The concept of probable cause is rooted in the Bill of Rights of the United States Constitution. Under the Fourth Amendment, “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….”

Generally speaking, probable cause is a standard of proof that law enforcement must meet before arresting, charging, or searching a criminal suspect. Neither the Constitution nor the Bill of Rights defines probable cause, however, leaving its interpretation up to courts.

Challenging the existence of probable cause is a common defense in criminal matters. But, the term is often misunderstood and misapplied. This guide explains the case law that, over time, has developed the modern conceptualization of probable cause, and compares it to other standards of proof in criminal cases.

Landmark Court Cases Addressing Probable Cause

Over the years, the Supreme Court of the United States (SCOTUS) has described the standard which equates to probable cause for law enforcement to take certain actions, but has consistently left application of the standard to lower courts. SCOTUS hesitates in trying to offer a checklist or quantifiable measure of probable cause, instead taking the position that whether evidence constitutes probable cause must be weighed on a case-by-case basis. Although many Supreme cases and lower court cases have addressed probable cause in one way or another, two landmark cases, in particular, inform the way this legal standard is applied today in Pennsylvania and across the nation:

Carroll et al v. United States (1925)

In this case, the court upheld the conviction of defendants who violated the National Prohibition Act. Federal prohibition agents and a state officer searched their vehicle near Grand Rapids, Michigan and found 68 quarts of whiskey and gin sewed inside the upholstery of the seats. The defendants appealed their case, arguing that law enforcement violated their Fourth Amendment rights by conducting a warrantless search and seizure. After citing several other examples of the application of probable cause, the Supreme Court offered the following opinion in regards to the presence of probable cause for this particular case:

… it is clear the officers here had justification for the search and seizure. This is to say that the facts and circumstances within their knowledge and of which they had reasonably trustworthy information were sufficient in themselves to warrant a man of reasonable caution in the belief that intoxicating liquor was being transported in the automobile which they stopped and searched.

Brinegar v. United States (1949)

More than 20 years after the Carroll decision, the Supreme Court applied the same standards to another case involving the transportation of liquor, a violation of Oklahoma’s Liquor Enforcement Act of 1936. The defendant argued that law enforcement violated his Fourth Amendment rights, but the Supreme Court upheld the lower court’s conviction, reasoning that the defendant’s criminal history and activities surrounding alcohol constituted probable cause. The opinion offered a more definitive description of probable cause:

Probable cause exists where the facts and circumstances within the officers’ knowledge, and of which they have reasonably trustworthy information, are sufficient in themselves to warrant a belief by a man of reasonable caution that a crime is being committed.

Probable Cause versus Other Evidentiary Standards

All criminal convictions rely on evidence, but various standards of evidence mediate the government’s power to investigate and prosecute crimes. Most Americans are familiar with the phrase “beyond a reasonable doubt,” which is the standard of proof needed for a jury to convict someone of a crime. “Beyond reasonable doubt” is the strictest evidentiary standard the criminal legal process, reflecting our collective belief that a person should not be deprived of liberty except when evidence of guilt is overwhelmingly clear.

On the other end of the spectrum, the least-strict evidentiary standards in criminal law are reasonable suspicion and probable cause. Legal professionals, journalists, and others often interchange these two ideas, but they are, in fact, different things.

Reasonable suspicion is the least-strict standard to which law enforcement must adhere. It arose out of a 1968 court case, Terry v. Ohio. Reasonable suspicion is the idea that a police officer must suspect that a crime has happened or might happen based on the circumstances of the situation and an officer’s training and skills, in order to take a particular action (i.e., to make a traffic stop). For example, a police officer who sees a car swerve over the centerline usually has reasonable suspicion to pull the car over and check whether the driver is drunk.

Although probable cause is also a fluid concept that depends on circumstances, it is a higher standard than reasonable suspicion. It is the evidentiary threshold law enforcement must meet before making an arrest, getting a search warrant, or performing a search.

In many cases, reasonable suspicion can lead to an opportunity for law enforcement to find probable cause. Take the traffic stop example above. Suppose the officer makes the traffic stop after seeing a vehicle swerve over the centerline. In speaking with the driver, the officer smells alcohol on the driver’s breath and notices that the driver’s speech is slurred. The officer now has more than reasonable suspicion a DUI offense has occurred, and can ask the driver to take sobriety tests and to blow into a Breathalyzer. If the driver cannot balance and registers a breath alcohol content (BAC) above the legal limit, the officer has enough evidence in hand to constitute probable cause to arrest the motorist for driving under the influence of alcohol.

Get the Legal Defense You Need

If you were arrested and charged with a crime, federal and state law afford you rights regardless of the nature of the crime. You need an experienced defense attorney who understands the nuances of the law, including evidentiary standards like probable cause.

Law enforcement must have probable cause to arrest you, obtain a search warrant, or search your vehicle. With no explicit legal definition, the presence of probable cause remains one of many areas that can serve as a viable defense in some criminal cases. Whether you are still detained, have been released on bail, or are searching for legal representation for a family member, contact The Zieger Firm at (215) 546-0340 right away to schedule a free consultation.

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