As a general rule, law enforcement officers must obtain warrants before conducting investigative searches or making arrests. In order to obtain a warrant, officers must show a judge that there is probable cause to justify the action. However, in the world of law enforcement, it is not always practical for an officer to obtain a warrant before stopping someone who may have been involved in a crime; therefore, police have been granted a certain level of discretionary power that allows them to conduct limited searches—and even initiate arrests—without having a warrant.
This does not mean that police may stop and search or arrest citizens for no good reason, though. Depending on the action, police must be able to give rational, coherent facts that satisfy the standards of reasonable suspicion or probable cause.
Reasonable suspicion and probable cause are legal standards by which officers may justify certain actions. Probable cause is a higher standard than reasonable suspicion, meaning that an officer may make an arrest based solely on probable cause, but not on reasonable suspicion. With reasonable suspicion, an officer may only stop and briefly detain a person. Reasonable suspicion also authorizes an officer to conduct an over-the-clothes pat-down to search for weapons, if the officer “reasonably” suspects that the person is armed.
In many cases, reasonable suspicion can lead to probable cause. Imagine the following scenario: a police officer hears the sound of glass shattering followed by a car alarm and observes a person running away from the car. The officer may now have reasonable suspicion to suspect that person of breaking into the car. The officer is then authorized to conduct a “stop and frisk.” After patting down the suspect’s clothes, the officer finds a crowbar, which he reasonably thinks was used to shatter the car window. The officer may now have sufficient probable cause to place the suspect under arrest.