Probable Cause: Definition, Legal Requirements, Example

What Is Probable Cause?

Probable cause is a requirement in criminal law that must be met before a police officer can make an arrest, conduct a search, seize property, or get a warrant.

Understanding Probable Cause

Probable cause requires that the police have more than just suspicion—but not to the extent of absolute certainty—that a suspect committed a crime. The police must have a reasonable basis in the context of the totality of the circumstances for believing that a crime was committed. The probable cause requirement stems from the Fourth Amendment of the U.S. Constitution, which provides for the right of citizens to be free from unreasonable government intrusion into their persons, homes, and businesses.

Probable cause is important in two aspects of criminal law:

1. Police must have probable cause before they search a person or property, and before they arrest a person.
2. The court must find that there is probable cause to believe the defendant committed the crime before they are prosecuted.

When a search warrant is in effect, police must generally search only for the items described in the warrant, although they can seize any contraband or evidence of other crimes that they find. However, if the search is deemed to be illegal, any evidence found becomes subject to the “exclusionary rule” and cannot be used against the defendant in court.

Example of Probable Cause

Illinois v. Gates is a landmark case in the evolution of probable cause and search warrants. In May 1978, the police department in Bloomingdale, Illinois, received an anonymous letter outlining in-depth details about plans by the defendants—named Gates, plus others—to transport drugs from Florida to Illinois. The police obtained a search warrant from a judge on the basis of a signed affidavit and the anonymous letter. When Gates arrived home, the Bloomingdale police searched his car, recovering over 350 pounds of marijuana, as well as more marijuana and weapons in the Gates residence.

The Illinois trial court suppressed all the seized items, and the Illinois Appellate Court and the state Supreme Court affirmed the trial court's ruling. It found that the search was unlawful since the affidavit did not provide enough evidence to establish sufficient cause, leading to the exclusion of the evidence obtained on the basis of the warrant. The case went up to the Supreme Court, which overturned the Illinois Supreme Court ruling.

In ruling in favor of the State of Illinois, the Supreme Court rejected the Aguilar-Spinelli test, a judicial guideline established by the Supreme Court for evaluating the validity of a search warrant, or an arrest without a search warrant based on information provided by a confidential informant or anonymous tip. The two prongs of the Aguilar-Spinelli test are that, when a magistrate signs a warrant sought by the police, they must be kept informed of:

1. The reasons to support the conclusion that the informant is reliable and credible.
2. Some of the underlying circumstances relied upon by the person providing the information.

Impact of Case on Probable Cause

The Supreme Court instead put into place a “totality-of-the-circumstances” standard, because there was more evidence that Gates was involved in drug trafficking than just the letter by itself. For instance, Florida was a known source for illegal drugs, and Gates’ stay at a motel for only one night and immediate return to Chicago was suspicious. The Court also agreed that the anonymous letter by itself would not be probable cause to get a warrant, while the “reliability” prong of the Aguilar-Spinelli was unlikely to ever be satisfied by an anonymous tip.

Overall, the Supreme Court decision, in this case, lowered the threshold of probable cause by ruling that it could be established by a “substantial chance” or “fair probability” of criminal activity, rather than a better-than-even chance.