A warrantless arrest is reasonable if, given the facts known to the officer, there is probable cause to believe that crime has been committed, regardless of whether the offense for which there is probable cause is “closely related” to the offense for which the officers actually undertake the arrest. Believing that a suspect was impersonating a police officer, a Washington State Patrol officer initiated a traffic stop. While questioning the suspect at the scene, another officer noticed that the suspect was taping the conversation and placed him under arrest for violating the state’s privacy laws. The state trial court subsequently dismissed criminal charges and the suspect filed suit against the two officers, claiming that the arrest had violated his Fourth Amendment rights. The jury was instructed that the plaintiff had to establish a lack of probable cause to arrest, and that taping officers at a traffic stop was not a crime. The jury found in favor of the officers, but the Ninth Circuit reversed, rejecting the officers’ argument that there was probable cause to arrest for impersonating and obstructing a law enforcement officer, because those offenses were not “closely related” to the offense for which officers actually made the arrest. The Supreme Court reversed, holding that the arresting officer’s state of mind was irrelevant so long as facts amounting to probable cause for the arrest were known to the officers at the time.