The "knock-notice" or "knock-and-announce" rule originates from the Fourth Amendment's protection against unreasonable searches and seizures. In general, even if officers have a warrant that justifies entering a home, they must announce themselves and their purpose before intruding. The Fourth Amendment requires this notice, as do many state laws.
Officers can enter a home without permission, either with or without force, only after they have been refused entry or waited a sufficient amount of time without receiving a response. But, as with many legal rules, there are exceptions.
Under the knock-and-announce rule, police officers must generally announce themselves and either explicitly or implicitly request entrance into a home. (An implicit request might take the form of, "Police—we're here to execute a search warrant!")
But courts aren't sticklers when it comes to what officers must communicate. As long as officers take reasonable measures to broadcast their presence, judges are likely to find that they satisfied the notice requirement. That's true even if those inside the home didn't hear and were otherwise unaware of the officers' request for admittance. (United States v. Thompson, 667 F. Supp. 2d 758 (S.D. Ohio 2009).)
In most instances, officers can't legally knock on a front door and yell, "Police—open up!" while simultaneously battering their way through it. They have to wait. How long, you ask? As with almost all legal questions, the answer depends. In fact, if the circumstances are pressing enough, they might not have to wait at all.
Courts generally require that law enforcement wait a "reasonable" period of time between knock-announcement and barging into a home. But what's reasonable to one officer or judge might be unreasonable to another. Further complicating the issue is the U.S. Supreme Court's watering down of the requirement.
In Hudson v. Michigan, the Supreme Court observed that officers don't need to knock and announce if there is a threat of violence or reason to believe providing notice will inspire occupants to destroy evidence. The Court commented that the showing that officers must make to justify ignoring the knock-and-announce rule "is not high." The result is a general perception that courts rubberstamp forcible residential entry by law enforcement. (547 U.S. 586 (2006).)
That perception isn't baseless, but courts actually do, from time to time, find that officers have unjustifiably violated the knock-notice rule. For example, in a 2010 opinion, an Indiana appellate court determined that the circumstances weren't sufficiently "exigent" (urgent) for the police to have impatiently barged their way into a residence to execute a search warrant. The prosecution argued that the criminal records of the occupants justified the officers ramming open the front door and declaring themselves only as they were entering. The court disagreed, and it found it appropriate to suppress (exclude) the evidence the officers found after busting into the home. (Lacey v. State, 931 N.E.2d 378 (Ind. Ct. App. 2010).)
The Supreme Court has held that knock-and-announce violations don't require suppression of evidence that officers find after entering a home. The Court reasoned that the purposes of the knock-and-announce rule—such as physical safety, property preservation, privacy, and dignity—"have nothing to do with the seizure of the evidence." Accordingly, it decided that the federal Constitution allows the government to use in court evidence obtained after a knock-notice violation. (Hudson v. Michigan.)
But that isn't the end of the story. State laws and constitutions can provide more personal liberty and privacy protection than their federal counterparts. The result is some state courts finding that judges must suppress evidence after an illegal unauthorized entry. That was the ruling in the above Indiana case. It's also what an Alaska appellate court found when certain officers had "seriously" violated the knock-and-announce rule. In that case, police officers knocked on the door of the defendant's hotel room for 20 or more seconds without announcing their identity. They then used a hotel key to open the door and enter. They identified themselves as police officers as they were entering but didn't disclose their purpose, which was to serve an arrest warrant. The appeals court ruled that the drugs the officers subsequently found were inadmissible in court. (Berumen v. State, 182 P.3d 635, 636 (Alaska Ct. App. 2008).)
Ultimately, courts vary at least somewhat in their interpretation of the knock-notice rule. And whether a judge will hold that a violation requires evidence suppression depends on the jurisdiction.
As noted above, officers don't need to knock and announce if providing notice would be dangerous for officers or likely result in the destruction of evidence. Sometimes officers don't become aware of those kinds of circumstances until arriving at the suspect's door. In that situation, exigent circumstances (described above) can excuse them from knocking and announcing before entering.
Other times, an officer might anticipate the danger of harm or evidence loss before going to a suspect's home and therefore ask a judge for a "no-knock warrant." In this situation, the officer is essentially asking for pre-approval to enter without announcing. Many states allow judges to issue these no-knock warrants in appropriate circumstances. Judges considering this type of warrant typically require proof that the police have a reasonable suspicion that announcing would be dangerous or result in the destruction of evidence.
No-knock warrants have gained public awareness, especially in the context of raids involving SWAT teams or paramilitary units. But even smaller raids can result in serious property damage and, worse, physical harm or death of suspects, officers, or innocent citizens. That kind of fallout has led to controversy regarding whether no-knock warrants actually increase safety.
Some argue that no-knock warrants increase officer safety by allowing for the element of surprise. But others argue the element of surprise can cause a suspect or innocent citizen who might have reacted calmly (or more calmly) to a knock and announce to instead react violently.
No-knock warrants also face scrutiny for their impact on people's constitutional rights, based on the arguments that no-knock warrants are used disproportionately against minorities, that the standard of proof to obtain a no-knock warrant is too low, and that the risk of entering the wrong residence is too high.
If you are a suspect in a criminal investigation or have questions about your Fourth Amendment rights, speak to a criminal defense attorney. For questions about a possible claim for violation of your rights, you might want to speak with an attorney who specializes in civil rights or personal injury lawsuits against the police.