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When is an Arrest a Legal Arrest?

Many times when people are questioned by the police, they don't feel that they are free to walk away from the officer. However, until you've been formally detained (as in a traffic stop) or arrested, the officer can't stop you from simply walking away. However, once the officer prevents you from walking away, an arrest has taken place. Whether the arrest is legal or not depends in large part on whether he or she had "probable cause".

What is Probable Cause?

The Fourth Amendment of the U.S. Constitution authorizes police to make an arrest as long as they have "probable cause". 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.

The idea behind probable cause is to prevent the sort of police states that exist in other countries, where officials can simply round up people they don't like as "undesirables" or "threats" without any justification. This standard is deliberately vague, but over time the interpretation of what constitutes probable cause has become fairly solidified:

  • Probable cause is established through factual evidence, and not just suspicions or hunches.
  • Probable cause can be established through observation alone (sight, smell, sound, etc), and includes observations that create suspicion based on a familiar pattern of criminal activity, such as when an officer sees a car circling around an area repeatedly or when someone is flashing their headlights.
  • Probable cause can be based on information derived from witnesses, victims and informants.
  • Probable cause can be based on police expertise, such as recognition of gang signs, detection of tools appropriate for committing certain crimes, or knowledge of movements and gestures that indicate criminal activity.
  • Probable cause can be based on circumstantial evidence that only indirectly indicates that a crime has occurred, such as a broken window.

Who Decides Whether an Officer had Probable Cause?

While probable cause needs to be established before an arrest by the police officer, probable cause can be determined after the fact by a judge. This means that an officer could have in good faith thought he or she had probable cause, but a judge can later determine that there was in fact no probable cause. If this occurs, evidence acquired without probable cause becomes inadmissible in court.

Also, even if it turns out that the police were wrong and a suspect is later acquitted, it doesn't affect the determination of probable cause. Probable cause is extremely important to the police, because even if the suspect is found to be innocent, the finding of probable cause shields the police from lawsuits.

How Much Evidence is Enough to Establish Probable Cause?

There is no set amount of information and no hard rule for what constitutes "enough" information to establish probable cause. The standard is meant to be flexible, and allows judges to balance the rights of the police to conduct investigations and protect the populace in good faith, against the rights of citizens to not be harassed by the police. Judges will often make their decision on whether probable cause existed based on their:

  • Interpretations of what the drafters of the Fourth Amendment meant to protect
  • Views on the police, government interference and defendant's rights
  • Experience in deciding similar factual situations
  • Experience with prior cases involving the arresting officer
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