Knowing When an Arrest is Legal

Learn when the police are permitted to make an arrest -- and how to tell if an arrest has actually taken place.

An arrest occurs when police officers take a suspect into custody. An arrest is complete the moment the suspect is no longer free to walk away from the arresting police officer. The U.S. Constitution's Fourth Amendment authorizes arrests only if the police have "probable cause" to believe that a crime was committed and that the suspect did it.

The probable cause requirement restrains the power of the police to deprive people of liberty. It prevents the random roundup of "undesirables" that sometimes occurs in other countries. Some principles of probable cause are well settled:

• To establish probable cause, police officers must be able to point to objective factual circumstances that lead them to believe that a suspect committed a crime. A police officer can't establish probable cause by saying something like, "I just had a hunch that the defendant was a burglar."

• Judges, not police officers, have the last word on whether probable cause exists. A police officer may be sincere in believing that enough factual information to constitute probable cause exists. But if a judge examines that same information and disagrees, then probable cause does not exist -- or did not exist, if the question is being decided after the arrest occurred.

• Probable cause to arrest may have existed at the time of the arrest, even if the police later turn out to be wrong. Put differently, an arrest is valid if it is based on probable cause, even if the arrested person is innocent. In this situation, probable cause protects the police against a civil suit for false arrest if the charges are later dismissed or the defendant is acquitted at trial.

These principles leave open the most important issue concerning probable cause: How much information do police officers need to convince a judge to issue an arrest warrant or to justify a warrantless arrest? In general, probable cause requires more than a "mere suspicion" that a suspect committed a crime, but not so much information that it proves a suspect guilty beyond a reasonable doubt. In the abstract, a firm definition of probable cause is impossible. The Fourth Amendment doesn't provide a definition, so it's up to judges to interpret the meaning of probable cause on a case-by-case basis, taking into account:

• what the judge thinks the Fourth Amendment's drafters meant by the term probable cause

• previous judges' interpretations in similar fact situations, and

• the judge's views about police rights v. defendants' rights.

Judges help to define the meaning of probable cause each time they issue a warrant or decide a case in which the issue arises.