The circumstances surrounding an arrest determine whether officers should announce themselves beforehand. (To read more about the arrest process, see Arrest: "Seizing" People.)
One of the primary purposes for the announcement requirement is to avoid a scenario where the suspect resists arrest. An officer making an arrest without a warrant generally must announce:
A detailed explanation of the offense isn’t necessary; just stating the type of crime is sufficient.
When the arrest is to be made at a person’s home, officers are generally required to:
(For more on police entry into a home, see Warrants and the Knock-Notice Rule.)
When an officer’s authority isn’t apparent, such as when she is working undercover, it’s typically crucial that she clearly state her authority to make the arrest. To determine if there was sufficient notice, courts consider whether a reasonable person under the same circumstances would understand that an arrest is taking place.
Sometimes circumstances alone can provide the required notice. For example, where the defendants knew the officers who approached them were narcotic officers and knew from the conversation with them that they were being taken to the station to be questioned and searched for transporting narcotics, there was sufficient notice. (United States v. Robinson, 325 F.2d 391 (2nd Cir. 1963).).
In certain situations, the need to arrest the suspect is immediate and apparent; an introduction and explanation aren’t necessary. An officer doesn’t have to give notice before a warrantless arrest when:
There are two primary types of arrest warrants: “knock and announce” and “no knock.”
With a no-knock warrant, officers announce themselves, then forcibly enter the property without knocking or waiting. This element of surprise is designed to protect the officers, to prevent occupants from destroying evidence, or both.
On the other hand, a knock-and-announce warrant, whether for a search or an arrest, typically requires that officers:
But if the circumstances, such as a threat to an officer’s safety, require immediate action, there’s no requirement to knock and announce.
For example, in one case, the officers had a knock-and-announce search warrant for the defendant’s home. The officers knocked on the door, then walked into it without announcing themselves or waiting for a response. There was no evidence that announcing their presence would have created danger, either of evidence destruction or to their safety. Because the police immediately entered the home after knocking and didn't announce themselves, the court found the evidence in the home inadmissible. (U.S. v. Holmes, 183 F.Supp.2d 108 (D. Maine 2002).) (Not all courts however, would necessarily reach the same conclusion given similar facts.)