The Fourth Amendment to the United States Constitution says that the police need “probable cause” to make arrests, conduct searches, and obtain warrants. Before proceeding to trial, prosecutors must prove to a judge that there is probable cause to even charge defendants with crimes.
This article provides a brief overview of probable cause: What is it? When do police and prosecutors need it, and how do they prove it? How much is enough?
The Fourth Amendment doesn't define probable cause. And there is no federal or state statute (law) that spells out the precise meaning of the term. Instead, courts have interpreted probable cause to mean that the police or judge must have an objective (reasonable) belief that the person to be arrested and charged has committed a crime; or that the place to be searched contains evidence of a crime. Officers and prosecutors must present evidence of specific facts, not unsupported conclusions, to support their probable cause conclusions. Courts will consider officers' opinions regarding the significance of facts if their opinions are based on training and experience.
Courts consider the “totality of the circumstances” (all of the facts and information) before deciding whether there is probable cause for a particular arrest, search, or prosecution. Judicial reviews of probable cause determinations are rooted in common sense, not legal technicalities.
People often want courts to give them a number: What probability of suspicion is enough for probable cause? Is it 75%? 51%? Can it be lower than 50%?
Courts are generally unwilling to describe probable cause—and other standards like reasonable suspicion and reasonable doubt—in terms of numbers. Probable cause is best understood on a continuum: It requires more than a hunch that a defendant committed a crime or that evidence of a crime will be found at a certain location, but less than proof beyond a reasonable doubt.
Courts apply the probable cause standard at several critical stages of the criminal process.
Warrants are written court orders that authorize police to make arrests or to search for particular objects or materials at a specified location and time. To get a warrant, officers must convince a judge that probable cause (a reasonable suspicion based on facts) exists for the arrest or search. Warrants don't guarantee that the subject of the warrant will eventually be charged, let alone convicted. They're simply a judge's decision that, at this point, there's probable cause to support the search or arrest. Judges issue warrants during investigations or after some probable cause hearings (see below).
Judges can issue another type of warrant, called a “bench warrant,” when defendants fail to appear for their court hearings. Bench warrants authorize the police to arrest the absent defendant. Sometimes the police go looking for the defendant, but most often, the warrant sits in the system. Police arrest the defendant later when they encounter the person for other reasons (such as a traffic stop) and a search of their database reveals the warrant.
As just explained, the general rule is that a valid warrant is required for an arrest or a search. But there are many exceptions to the warrant requirement. It isn't reasonable for courts to require officers to get arrest warrants for every cell phone-snatcher they encounter or search warrants during traffic stops when they suspect a vehicle contains drugs or weapons (see below).
When defendants remain in jail after a warrantless arrest, judges typically review whether officers had probable cause for the arrest within 48 hours. Defendants (usually in federal court) might also be entitled to have a grand jury decide whether there is probable cause to charge them with a crime. In many states, defendants charged with felonies have a right to a preliminary hearing, where a judge hears evidence and decides whether there is probable cause for the case to go to trial.
Procedures vary depending on charges and jurisdiction, but the bottom line is that defendants usually have a right to a finding by a neutral third party (grand jury or judge) that probable cause exists for criminal charges at some point in the process.
Courts can rely on any source of information when they evaluate probable cause, but some information is considered more reliable than others.
Law enforcement officers. At the top of the hierarchy of reliability are personal observations made by law enforcement officers. But law enforcement officers are also considered to be reliable transmitters of information from official channels. The “official channels rule” allows officers to detain, arrest, and sometimes search suspects based on an official request to do so from another officer or agency, including law enforcement databases.
Official and business records. Courts generally consider information gathered by government agencies and some businesses to be reliable. Common examples are DMV records, parole and probation records, phone records, and public utility records.
Citizens. Typically, courts consider crime victims and witnesses to be reliable if they identify themselves to officers or at least risk having their identity revealed. Anonymous 9-1-1 callers are more complicated. Is an anonymous voice on the phone enough to justify a detention or arrest? The answer often turns on whether the caller is reporting an immediate threat to people or property and whether the officer has other evidence of the caller's reliability.
Informants. Informants are people who provide information to police and prosecutors for their own benefit. For example, they might want a lighter sentence for their own crime or immunity from prosecution. Informants who have previously worked with police and have supplied reliable information are considered more reliable than untested informants. The identity of informants is often kept confidential (secret) from defendants.
If you think you might be the target of an arrest or search warrant, or if you have been charged with a crime, talk to an experienced criminal defense lawyer as soon as possible. An experienced lawyer can help you understand the law and how it applies to your case.
A lawyer can help you challenge unlawful arrests and searches by filing a motion to suppress (exclude) evidence. A lawyer can also make a motion to set aside (dismiss) an information or indictment if the lawyer thinks a judge might find that there is not enough evidence (less than probable cause) to support the charges.