Illinois v. Gates 462 U.S. 213, 103 S. Ct. 2317, 76 L. Ed.2d 527 (1983) The police received an anonymous letter saying that the Gates' were traveling between Illinois and Florida transporting drugs. The police found out that Mr. Gates had indeed bought a plane ticket to Florida. They followed him and saw him meet with someone and then drive back to Illinois. Gates' behavior was mostly similar to what was described in the letter. Based on the letter and their surveillance, the police swore out an affidavit for a search warrant. The warrant was granted, the police searched Gates' car and found a large quantity of drugs. He was arrested. The Trial Court found that there was no probable cause and so the warrant was insufficient and therefore the drug evidence was inadmissible. The prosecutor appealed. Gates argued that the police did not have sufficient probable cause for the warrant, and therefore the search was a violation of the 4th Amendment. The Appellate Court affirmed. The prosecutor appealed. The Appellate Court looked to Spinelli v. United States (393 U.S. 410 (1969)), which had a two-pronged test that basically said that a warrant can't be based on an informer's information unless the affidavit for the warrant also explained why the informer was reliable, and what the underlying circumstances of the informer's information were. Known as the Aguilar-Spinelli Doctrine. The letter provided nothing that would let a magistrate conclude that the author is honest or that his information is reliable. It also gave no indication of how the author came upon the information. The Illinois Supreme Court affirmed. The prosecutor appealed. The US Supreme Court reversed and found that there was sufficient probable cause. The US Supreme Court reversed their decision is Spinelli, finding that the courts should look at the totality of the circumstances when deciding if there is probable cause. "The task of the 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 places." The Court found that while the veracity and basis of knowledge of an anonymous tipster cannot be measured, when corroborated by the actions of the Gates, it satisfies the requirement to establish probable cause. "We agree with the Illinois Supreme Court that an informant's 'veracity,' 'reliability,' and 'basis of knowledge' are all highly relevant in determining the value of his report. We do not agree, however, that these elements should be understood as entirely separate and independent requirements to be rigidly exacted in every case. They should be understood simply as closely intertwined issues that may usefully illuminate the common sense, practical question whether there is probable cause to believe that contraband or evidence is located in a particular place." The Court noted that if the only thing that the police had was the anonymous letter, that alone would not be sufficient to establish probable cause. However, the letter combined with their other police work was enough. Basically, in this case, the Court was saying that the requirement for probable cause is not as high as for admissibility at a formal trial. It is enough for the magistrate to decide that there is a fair probability that the informer is telling the truth. In this case, the corroboration of much of the letter's contents by the police investigation should be sufficient to establish that the letter writer was reliable and had a good basis for their information. At the end of the day, this ruling wasn't much different than Spinelli. It just makes the analysis less formalistic and somewhat combines the two prongs. The main differences between the Aguillar-Spinelli Doctrine and the Totality of the Circumstances Test are that under the Totality of the Circumstances Test, a deficiency in one prong can be made up for by a strong showing in the other prong, and there is no bright line rule about how much is required, it is an open-ended standard. If one prong is completely missing, then there is not enough to give probable cause.