You're walking down the street when a police officer orders you to halt and begins asking you questions. You're pretty sure that you're not free to leave. Does that mean that you've been arrested, or are you simply being detained?
In general, if a reasonable person in the suspect's shoes wouldn't feel free to leave an encounter with the police, then there's been either a detention or an arrest. Determining which can be tough—and sometimes crucial. Suppose, for instance, that an officer has reasonable suspicion to detain someone, but not probable cause to arrest them. In the course of the encounter, the officer discovers incriminating evidence. In this situation, if the defense attorney persuades the court that, instead of merely detaining her, the officer arrested the suspect without probable cause, then the evidence may be inadmissible in court.
(For information about the level of suspicion police officers must have in order to detain or arrest, see What's the difference between an arrest and a detention or "stop and frisk"?)
An officer's "brief and cursory" holding and questioning someone is a detention. An example is a cop stopping someone who is behaving suspiciously in order to ask a few questions. The suspect isn't free to leave, but he also isn't under arrest, at least until the officer develops probable cause. Another common example is an officer pulling over a driver for some kind of traffic or equipment violation.
An arrest, on the other hand, involves the police taking someone into custody through a more significant restraint on movement. The quintessential example involves the use of handcuffs and an advisement that the suspect is under arrest. (See How do I know if I've been arrested?)
Investigatory stops (or "detentions") must be no longer than necessary and officers must investigate with the least intrusive means that are reasonably available. When an officer prolongs a detention beyond what is brief and cursory and broadens it, then the detention may turn into a de facto arrest—that is, an actual but not official arrest.
If a reasonable person in the suspect's position would have considered the police's behavior to constitute the kind of restraint that's typical of formal arrest, then an arrest has occurred. Some courts phrase the issue as depending on whether, after brief questioning, a reasonable innocent person would have felt free to leave—if not, there's been an arrest. (Johnson v. Bay Area Rapid Transit Dist., 724 F.3d 1159 (9th Cir. 2013).)
Courts consider a variety of factors in determining whether a detention has ripened into an arrest, among them:
(U.S. v. Vargas, 369 F.3d 98 (2d Cir. 2004), In re Hoch, 82 A.3d 1167 (Vt. 2013).)
Although the extent to which officers restrain and intrude upon the suspect are key to the determination, there's no bright line indicating the point at which a detention becomes an arrest. For instance, the use of handcuffs doesn't automatically signal an arrest where there are concerns for officer or public safety.
In one case, officers handcuffed a suspect and placed him in the back of a squad car while they searched a house he had just visited. The appeals court held that their actions didn't turn the detention into an arrest because they needed to avoid an escape attempt and to take precautions against potential violence. The court also found that it made sense to take the suspect back to the house because they knew that the search they were about to begin could implicate him. (United States v. Bullock, 632 F.3d 1004 (7th Cir. 2011).)
Two plain-clothes detectives at Miami International Airport notice Brandon. His clothes, luggage, and behavior cause them to suspect that he's a drug courier. The detectives approach him and identify themselves, and Brandon agrees to speak with them. The detectives determine that Brandon is flying under an assumed name and notice that he is becoming increasingly nervous. They then inform him that they are actually narcotics investigators and that they suspect him of transporting drugs.
The detectives, with Brandon's airline ticket and driver's license in hand, ask their suspect to accompany them to a room 40 feet away. Brandon says nothing but goes with the officers. The detectives, without permission, take Brandon's luggage and bring it to the small interrogation room, where they ask whether he'll consent to a search. Brandon acquiesces, and the officers find cocaine in each suitcase. Approximately 15 minutes had elapsed from the time the detectives first approached Brandon until they found the drugs.
Under these circumstances, the officers' removing Brandon from the public area and putting him in the investigation room constituted an arrest. At that point, the facts supported reasonable suspicion to justify a temporary detention, but not probable cause for an arrest. And since there wasn't probable cause for the arrest, Brandon's consent to the search was invalid, in turn meaning that the cocaine evidence is inadmissible.
The detectives could have lawfully continued the initial encounter with Brandon by questioning him on the spot. (Even if it wasn't a consensual encounter at that point, and it was a detention, the officers were justified because they had reasonable suspicion.) But, by taking his ticket, driver's license, and luggage, and transferring him to another, isolated area in order to gain his consent to a search of the luggage, the detectives arrested him. Had they continued to question him in the public area and had he then consented to a search, or had they used sniffing dogs to inspect his luggage, then there wouldn't have been an arrest and the evidence would have been admissible. But since it was an arrest, and there wasn't yet probable cause to justify it, the court must suppress the cocaine evidence. (Florida v. Royer, 460 U.S. 491 (1983).)