Investigations | Section 1.2

Fundamentals of Criminal Investigations

Adam J. McKee, Ph.D.

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Section 1.2: Evidence Law & Investigations

Uses of Evidence in Court

Evidence can be divided into two major types.  The first of these major categories is real evidence.  Real evidence means any evidence that consists of a tangible item that can be perceived with the five senses.  Physical evidence is the first thing that comes to mind when we think of evidence, but documents, photographs, videos, sound recordings, and even computer files fall into this category.  Demonstrative evidence also falls into this category, even though it is usually generated by investigators.  Demonstrative evidence consists of aids developed by investigators such as diagrams, sketches, computer simulations, and the like that are intended to help jurors sort through complex evidence and reconstruct a crime in their mind’s eye.

The second major type of evidence is testimonial evidence.  Testimonial evidence refers to things that a person communicates to the court under oath.  Usually, this means courtroom testimony, but can also include things such as affidavits and depositions take out of court.  As a general rule, testimonial evidence not given under oath is inadmissible. Certain exceptions, however, do apply.

Another important way to classify evidence is by whether it is direct or circumstantial.  Direct evidence is evidence that proves a fact without the need for the jury to make any inferences or presumptions.   That is, it directly proves a certain fact at issue.  Circumstantial evidence, on the other hand, does not prove things directly.  Rather, it requires the finders of fact to make inferences and draw their own conclusions.

Direct evidence can be considered the most compelling because, if it is believed, leaves no doubt in the fact finder’s mind.  A common example of direct evidence is when an eye witness testifies that the defendant committed the crime. So long as the witness is credible, there can be little doubt as to guilt, and no inferences are required.  Crimes being committed on videotape is a good example of direct evidence. Assuming that the person on the tape is clearly the defendant and that the tape is believed to be authentic, then there can be little doubt. Unfortunately for the investigator, direct evidence is not very common.  Most cases are built on circumstantial evidence.

While perhaps less compelling, circumstantial evidence is still valuable and can be used to secure convictions.  Some circumstantial evidence can be very compelling, being almost direct in nature.  For example, if a jury hears compelling evidence that the defendant’s fingerprints were found on the murder weapon, then they may conclude that the defendant was, in fact, the person that used the weapon.  Often, circumstantial evidence is used to demonstrate the following:

Ability to commit the crime.  It is a common defense tactic to try and convince the jury that the defendant could not commit a particular crime based on some personal deficiency.  Often, physical ability, mental ability, and means come into play.  Investigators must be prepared to combat these assertions in court by offering evidence that the defendant did, in fact, possess the necessary means, as well as the necessary physical and mental ability to commit the crime.

For example, let us say that a school resource officer finds a bomb in a disgruntled high school student’s locker.  The defense could argue that obviously, the bomb was not the defendants, because he lacked the means to acquire bomb-making supplies and lacked the necessary knowledge to construct the bomb even if he had the means.  If the investigators introduce the U.S. Army’s “Improvised Munitions Handbook” taken from the student’s home into evidence, then the defense’s argument crumbles.

Intent.  Circumstantial evidence is usually used to show intent because only direct testimonial evidence can reveal the inner workings of the defendant’s mind.  So, absent a detailed confession, we are forced to piece together his or her mental state using circumstantial evidence. Most commonly, modus operandi, motive, and threats are used for this purpose.

The phrase modus operandi (MO) literally means “method of operation.”  That is, criminals have patterns in how the accomplish their criminal acts.  If a new crime is committed using the same behavior patterns that the defendant has used in the past, it is circumstantial evidence that the defendant committed this crime as well.  The more novel or bizarre the method, the more compelling the evidence will be. Bizarre behaviors being repeated in multiple crimes are common among sadistic killers and rapists. It can also be important in more mundane crimes, however, such as a burglars signature method of gaining entry into a dwelling.

Motive can also be used to provide the jury with circumstantial evidence of guilt.  Usually, motive is not an element of the crime that needs to be proven in order to secure a conviction.  All that is strictly necessary is that the defendant did it purposely; the reason is of no legal significance.  The reason for committing a crime, however, can be a great aid in helping juries determine guilt. Money, especially large insurance policies, are a common motive for murder and arson.  Do not, however, limit possible motives to money alone. Depending on the nature of the crime, motives can be almost limitless. Revenge, retaliation, racial hatred, jealousy, and even entertainment can be motives for crime.

Threats are also useful in proving guilt.  When a defendant makes a threat to do something criminal, and then that crime happens, a jury may infer that the defendant did the act.  This is not as valuable as it first seems. People often make threats in jest or to “blow off steam,” never intending to actually carry out the threat.  Ultimately, the credibility of a threat or a pattern of threatening behavior is up to the jury.

Acting guilty.  If the prosecution can show that the defendant acted in a way that is consistent with a guilty person, then the jury may infer guilt.  Flight at any stage in the criminal process is often a sign of guilt. Why would an innocent person run from the police or jump bail? If the jury infers that they would not do so, then they might also infer guilt.  Juries may also infer guilt from the defendant trying to conceal evidence. For example, if a knife used in an aggravated battery is found in a dumpster with the defendant’s fingerprints on it, the jury may infer guilt.  Why else would someone discard a perfectly good knife?

Being caught with the fruits of a crime is a powerful indicator of guilt.  Juries can reasonably infer, for example, that the possession of a stolen laptop by the suspect reasonably demonstrates that the person in possession stole it.  The possession of wealth that has no legitimate origin can also be used to suggest guilt, at least in financially lucrative crimes such as dealing in narcotics.

