A warrant is a writ by a judge or magistrate authorizing police to conduct an arrest or search.
A valid warrant must be based on probable cause.
Brinegar v. United States
Probable cause exists where "the facts and circumstances within their [the officers'] knowledge and of which they had reasonably trustworthy information [are] sufficient in themselves to warrant a man of reasonable caution in the belief that" an offense has been or is being committed.
An officer's subjective intent or belief is irrelevant in determining probable cause. Only what a reasonable officer would believe matters.
A mistake of fact or law is okay as long as it is reasonable for an officer to make.
Probable cause requires more than just reasonable suspicion, but it does not require evidence that would justify a conviction.
Anonymous tips alone cannot establish probable cause, but they can when combined with the totality of the circumstances. Especially important in such determinations are whether the informant is credible—likely to be telling the truth—and reliable—likely to have knowledge. Thus, the veracity of the informant and his basis of knowledge should be analyzed.
Note that just because an anonymous tip turned out to be true does not mean that it was credible. The totality of the circumstances must establish probable cause before conducting the search, not after. This can be done by corroborating parts of a tip that do not require infringing upon one's reasonable expectation of privacy.
General warrants are not constitutional. They must be supported by oath or affirmation and particularly describe the place to be searched and the persons or things to be seized.
- A warrant must relate to a specific crime.
Executing a Warrant
Officers can detain and question people in the immediate vicinity while executing a warrant for the purpose of ensuring the safety officers and others.
Generally, officers must knock and announce their presence when entering a home while executing a warrant. However, courts apply a reasonableness test to this requirement which allows exceptions for several reasons where knocking and announcing would cause more harm than good:
- Exigent circumstances
- Officer safety
- Preventing the destruction of evidence
Note that the exclusionary rule does not apply to evidence obtained in violation of the knock-and-announce rule anyway.
If a mistake is made in executing a search warrant, the search will still be valid as long as the police acted objectively reasonably in their efforts to identify the place to be searched.
Courts are very lenient on mistakes as long as the officers acted in good faith.