Shannon Challender of Jacoby and Meyers writes:

A crime has been committed. A law enforcement officer plans to make an arrest based on an investigation. The Constitution requires the officer to present a sworn statement before a detached magistrate in order to determine probable cause.That is, a magistrate, without any bias or attachment to the parties involved, must review the facts of the investigation to determine the suspected culprit more than likely committed the crime before an officer may Constitutionally arrest a suspect. This process results in an arrest warrant. However, sometimes circumstances arrive when an officer is called to the scene of an ongoing crime or witnesses a crime in which the officer must make an arrest to protect the peace. The officer must however only make the arrest if there is probable cause the arrestee committed the crime. An officer cannot arrest someone for less than probable cause, e. g. spite. However, because the officer is not a detached magistrate, the US Constitution still requires that a magistrate determine probable cause after an arrest made without a warrant. This is to prevent circumventing the arrest warrant protection afforded all Americans.  

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