Investigations: Understanding Probable Cause

Criminal investigation demands a thorough understanding of probable cause. Detainments, arrests, searches, seizures, interviews and interrogations are legally judged on the reasonableness of police action based on the totality of circumstance known to the police prior to taking an action. The power of the police in our free society is intentionally limited by several of the first 10 amendments to the U.S. Constitution, along with decisions from federal, state and local court cases. The mechanism used to explain a police officer's legal justification for interfering with a person's liberty (e.g., effectuating an arrest with or without a warrant) is written communication, such as a police report and/or affidavit, followed by oral communication, such as a deposition and/or court testimony. Ability to skillfully document in writing facts and circumstances that lead to logical inferences and reasonable conclusions remains a professional requirement in criminal investigation. Excellent investigative work is negated and the guilty may walk free if the legal framework on which it was based can't be adequately explained.

Our forefathers feared strong central government. In 1789, two years after the signing of the Constitution, James Madison of Virginia proposed a dozen amendments to the Constitution. Congress approved 10 of them in September 1791, and they took effect in December of that year.1 These first 10 amendments to the Constitution became known as the Bill of Rights. To understand the investigative process, you must know that the Bill of Rights restricts government actions against individuals. The Bill of Rights, particularly the Fourth, Fifth and Sixth Amendments and their associative court cases handed down through the years, is the engine that drives what the police can and can't do.

Probable Cause & the Fourth Amendment
The laws related to search and arrest are covered by the Fourth Amendment, which states, The right of the people to be secure in their persons, houses, papers and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

In order to make a valid arrest (seizure) or search a particular person, place or thing, a police officer must have probable cause facts and circumstances that would lead a reasonable person to believe a crime is being, has been or will be committed. Note the use of the present, past and future tense in this language. If a police officer observes a crime committed in their presence, such as John strikes Paul in the face with his fist, they may, in most states, arrest John without an arrest warrant signed by a judge.

If John strikes Paul in the face with his fist and a police officer was not present but called to the scene a short time after this occurred, a crime has been committed. The officer did not see the crime occur. In most states, if John can be located a short time after the incident based on speedy information from Paul, the officer can arrest John without an arrest warrant based on probable cause. This ability of the police in most states to arrest without an arrest warrant signed by a judge is especially true if the assault was a felony.

Here's an example of a crime that will be committed: A confidential, reliable informant was recently in an apartment and heard three other men discussing robbing a bank. He also observed the men viewing diagrams of the bank, alarm codes and escape routes, and he saw weapons in the apartment to be used in the bank robbery. The informant tells a detective a bank robbery is being planned.

The definitions of what constitutes a crime, felony, misdemeanor, violation, corpus delecti and elements of a crime prove crucial to understanding the circumstances under which an officer can make an arrest with or without an arrest warrant based on probable cause.

Probable cause For a police officer to make an arrest with or without an arrest warrant signed by a judge, the officer must be able to conclude to a reasonable probability an offense has been committed and the person is in fact a criminal participant.2 A shortened version of the definition of probable cause often used to train police officers states less than proof, but more than mere suspicion that a crime is being, has been or will be committed. Thus, probable cause requires a higher standard than reasonable suspicion, but less than the proof beyond a reasonable doubt required for conviction in court.

Probable cause is a series of facts and circumstances in a step-by-step progression that by themselves or in combination may cause a reasonably prudent person to believe a crime was committed and therefore allow the suspect s arrest. Another definition: Rational grounds of suspicion, supported by circumstances sufficiently strong in themselves to warrant a cautious man in believing the accused to be guilty.3 This may take seconds or years.

Here s a partial list of guilt-laden or other facts that may help build probable cause:

  • Flight;
  • Furtive movements;
  • Hiding;
  • An attempt to destroy evidence;
  • Resistance to officers;
  • Admissions or confessions;
  • Evasive answers;
  • Unreasonable explanations;
  • Fingerprint or DNA identification;
  • Hair-follicle identification;
  • Handwriting comparisons;
  • Fabric comparisons;
  • Identification of a suspects by a witness;
  • Ballistics;
  • Contraband or weapons in plain view;
  • A criminal record;
  • Hearsay information;
  • Police training and experience;
  • Unusual or suspicious behavior; and
  • Footprints, blood-splatter evidence, tire impressions, etc.

Arrest There are many different definitions of what constitutes an arrest. One definition: the taking of a person into custody for violation of any law, ordinance, regulation or bylaw of the state. Another definition states, the action of taking a person into custody for the purpose of charging him or her with a crime. 4 Whichever definition we use, arrest is a seizure of a person under the Fourth Amendment.

