Pin During the course of any criminal investigation, someone refers to the concept of probable cause with respect to the investigation. What, exactly, is probable cause? The Fourth Amendment states that “no warrants shall issue, but upon probable cause,” but it fails to clarify what, exactly, probable cause is. The US Supreme Court has tried to offer an explanation to this very fluid, inexact, and context-based term.

The word “reasonable” is a key word when attempting to explain the concept of probable cause. Thus, most definitions of probable cause involves a reasonable suspicion that is supported by circumstances strong enough to justify a prudent person’s belief that certain facts are probably true.

The history of probable cause

The concept of probable cause has its roots in English common law wherein people believed that a man’s home is his castle. By the 1600s, this saying expanded to legally protect landowners from casual searches by law enforcement or other government officials. By the 1700s, the English began to use general warrants and writs of assistance in order to lawfully search areas in which government officials believed held evidence of a crime.

In the US, probable cause has its roots in the Fourth Amendment of the US Constitution that protects citizens from unreasonable searches and seizures of their “persons, houses, papers, and effects.” In other words, police officers may not simply search your home or your place of business looking for evidence against you.

Further, law enforcement may not arrest you without probable cause either. Consequently, when police obtain a warrant—whether a search or arrest warrant—they must prove to the judge that there is probable cause for its issuance.

Thus, the court will find probable cause exists—and issue a warrant—when there is a reasonable basis for believing that a crime may have been committed (arrest) and/or evidence of a crime may be present in the area(s) to be searched (search).

Sources of probable cause

In order to prove probable cause, a police officer must have sufficient knowledge of fact that warrants a reasonable belief that a suspect has committed or is committing a crime.

Probable cause is one of the legal standards used during the course of a criminal investigation and trial. Probable cause requires more than simply a suspicion that someone committed a crime but not as much to prove beyond a reasonable doubt that a suspect is, indeed, guilty. Beyond a reasonable doubt is a standard is reserved for the courts.

There are four general categories of evidence police may use to establish probable cause for an arrest and/or perform a search.

• Observation: Anything an officer directly sees, hears, smells, or feels is considered observation and may be used to establish probable cause.
• Circumstantial evidence: This is indirect evidence that implies—but does not prove—an incident occurred but provides probable cause for an officer to conduct a search.
• Information: Information obtained from a reliable source such as a witness, victim, informant, or the like can be used to establish probable cause.
• Expertise: Police officers may rely on their training, expertise, and professional knowledge to identify criminal activity and establish probable cause.

When a warrant isn’t necessary

A warrant is not always necessary. A warrant isn’t necessary if a person consents to the search of their person and/or property. In such cases, probable cause is automatically created; however, the search area is limited to only the area where the consent was given. If a roommate or other resident consents to a search of the property, this search area is also limited to areas and places that the person who provided the consent has access to.

Probable cause can also serve as the justification for a warrantless search and seizure when exigent circumstances—circumstances that would cause a reasonable person to believe that entry was necessary to prevent destruction of evidence, physical harm to another, the suspect’s escape, or other similar situation—exists.

Without a warrant—and without probable cause—any evidence seized from a warrantless search can be excluded from court via the exclusionary rule of the Fourth Amendment. Oftentimes, such evidence is known as “fruit of the poisonous tree.”