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In the United States, a no knock warrant is a warrant issued by a judge that allows law enforcement officers to enter a property without immediate prior notification of the residents, such as by knocking or ringing a doorbell. In most cases, law enforcement will identify themselves just before they forcefully enter the property. It is issued under the belief that any evidence they hope to find can be destroyed during the time that police identify themselves and the time they secure the area, or in the event where there is a large perceived threat to officer safety during the execution of the warrant.
The Department of Justice writes:
Federal judges and magistrates may lawfully and constitutionally issue "no-knock" warrants where circumstances justify a no-knock entry, and federal law enforcement officers may lawfully apply for such warrants under such circumstances. Although officers need not take affirmative steps to make an independent re-verification of the circumstances already recognized by a magistrate in issuing a no-knock warrant, such a warrant does not entitle officers to disregard reliable information clearly negating the existence of exigent circumstances when they actually receive such information before execution of the warrant.
The number of no-knock raids has increased from 3,000 in 1981 to more than 50,000 in 2005, according to Peter Kraska, a criminologist at Eastern Kentucky University in Richmond. Raids that lead to deaths of innocent people are increasingly common; since the early 1980s, 40 bystanders have been killed, according to the Cato Institute in Washington, D.C.
No-knock warrants have been controversial for various reasons. Some consider them to be unconstitutional. In addition, there have been cases where burglars have robbed homes by pretending to be officers with a no knock warrant. There have been many cases where armed homeowners, believing that they are being invaded, have shot at officers, resulting in deaths on both sides. While it is legal to shoot a homeowner's dog when an officer fears for his/her life, there have been numerous high-profile cases in which family pets lacking the size, strength, or demeanor to attack officers have been shot, greatly increasing the risk of additional casualties in neighboring houses via overpenetrating bullets.
Police and prosecutors are criticized for the use of no-knock search warrants. The Fourth Amendment to the United States Constitution assures and protects individuals from unreasonable searches and seizures. Police, civil libertarians, and/or criminals may interpret the phrase “unreasonable searches and seizures” in different ways. Police view unreasonable searches and seizures as entering a home or property without obtaining what is deemed as probable cause. Probable cause is based on the officer having reason to believe a crime has been committed or exigent circumstances defined in United States v. McConney: Emergency conditions. "Those circumstances that would cause a reasonable person to believe that entry (or other relevant prompt action) was necessary to prevent physical harm to the officers or other persons, the destruction of relevant evidence, the escape of a suspect, or some other consequence improperly frustrating legitimate law enforcement efforts."
- Kathryn Johnston (1914–2006) was an elderly Atlanta, Georgia, woman shot by three undercover police officers in her home on November 21, 2006, after she fired one shot at the ceiling, assuming her home was being invaded. While the officers were wounded by friendly fire, none of the officers received life-threatening injuries, but Johnston was killed by their gunfire.
- Two former Los Angeles Police Department officers, along with 13 others, have plead guilty to running a robbery ring, which used fake no-knock raids as a ruse to catch victims off guard. The defendants would then steal cash and drugs to sell on the street.
- Tracy Ingle was shot in his house five times during a no-knock raid in North Little Rock, Arkansas. After the police entered the house Tracy thought armed robbers had entered the house and intended to scare them away with a non-working gun. The police expected to find drugs, but none were found. He was brought to the intensive care, but police pulled him out of intensive care for questioning, after which they arrested him and charged him with assault on the officers who shot him.
- Ismael Mena was shot and killed by SWAT team officers in Denver, Colorado, who were performing a no-knock raid that was approved by a judge acting on false information contained in a search warrant. The police believed there to be drugs in the house, but no drugs were found on the premises, and it was later revealed that the address given to the SWAT team by officer Joseph Bini was the wrong one. Jefferson County District Attorney Dave Thomas investigated the matter and cleared the officers involved with the raid on the grounds that Mena had pointed a gun and fired it at SWAT officers, although who fired first remains unknown. However, many have objected to the investigation's findings due to inconsistencies in the various officers' account of what happened. The American Civil Liberties Union, and others, have objected to the Denver Police Department's request for a no-knock raid and the judge's decision to allow such a raid on the grounds that they failed to meet the criteria necessary for a no-knock raid.
- 19 month old Bounkham “Bou Bou” Phonesavanh was injured when police tossed a flash-bang stun grenade into a home during a 3 a.m. raid, and it landed on his pillow in his playpen where he was sleeping. The search yielded no drugs, no drug dealer and no weapons. 
- A Georgia SWAT team shot and killed an armed homeowner, David Hooks, during a drug raid sparked by the word of a self-confessed meth addict and burglar who had robbed the property the previous day. David Hooks' wife, Teresa, looked outside and saw people with hoods on the evening of the raid and woke up her husband. Fearing the burglar or burglars who had struck two nights earlier had returned, Hooks armed himself. Despite the fact that the illegal search warrant did not have a 'no knock' clause, the Drug Task Force and SRT members broke down the back door of the family's home and entered firing in excess of 16 shots. There is no evidence that David Hooks ever fired a weapon, nor was there any evidence he was involved in drugs. The Georgia Bureau of Investigation conducted an intensive 44-hour search of the property and came up with not one item of contraband. 
- Patrik Jonsson (November 29, 2006). "After Atlanta raid tragedy, new scrutiny of police tactics. Police are reviewing their use of 'no-knock' warrants after an octogenarian was killed after officers burst into her home.". Christian Science Monitor. Retrieved 2007-02-14.
- Balko, Radley (April 6, 2006). "No SWAT". Cato Institute.
- United States v. McConney, 728 F.2d 1195, 1199 (9th Cir.), cert. denied, 469 U.S. 824 (1984)
- "Robbery Ring Disguised as Drug Raids Nets Convictions for Former LA Cops". Fox News. 2008-01-30. Retrieved 2008-01-31.
- "Release No. 08-067". Retrieved 2009-02-02.
- Balko, Radley (May 7, 2008). "Tracy Ingle: Another Drug War Outrage". Reason (Hit & Run blog).
- Koon, David (April 24, 2008). "Shot in the dark". Arkansas Times.
- Prendergast, Alan (February 24, 2000). "Unlawful Entry". Denver Westword News.
- Chen, Tina (May 30th, 2014). "Baby in Coma After Polive Grenade Dropped in Crib During Drug Raid". ABC News. Check date values in:
- Balko, Radley (October 6th, 2014). "Meet 59 Year Old David Hooks the Latest Drug Raid Fatality". Washington Post. Check date values in:
- Donald E. Wilkes Jr, No Knock Search Warrants in Georgia (1996) & Explosive Dynamic Entry (2003) & SWATstika Policing (2006).
- Epstein, Dimitri (2009). "Cops or Robbers? How Georgia's Defense of Habitation Statute Applies to No-Knock Raids by Police". Georgia State University Law Review 26 (2): 5.