The Aguilar–Spinelli test was a judicial guideline set down by the U.S. Supreme Court

Supreme Court of the United States

The Supreme Court of the United States is the highest court in the United States. It has ultimate appellate jurisdiction over all state and federal courts, and original jurisdiction over a small range of cases...

 for evaluating the validity of a search warrant

Search warrant

A search warrant is a court order issued by a Magistrate, judge or Supreme Court Official that authorizes law enforcement officers to conduct a search of a person or location for evidence of a crime and to confiscate evidence if it is found....

 based on information provided by a confidential informant or an anonymous tip. The Supreme Court abandoned the Aguilar–Spinelli test in Illinois v. Gates

Illinois v. Gates

Illinois v. Gates, , is a Fourth Amendment case. Gates overruled Aguilar v. Texas, and Spinelli v. United States, , thereby replacing the Aguilar–Spinelli test for probable cause with the "totality of the circumstances" test....

, 462 U.S. 213 (1983), in favor of a rule that evaluates the reliability of the information under the "totality of the circumstances." However, Alaska, Massachusetts, New York, Tennessee, Vermont, and Washington have retained the Aguilar–Spinelli test, based on their own state constitutions.

The two "prongs" of the test are that, when law enforcement seeks a search warrant and a magistrate signs a warrant:

1. The magistrate must be informed of the reasons to support the conclusion that such an informant is reliable and credible.
2. The magistrate must be informed of some of the underlying circumstances relied on by the person providing the information.

This information provided to a magistrate will allow the magistrate to make an independent evaluation of the probable cause

Probable cause

In United States criminal law, probable cause is the standard by which an officer or agent of the law has the grounds to make an arrest, to conduct a personal or property search, or to obtain a warrant for arrest, etc. when criminal charges are being considered. It is also used to refer to the...

 that a crime has been or will be committed.
According to the Fourth Amendment

Fourth Amendment to the United States Constitution

The Fourth Amendment to the United States Constitution is the part of the Bill of Rights which guards against unreasonable searches and seizures, along with requiring any warrant to be judicially sanctioned and supported by probable cause...

 to the U.S. Constitution

United States Constitution

The Constitution of the United States is the supreme law of the United States of America. It is the framework for the organization of the United States government and for the relationship of the federal government with the states, citizens, and all people within the United States.The first three...

:

The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

Historically in the United States, if the police made an illegal search and seizure of evidence, the evidence, once obtained, could often be used against a defendant in a criminal trial regardless of its illegality.

By a unanimous decision in the case of Weeks v. United States

Weeks v. United States

In Weeks v. United States, 232 U.S. 383 , the United States Supreme Court unanimously held that the warrantless seizure of items from a private residence constitutes a violation of the Fourth Amendment...

, 232 U.S.