In addition to the probable-cause requirements, an arrest has three basic elements: authority, intention and custody. The episode below illustrates all three elements.

War Story
Many years ago, I was walking a beat in Hartford, Conn., that included a large park. I was strolling along one of its paths in full uniform, swinging my nightstick to and fro in celebration of a gorgeous spring day. (I was a police officer in this jurisdiction with the authority to arrest.) An elderly woman sitting on a park bench feeding the pigeons waved me over to her.

Top of the morning to you, ma am, I said to the lady, tipping my hat.

She cocked her head, motioned me closer and pointed. You see that man over there by the fountain? He has a .45 automatic under his coat in a holster just over his back pocket. I looked over to the man about 20 feet away and slowly walking in the opposite direction.

How do you know that? I asked.

I saw it. I was in the army for 35 years and I carried a .45 every day. I know a gun when I see one.

I nodded, viewing the elderly women in a new light. Are you sure? I asked.

She made a derisive noise in her throat and said, Absolutely!

I rapidly made my way along the path, and as I closed on the man, he looked over his shoulder with an alarmed expression and increased his pace almost to a trot. I yelled at him, Hey buddy, I want to talk with you.

The man began to run, and I gave chase. The man ran out of the park into a line of traffic, narrowly missing being hit by several cars. I saw the man take out a gun, and as he ran by a mailbox, he opened the lid and tossed the gun in. Yelling, You're under arrest, I tackled him, and after a brief struggle, I managed to get him into handcuffs.

For this scenario, a list of the building blocks of probable cause that gave me the authority to arrest might look something like this:

1. I was a readily identifiable police officer on duty in my jurisdiction;

2. It was daytime, with no vision obstructions;

3. I was contacted by a citizen who reported a possible crime;

4. The citizen appeared to be credible; she related information about a possible crime based on her prior experience (army, guns) and specific personal observations (man carrying gun);

5. The citizen had no discernible reason to lie or gain anything from reporting her observations;

6. As I approached the man, he looked at me with an expression of alarm, increased his pace and began moving quickly away;

7. I told the man I wanted to speak with him, at which point he ran away from me out of the park into a line of traffic, narrowly missing being hit by several cars; and

8. I saw the man take out a gun and, as he ran by a mailbox, open the lid and toss it in.

Whether probable cause exists in each case is determined by the totality of circumstance surrounding the act or acts in question.5 Did I have facts and circumstance that would lead a reasonable person to believe a crime had been committed and that this person committed the crime? You bet! Did I have the authority and intention to arrest the man? I did. Did I take the man into custody with the intent of bringing him before a court to answer for a crime? Absolutely. Did I have a warrant for the man s arrest signed by a judge? No. However, a crime was committed in my presence.

It's important to note that the facts gathered prior to an arrest in support of probable cause cannot be used post facto. Had I not observed the suspect place the gun in the mailbox and arrested him prior to its discovery, I could not use the later discovery of the gun in support of the probable cause to arrest him.

Reasonable & Prudent
There are many intricate layers in the law relative to authority, intention and custody. For example, what if a police officer pursues a fleeing felon in a motor vehicle, and the felon drives the car over the city or state line out of the officer s normal jurisdiction? In most states this situation is covered as hot pursuit, and the officer carries jurisdictional authority to arrest into the city or state. What if the officer is in another jurisdiction in hot pursuit and loses sight of the suspect? Once again, the standard is what would be reasonable and prudent under the totality of circumstances. How serious is the crime? Would 10 minutes of searching and then finding the felon be reasonable? How about 30 minutes, an hour, five hours?

Time and again throughout legal interpretations of the law, you'll encounter the phrase reasonable and prudent or the question, Were the officer's actions reasonable given the circumstances? Remember: Clearly outline in your report every detail known to you at the time so that a subsequent reviewing authority has the full situation in mind when deciding whether your actions were reasonable given the totality of the circumstances.

My next article will deal with another basic precept of the investigation process: searching a person, place or thing without a warrant.


1. Inciardi, James. Introduction to Criminal Justice. 7th Edition. Orlando, Florida: Harcourt College Publishers, 2002. p. 126.

2. State v. Gant, 231 Conn. 43 (1994).

3. Brinegar v. United States, 338 U.S. 161 (1949).

4. Inciardi, p. 141.

5. State v. Atkinson, 235 Conn. 748 (1996).


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