The task of the issuing magistrate is simply to make a practical, common-sense decision whether, given all the circumstances set forth in the affidavit before him, including the "veracity" and "basis of knowledge" of persons supplying hearsay information, there is a fair probability that contraband or evidence of a crime will be found in a particular place. And the duty of a reviewing court is simply to ensure that the magistrate had a "substantial basis for concluding" that probable cause existed.
Prior to the defendants' trial on charges of violating state drug laws, a Circuit Court in Illinois ordered the suppression of all items seized from the defendant's home and automobile on the ground that the affidavit submitted in support of the application for a search warrant was insufficient to show a substantial basis for concluding that probable cause existed for the issuance of the warrant. A copy of an anonymous letter informing the police of the defendants' alleged drug trafficking activities and details as to an alleged imminent transaction was attached to the affidavit which contained corroborating data collected by the police in an independent investigation. The Illinois Appellate Court affirmed the suppression order. The Illinois Supreme Court also affirmed, holding that the letter and affidavit were inadequate to sustain a determination of probable cause for the issuance of a search warrant. According to the Illinois Supreme Court, the search warrant was based on a confidential informant's tip that did not satisfy the purported "veracity" and "basis of knowledge" prongs for probable cause, thereby violating the Fourth Amendment.
Did the lower courts err in their decision to suppress evidence alleged to be obtained in violation of the Fourth Amendment?
Holding instead that probable cause was determined by a traditional totality-of-the-circumstances analysis, the Court reversed the judgment of the lower courts. The Court posited that probable cause determinations were not susceptible to the rigid, technical methodology that had been read into Fourth Amendment jurisprudence. Furthermore, the Court held that the elements of an informant's veracity and knowledge should be understood simply as issues that could illuminate the common sense inquiry of whether there was probable cause to issue a search warrant. The Court concluded that the informant's recitation of detailed facts, though relating to innocent activities, when corroborated by observation by police officers, afforded probable cause to believe that respondents had drugs in their possession; therefore, the evidence obtained should not be suppressed.