Aguilar v. Texas: The Two-Pronged Test for Probable Cause

The Aguilar v. Texas two-pronged test was a 1964 Supreme Court rule requiring that any search warrant affidavit built on an informant’s tip show two things: how the informant got the information (the basis of knowledge) and why the informant should be believed (veracity or reliability).¹ The Supreme Court later abandoned that rigid two-part framework in Illinois v. Gates, replacing it with a totality of the circumstances test, though the two prongs still function as useful factors in that broader analysis.²

What the Two Prongs Required

Under Aguilar, a magistrate could not issue a warrant based on an officer’s bare assertion that a confidential source was reliable. The affidavit had to lay out enough underlying facts for the magistrate to independently judge probable cause on two separate points:²

Basis of knowledge. The affidavit needed to explain how the informant learned what they claimed, for example by personally witnessing the activity described.
Veracity or reliability. The affidavit needed to give the magistrate specific reasons to trust the informant or the information itself.

Conclusory language failed the test. An officer stating that a source was “credible” or that information was “reliable,” without more, did not give the magistrate anything to evaluate.¹

The Affidavit That Produced the Rule

In January 1960, two Houston police officers applied to a local justice of the peace for a warrant to search Nick Aguilar’s home. Their sworn affidavit said they had received information from a credible person who believed heroin, marijuana, and other narcotics were being kept at the residence. The search followed, and Aguilar was convicted of illegal possession of heroin.¹

Why the Supreme Court Reversed the Conviction

The Court held that the affidavit failed on both prongs. The officers said their source was reliable but gave no reason why. They also did not describe how the informant knew that drugs were actually inside the house. Because the warrant rested on bare conclusions rather than specific facts, the search violated the Fourth Amendment, and Aguilar’s conviction was reversed.¹

How the Test Works Today

The two prongs are no longer treated as separate, mandatory requirements. In Illinois v. Gates, the Supreme Court held that basis of knowledge and veracity are closely related considerations that feed into a single common-sense question: looking at everything in the affidavit, is there a fair probability that evidence of a crime will be found? A strong showing on one prong can make up for a weaker showing on the other.²

An informant’s track record of accurate tips still helps establish credibility, but it is not a categorical requirement. The magistrate needs only a substantial basis for concluding that probable cause exists.² Some state courts have kept the stricter Aguilar framework as a matter of state constitutional law, so the two prongs remain worth understanding even where federal doctrine has moved on.

When a Bad Warrant Does Not Mean Suppression

A warrant that fails the probable cause standard does not automatically keep the evidence out of court. Under the good-faith exception recognized in United States v. Leon, evidence obtained through a defective warrant may still be admissible if the officers relied on the warrant in objectively reasonable good faith. Suppression is generally reserved for cases where the officers were dishonest in the application or where the affidavit was so lacking in probable cause that no reasonable officer would have relied on it.³