What is a warrantless arrest?
As the name implies, a warrantless arrest is simply an arrest without a warrant. When police officers make a warrantless arrest, a judge does not have a chance to determine ahead of time whether the police have probable cause to make the arrest. Nevertheless, the Fourth amendment probable cause requirement remains the same. For a suspect to remain in custody following an arrest, the police must speedily satisfy a judge or magistrate that they had probable cause to make the arrest. (Gerstein v. Pugh, U.S. Sup. Ct. 1975)
When can a police officer legally make a warrantless arrest?
Assuming that they have probable cause to make an arrest, police officers can legally make warrantless arrests in these two circumstances:
When the crime is committed in the officer's presence. For example, a police officer, on routine patrol, sees a driver strike a pedestrian and drive off without stopping (the crime of "hit and run"). The police officer can pursue the driver and place him in custody.
When the officer has probable cause to believe that the suspect committed a felony, whether or not the deed was done in the officer's presence.
The bottom line: Warrantless arrests are generally okay if probable cause exists, except if a police officer arrests a suspect for a misdemeanor not committed in the officer's presence.
Can the police make a warrantless arrest for an offense that is punishable only by a small fine?
Yes. If a police officer has probable cause to believe that an offense has been committed, the officer can make an arrest even if the crime is a very minor one that is punishable only by a small fine (Atwater v. Lago Vista, U.S. Sup. Ct. 2001). As a practical matter, police officers rarely make arrests in these situations. However, in the Supreme Court's opinion, a rule making the validity of an arrest depend on the seriousness of an offense would be too difficult for police officers to follow because they would have to know the punishment for every criminal offense.
Can the police make a warrantless entry into my home to arrest me?
Police officers generally need to obtain arrest warrants before arresting suspects in their dwellings (Payton v. New York, U.S. Sup. Ct. 1980). If necessary, another officer can be posted outside a home to prevent a suspect's escape during the time it takes to obtain a warrant.
However, warrantless in-home arrests are valid under certain circumstances if "exigent circumstances" exist that make it impracticable for the police to obtain a warrant. Examples of exigent circumstances are:
A police officer who is in hot pursuit of a fleeing suspect who runs into a house or apartment will not generally be required to break off the chase and obtain a warrant.
A police officer who believes that someone in the home is in danger and gains entry for that reason may then arrest the owner without a warrant.
A police officer who is let into the home by someone answering the door may make the arrest without a warrant.