Police have enormous power under the law, but the law also places important limits on that power. Some of the most important of these limits apply to situations in which the police want to take custody of a person.
The Fourth Amendment to the U.S. Constitution protects the public from “unreasonable search and seizure.” In this context, “seizure” refers to a law enforcement officer’s ability to take a person into custody. However, the courts have disagreed often over the years about when a seizure is “unreasonable” under the meaning of the law. Generally, courts have decided that there are two categories in which the police can take custody of person: they can arrest the person or they can detain them.
How the police exercise — or abuse — these powers in any given case can have enormous consequences for a person’s criminal defense.
Detaining
When police detain a person on the street, they have the power to stop a person temporarily and even conduct a pat-down search of their outer clothing. This type of stop is sometimes known as a Terry stop or a stop-and-frisk. It is considered and investigatory stop and it must be brief. However, it can easily turn into a bigger ordeal if the officer has enough evidence to justify turning a Terry stop into an arrest.
Generally, a police officer may detain an individual only when the officer has a reasonable suspicion that the the person is armed, or is engaged in, or will momentarily be engaged in criminal conduct. Police have somewhat more power to detain a car and its occupants in a traffic stop.
Arrest
Being arrested is much more serious than being detained. It is a formal process in which a person is legally and physically restrained. They cannot be released without another formal process.
To arrest a person, police need more than a reasonable suspicion. Generally police can only arrest a person when:
A law enforcement officer has personally witnessed the person committing a crime, This is often the case in traffic moving violations, although police usually have discretion to either arrest a driver or issue them a citation based on the traffic violation involved.
Police have a warrant for the person’s arrest. A warrant is a court order that allows police to arrest a person. Typically, police obtain a warrant by showing a court why they have probable cause to believe the person has committed a crime. “Probable cause” in this context means the police have reason to believe it’s more likely than not that the person committed the crime.