Once the Fourth Amendment applies to a particular search or seizure, the next question is under what circumstances is a warrant required. The Supreme Court has ruled that the Constitution expresses a preference for searches, seizures, and arrests conducted pursuant to a lawfully executed warrant (see Mincey v. Arizona, 437 U.S. 385 ). A warrant is a written order signed by a court authorizing a law-enforcement officer to conduct a search, seizure, or arrest. Searches, seizures, and arrests performed without a valid warrant are deemed presumptively invalid, and any evidence seized without a warrant will be suppressed unless a court finds that the search was reasonable under the circumstances.
An application for a warrant must be supported by a sworn, detailed statement made by a law enforcement officer appearing before a neutral judge or magistrate. The Supreme Court has said that probable cause exists when the facts and circumstances within the police officer’s knowledge provide a reasonably trustworthy basis for a man of reasonable caution to believe that a criminal offense has been committed or is about to take place (see Carroll v. United States, 267 U.S. 132 ). Probable cause can be established by out-of-court statements made by reliable police informants, even though those statements cannot be tested by the magistrate. However, probable cause will not lie where the only evidence of criminal activity is an officer’s affirmation of suspicion or belief (see Aguilar v. Texas, 378 U.S. 108 ). On the other hand, an officer’s subjective reason for making an arrest does not need to be the same criminal offense for which the facts indicate. (Devenpeck v. Alford, 543 U.S. 146 ).
Probable cause will not lie unless the facts supporting the warrant are sworn by the officer as true to the best of his or her knowledge. The officer’s oath can be written or oral, but the officer must typically swear that no knowing or intentionally false statement has been submitted in support of the warrant and that no statement has been made in reckless disregard of the truth. Inaccuracies due to an officer’s negligence or innocent omission do not jeopardize a warrant’s validity.
The Fourth Amendment requires not only that warrants be supported by probable cause offered by a sworn police officer, but it also requires that a war-rant “particularly” describe the person or place to be searched or seized. Warrants must provide enough detail so that an officer with the warrant can ascertain with reasonable effort the persons and places identified in the warrant. For most residences a street address usually satisfies the particularity requirement, unless the warrant designates an apartment complex, hotel, or other multiple-unit building, in which case the warrant must describe the specific sub-unit to be searched. Warrants must describe individuals with sufficient particularity so that a person of average intelligence can distinguish them from others in the general population.
The magistrate before whom an officer applies for a warrant must be neutral and detached. This qualification means that the magistrate must be impartial and not a member of the “competitive enterprise” of law enforcement (see California v. Acevedo, 500 U.S. 565 ). Thus, police officers, prosecutors, and attorney generals are disqualified from becoming a magistrate. States vary as to the requirements that candidates must possess before they will be considered qualified for the job of magistrate. Some states require that magistrates have an attorney’s license, while others require only that their magistrates be literate.