Probable Cause in US Criminal Justice

'Reasonable Suspicion' vs. 'Probable Cause'

People holding sign protesting police stop and frisk searches
Activists Organize March and Rally Against Police Stop And Frisks\. Mario Tama / Getty Images

In the U.S. criminal justice system, the police cannot arrest people unless they have "probable cause" to do so. While TV cops rarely have trouble finding it, "probable cause" in the real world is far more complicated.

Probable cause is a standard created by the Fourth Amendment to the United States Constitution that must usually be proven before police can make arrests, conduct investigative searches, or be issued warrants to do so. The Fourth Amendment 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." [Emphasis added].

In practice, judges and courts typically find probable cause for making arrests exists when there is a reasonable belief that a crime may have been committed or for conducting searches when evidence of the crime is believed to be present in the place to be searched.

In exceptional cases, probable cause can also be used to justify arrests, searches, and seizures without a warrant. For example, a "warrantless" arrest may be allowed when a police officer has probable cause but not enough time to request and be issued a warrant. However, suspects arrested without a warrant must be given a hearing before a judge shortly after the arrest for an official judicial finding of probable cause.

The Constitutional Quandary of Probable Cause

While the Fourth Amendment requires “probable cause,” it fails to explain exactly what the term means. So, in an example of the "other" ways the Constitution can be amended, the U.S. Supreme Court has tried to clarify the practical meaning of probable cause.

Perhaps most importantly, the Court in 1983, finally concluded that the very concept of probable cause is imprecise and depends largely on the circumstances of the particular criminal act involved. In its decision in the case of Illinois v. Gates, the Court declared probable cause to be a "practical, non-technical" standard that depends on the "factual and practical considerations of everyday life on which reasonable and prudent men [...] act." In practice, courts and judges often allow police greater leeway in the determination of probable cause when the alleged crimes are serious in nature, such as homicide.

As an example of "leeway" in determining the existence of probable cause, consider the case of Sam Wardlow.

Probable Cause in Searches and Arrests: Illinois v. Wardlow

‘Flight is the Consummate Act of Evasion’

Is running from a police officer for no apparent reason probable cause for being arrested?

On a night in 1995, Sam Wardlow, who was holding an opaque bag at the time, was standing on a Chicago street known for being in a high drug trafficking area. Noticing two police officers driving down the street, Wardlow fled on foot. When the officers caught Wardlow, one of them patted him down looking for weapons. The officer conducted the pat-down search based on his experience that weapons and illegal drug sales often went together. After finding that the bag Wardlow was holding contained a loaded .38 caliber handgun, the officers placed him under arrest.

In his trial, Wardlow’s lawyers filed a motion to block the gun from being admitted as evidence claiming that in order to legally detain an individual, short of actually arresting the person, police first had to point to "specific reasonable inferences" (probable cause) why the detention was necessary. The trial judge rejected the motion, ruling that the gun had been discovered during a lawful stop and frisk. Wardlow was convicted of unlawful use of a weapon by a felon. However, the Illinois Court of Appeals overturned the conviction finding that the officers did not have probable cause to detain Wardlow. The Illinois Supreme Court agreed, ruling that fleeing from a high crime area does not create a reasonable suspicion to justify a police stop because fleeing may simply be an exercise of the right to "go on one's way." So, the case of Illinois v Wardlow went to the U.S. Supreme Court.

In considering Illinois v Wardlow, the Supreme Court had to decide, "Is a person’s sudden and unprovoked flight from identifiable police officers, patrolling a high crime area, sufficiently suspicious to justify the officers' stop of that person?"

Yes, it is, ruled the Supreme Court. In a 5-4 decision delivered by Chief Justice William H. Rehnquist, the Court ruled that the police officers had not violated the Fourth Amendment when they stopped Wardlow because it was reasonable to suspect that he was involved in criminal activity. Chief Justice Rehnquist wrote "[n]ervous, evasive behavior is a pertinent factor in determining reasonable suspicion" to justify further investigation. As Rehnquist further noted, "flight is the consummate act of evasion."

The Terry Stop: Reasonable Suspicion Vs. Probable Cause

Whenever the police pull you over for a traffic stop, you and any passengers with you have essentially been "seized" by the police within the meaning of the Fourth Amendment. According to decisions of the U.S. Supreme Court, police officers can order all occupants out of the vehicle without violating the Fourth Amendment’s prohibition of "unreasonable" searches and seizures. In addition, the police are allowed, for their own protection, to search the occupants of the vehicle for weapons if they have "reasonable suspicion" to believe that they are armed or might be engaged in criminal activity. In addition, if the police have reasonable suspicion that any of the occupants of the vehicle might be dangerous and that the vehicle might contain a weapon, they may search the vehicle.

Any traffic stops that escalates into a search and potential seizure is now popularly known as a "Terry stop," from a legal standard established by the U.S. Supreme Court in its 1968 Terry v. Ohio decision.

In essence, in Terry v. Ohio, the Supreme Court established the legal standard that a person may be detained and searched by police based on a "reasonable suspicion" that the person may have been engaged in criminal activity, whereas an actual arrest requires the police to have "probable cause" to believe that the person had actually committed a crime.

In Terry v. Ohio, the Supreme Court had to decide whether the police are allowed under the Fourth Amendment to temporarily detain people and search them for weapons without having probable cause to arrest them.

In an 8-1 decision, the Supreme Court ruled that police may perform a limited surface check of a person’s outer clothing – a "stop and frisk" pat-down search – for weapons that could endanger the officers or bystanders, even without probable cause for an arrest. In addition, the Court ruled that any weapons found may be seized and used as evidence in court.

Rights-wise, the bottom line is that when police officers observe unusual behavior causing them to reasonably suspect criminal activity may be occurring and that the people being observed may be armed and dangerous, the officers may briefly detain the subjects for the purpose of conducting a limited initial investigation. If after this limited investigation, the officers still have a "reasonable suspicion" that the person may threaten the safety of themselves or others, the police may search the subject’s outer clothing for weapons.

However, the officers must identify themselves as police officers before beginning the initial investigation.