Rules of Evidence

As we have seen, evidence serves many purposes in court.  But before evidence can be used in court, it must be admitted.  For evidence to be admitted by the judge, it must conform to the rules of evidence.  The rules of evidence are rules that determine the admissibility of evidence at hearings and trials.  As a practical matter, almost anything can be admitted into evidence unless there is a specific rule that prohibits it.  Obviously, the competent investigator will have a working knowledge of these rules.

The most important rule of evidence to the investigator is the exclusionary rule.  The exclusionary rule is a rule developed by the U.S. Supreme Court that prohibits the admission of illegally obtained evidence in court.  This rule is grounded in the Fourth Amendment, which seeks to protect American citizens from unreasonable searches and seizures.  As a practical matter, investigators must make sure that evidence is obtained within the parameters set forth by the courts in Fourth Amendment cases.  The rules about searches and seizures are voluminous and complex. Beyond dedication and long study to learn these procedural laws, the best advice is this:  When in doubt, get a warrant.

A related rule is the fruits of the poisonous tree doctrine.  This rule says that when illegally obtained evidence leads an investigator to new evidence, then that evidence (the “fruits” of the original illegal search) is also inadmissible.  When these two doctrines are taken together, we can quickly see just how bad it would be if it was determined that a crime scene was illegally searched. The guilty party would almost certainly go free.  Thus, we see that the success of our investigations, the best interest of justice, and the public safety all demand that investigations be conducted “by the book.”

Assuming that evidence was legally obtained by the police, it is still not automatically admissible in court.  To be admissible, the judge must determine that the evidence is 1) relevant, 2) material, and 3) competent.

Relevancy means that a piece of evidence goes to demonstrate the truth or falsity of an issue before the court.  What this basically means is that the evidence must be doing something useful in proving a disputed fact.  Evidence that is not relevant would waste the court’s time and potentially prejudice the defendant.

Materiality refers to the strength or importance of the evidence in supporting or falsifying an alleged fact.

Competency refers to the quality or trustworthiness of the evidence.  If the credibility of a piece of evidence or the person offering it is in question, then the value of the evidence is discerning the truth severely diminished.  The judge may determine that the evidence is so unreliable that it will not be admitted.

Chain of Custody

Chain of custody is a legal term that refers to the ability to guarantee the identity and integrity of evidence from collection through the presentation in court. It is a process used to maintain and document the chronological history of the evidence. Chain-of-custody documents should include the name of the person collecting the sample, each person or entity subsequently having custody of it, dates the items were collected or transferred, the collection location, a brief description of the item, and an identification number.

Evidence is considered to be in your custody when:

  • The evidence is in your physical possession;
  • The evidence is in your view, after being in your physical possession;
  • The evidence is in your physical possession and then locked up so that tampering cannot occur;
  • The evidence is kept in a secured area, with access restricted to authorized personnel only.

The exact chain of custody requirements will vary from jurisdiction to jurisdiction.  Most departments and crime labs will have developed protocols that must be followed.  The following are some general rules to keep in mind:

  • Keep the number of people involved in collecting and handling the evidence to a minimum.
  • Only allow people associated with the investigation to handle evidence.
  • Always document the transfer of evidence from one person to another on chain-of-custody forms.
  • Always accompany samples and data with their chain-of-custody forms.
  • Give evidence items positive identification at all times that is legible and written with permanent ink.

Chain of custody is one of the most important tasks of the criminal investigator because if the chain of custody is broken, the evidence cannot be used in court.

Linkage Concepts

Comparison samples: A generic term used to describe physical material/ evidence discovered at crime scenes that may be compared with samples from persons, tools, and physical locations. Comparison samples may be from either an unknown/questioned or a known source. Samples whose source is unknown/questioned are of three basic types: Recovered crime scene samples whose source is in question (e.g., evidence left by suspects, victims).

Questioned evidence that may have been transferred to an offender during the commission of the crime and taken away by him or her. Such questioned evidence can be compared with evidence of a known source and can thereby be associated/linked to a person/ vehicle/tool of a crime. Evidence of an unknown/questioned source recovered from several crime scenes may also be used to associate multiple offenses that were committed by the same person and/or with the same tool or weapon.

Samples whose source is known are of three basic types: A standard/reference sample is material of a verifiable/documented source which, when compared with evidence of an unknown source, shows an association or linkage between an offender, crime scene, and/or victim (e.g., a carpet cutting taken from a location suspected as the point of transfer for comparison with the fibers recovered from the suspect’s shoes, a sample of paint removed from a suspect’s vehicle to be compared with paint found on a victim’s vehicle following an accident, or a sample of the suspect’s and/or victim’s blood submitted for comparison with a bloodstained shirt recovered as evidence).

A control/blank sample is material of a known source that presumably was uncontaminated during the commission of the crime (e.g., a sample to be used in laboratory testing to ensure that the surface on which the sample is deposited does not interfere with testing. For example, when a bloodstain is collected from a carpet, a segment of unstained carpet must be collected for use as a blank or elimination sample).

An elimination sample is one of known source taken from a person who had lawful access to the scene (e.g., fingerprints from occupants, tire tread impressions from police vehicles, footwear impressions from emergency medical personnel) to be used for comparison with evidence of the same type.

Key Terms

References and Further Reading


Modification History

File Created:  05/02/2019

Last Modified:  05/02/2019

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This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.

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