation, the crime is unfounded, there are no more leads available, or an arrest is made). If an arrest is made, or an arrest warrant is issued, primary responsibility for the case typically shifts to the prosecutor's office. The detective then assists the prosecutor in preparing the case for further processing.

With regard to proactive criminal investigations, undercover investigations are of most significance (Marx). Perhaps the most well-known type of undercover strategy is the sting or buy-bust strategy that usually involves a police officer posing as someone who wishes to buy some illicit goods (e.g., sex, drugs). Once a seller is identified and the particulars of the illicit transaction are determined, police officers waiting nearby can execute an arrest. Another common strategy involves undercover police officers acting as decoys where the attempt is to attract street crime by presenting an opportunity to an offender to commit such crime (e.g., a police officer poses as a stranded motorist in a high crime area; when a robbery attempt is made, nearby officers can make an arrest). Undercover strategies are controversial primarily because of the possibility of entrapment. Although a multitude of court cases have dealt with this issue, the basic rule is that the police can provide the opportunity or can encourage the offender to act but cannot compel the behaviora fine line indeed.

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 regardsome 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 prooffor the police, prosecutor, judge, and jurybut the problem is that eyewitness identifications are often quite inaccurate and unreliable (Loftus et al.). Research has shown that many factorssuch 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 describedcan 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).

Other sources of information

Along with physical evidence, witnesses, and suspects, there are a number of other sources that can provide useful information in a criminal investigation. These include psychological profiling, crime analysis, and the general public.

In the last two decades, psychological profiling has received much media attention. It is often portrayed as a complicated yet foolproof method of crime solving. In reality, psychological profiling is not all that mysterious. Psychological profiling is a technique for identifying the major personality, behavioral, and background characteristics of an individual based upon an analysis of the crime(s) he or she has committed. The basic theory behind psychological profiling is that the crime reflects the personality and characteristics of the offender much like how clothes, home decorations, and the car you drive reflects, to some degree, your personality. And these preferences do not change, or do not change very much over time.

In constructing a psychological profile, the characteristics of the offender are inferred from the nature of the crime and the behaviors displayed. The elements of a psychological profile are essentially statements of probability as determined from previous crimes and crime patterns (Holmes). Profiles are best suited and most easily constructed in cases where the perpetrator shows indications of psychopathology such as lust and mutilation murder, sadistic rape, and motiveless fire setting.

The value of a psychological profile is that it can help focus an investigation or reduce the number of suspects being considered. In this respect, a psychological profile is much like most physical evidence; a psychological profile cannot identify a suspect when one is not already known. There has been very little systematic research that has documented the actual impact of psychological profiles on criminal investigations. In an internal examination by the F.B.I. (as reported in Pinizzotto, 1984), analysts examined 192 cases where profiling was conducted. Of these, 88 were solved. Of these 88 solved cases, in only 17 percent did a profile offer significant help. Given the limitations of psychological profiles, it is clear that they are not as useful as media depictions might suggest.

Crime analysis is another potentially powerful source of information in a criminal investigation. Simply defined, crime analysis is the process of identifying patterns or trends in criminal incidents. Various means can be used to reach such ends, from computer mapping technology to computer data banks. The Violent Criminal Apprehension Program (VI-CAP) of the Federal Bureau of Investigation is an example of an elaborate and sophisticated crime analysis system. When police departments are confronted with unsolved homicides, missing persons, or unidentified dead bodies, personnel in the department may complete a VI-CAP questionnaire, which asks for detailed information regarding the nature of the incident. These data are then sent to the F.B.I. and entered into the VI-CAP computer system, which is able to collate the data and possibly link crimes that occurred in different jurisdictions based on similarities in the crimes. This system, and similar intrastate systems, are designed to facilitate communication and sharing of information across agencies.

Finally, the general public is a potentially useful source of information in criminal investigations. As defined here, the public consists of people who have information relating to a particular crime or criminal but often cannot be identified through traditional methods (like a neighborhood canvass). Crime Solvers (or Crime Stoppers) tip lines and television shows such as America's Most Wanted provide a method of disseminating information and encouraging individuals to come forward with information relating to particular crimes. Although once again, little systematic research has examined the actual effects of such strategies, information that has come from the general public through these sources has led to the solving of many crimes across the county (Rosenbaum; Nelson).


This discussion has provided an overview of the criminal investigation process and the role, function, and utility of various types and sources of evidence within the process. With these understandings, one may be able to better appreciate the complexities of criminal evidence and the criminal investigation process.

Steven G. Brandl

See also Criminal Procedure: Constitutional Aspects; Entrapment; Federal Bureau of Investigation: History; Police: Handling of Juveniles; Police: History; Police: Organization and Management; Police: Police Officer Behavior; Police: Policing Complainantless Crimes; Police: Private Police and Industrial Security; Police: Special Weapons and Tactics (SWAT) Teams; Scientific Evidence; Urban Police.


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