What Is Probable Cause?

The requirement for probable cause before police make an arrest is one of the cornerstones of constitutional law. The Fourth Amendment provides that police cannot make an arrest, make a search, or obtain a warrant without having probable cause. Unfortunately, the Constitution doesn’t spell out what constitutes probable cause, leaving that particular task to the courts. Over the years, courts have established that probable cause means there is a basis for a reasonable person to believe that a crime has been committed—which can enable the police to make an arrest—or that evidence of a crime is present in a particular location—enabling police to obtain a search warrant.

Under the Fourth Amendment, “no warrants shall issue, but upon probable cause.” In the years since that pithy statement, the Supreme Court has tried to define the term, stating that a finding of probable cause relies upon a “factual and practical considerations of everyday life on which reasonable and prudent men act.” Basically, that means there has to be what a reasonable person would think were good reasons for thinking a particular person was involved in a crime.

Based on that standard, police who can show probable cause can make an arrest without a warrant. However, after such an arrest, the prosecutors quickly face a hearing in which they must establish that there was, in fact, probable cause for the arrest.

How Does Probable Cause Affect an Arrest?

If the police don’t have probable cause, they can’t obtain a warrant to make a search or an arrest. There has to be a reason to believe that a crime has been committed. Likewise, if they make an arrest on the street without probable cause, the court can throw out that arrest at the hearing during which the prosecution is required to show that probable cause existed to make the arrest. They must show that the police officers reasonably believed, based on credible evidence, that there was a crime committed by the person the police are arresting, or that the person arrested was involved in a crime. A failure by the prosecution to show that probable cause existed at the time of an arrest will result in the dismissal of the arrest.

If the arrest is invalid for lack of probable cause, any evidence gathered as a result of that arrest likewise is invalid. Known as the exclusionary rule, evidence gathered in the absence of probable cause cannot be used in the prosecution of an alleged crime. Often, the exclusion of such evidence leads to the dropping of charges.

How Does Probable Cause Work in the Real World?

Probable cause does not mean that the police have to actually see a crime happen. Probable cause can take many forms. Frequently, probable cause arises from simple and obvious circumstances. These could include:

  • Evidence of a crime in plain sight of the public
  • The smell of drugs surrounding a suspect, such as marijuana or other drugs with a distinctive smell—especially when smoked—also can serve as probable cause. A police officer need not observe someone smoking marijuana if that person reeks of marijuana when the officer approaches.
  • Admissions of guilt always are probable cause for an arrest.

In each of these circumstances, the police officer could make an arrest and conduct a search, even without a warrant, because the office would be able to show sufficient probable cause to support a warrant after the arrest.

Setting aside such specific instances of probable cause, there are broader categories of evidence that can serve to establish probable cause. Broadly speaking, these categories may be observational, circumstantial, based on expertise, or information. These are characterized by:

  • Observational evidence is self-explanatory. The evidence consists of what the police officer sees, smells, or hears. If a police officer sees a person carrying a crowbar and walking up to one car after another and looking through the windows at night would fall into this category.
  • Circumstantial evidence is not direct evidence, such as actually observing a crime in progress. Circumstantial evidence consists of a compilation of facts and circumstances that, taken together, make it appear that a crime has been committed. Even where there is no direct evidence, circumstantial evidence can be sufficient. In general, circumstantial evidence must be either very convincing or accompanied by direct evidence to serve as probable cause.
  • When a police officer is investigating a crime and gathering evidence, that officer’s experience is relevant. For instance, an officer might recognize scratch marks on a doorway that indicate a lock was pried open or picked, or be able to recognize and interpret gang graffiti and realize that a crime has been committed.
  • Information from confidential informants or members of the public who call in with tips can form the basis of probable cause.

Of course, consent will override the need for probable cause. For instance, most traffic violations—such as malfunction taillights or headlights, speeding, or having an expired registration—are not considered probable cause. During a stop for a traffic violation, police can search your vehicle if they have probable cause. But if minor traffic violations—the cause of most traffic stops—are not probable cause, how do the police get to search your car?

It’s simple. You let them. Believing you have nothing to hide, when the police ask if they can search your car, you say yes.

Don’t do that.

You always have the right to refuse to allow a search not based on probable cause. If the police are asking for permission to search, they don’t have probable cause. Don’t give them permission. Once you consent, they don’t need probable cause, but if you refuse consent and they search anyway, anything they find should not be admissible as evidence.

After an Arrest in DuPage, Kane, or Cook County, You Should Consult With the Criminal Attorneys of Martin & Kent, L.L.C.

If police arrest you in DuPage, Kane, or Cook County, call the criminal attorneys of Martin & Kent, L.L.C., whose experience includes service as felony prosecutors. They will mount a tough defense on your behalf and are committed to delivering the best possible result. For a consultation in DuPage, Kane, or Cook County, contact them 24 hours a day at (630) 474-8000.



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