The Fourth Amendment: The Requirement of Probable Cause

Fourth Amendment: The Requirement of Probable Cause

            In the United States Constitution’s Fourth Amendment, we’re introduced to the concept of “probable cause,” as that amendment states that “no warrants shall issue but upon probable cause.” But, what exactly does that mean? Unfortunately, the Constitution doesn’t define it and for much of our nation’s history, judges, law enforcement, and lawyers have struggled to interpret this idea.[1]

            In this presentation, we’ll look into the development of probable cause, its purpose, and what law enforcement needs to do to establish probable cause for an arrest or search warrant. 

Probable Cause Basics

            The Fourth Amendment guarantees the right to be free from unreasonable searches and seizures by the government.[2] Also, before a search warrant can be issued the government must demonstrate probable cause to believe that a crime was committed and that the proposed search area contains the fruits of illegal activity.[3] A probable cause determination balances the right to liberty with the government’s interest in preventing and investigating crime; it gives law enforcement flexibility in enforcing the law and protects people from “rash and unreasonable interferences with privacy and unfounded charges of crime.” Mere “suspicion” on the part of the police is insufficient. Law enforcement can establish probable cause through live, sworn testimony or by a detailed affidavit describing why a warrant is necessary.

            In 1925, the United States Supreme Court decided Carroll v. United States, and provided the contours of probable cause. In that case, defendants were convicted of violating the National Prohibition Act after police searched their vehicle and discovered that they were transporting 68 quarters of whiskey and gin.[4] In determining that the search was supported by probable cause, the Court wrote, “good faith is not enough to constitute probable cause. That faith must be grounded on facts within the knowledge of the law enforcement agent, which in the judgment of the court would make his faith reasonable.”[5]

            The Court further expounded upon Carroll’s definition of probable cause in Brinegar v. United States, another case that dealt with the illegal transportation of liquor. In finding that the police had probable cause to arrest the defendant, who had a reputation for hauling liquor, after they noticed that his vehicle was “heavily loaded,” the Court reasoned, “In dealing with probable cause . . . we deal with probabilities. These are not technical; they are the factual and practical considerations of everyday life on which reasonable and prudent men, not legal technicians, act.”[6] Thus, probable cause exists where the facts and circumstances within the officers' knowledge are sufficient to allow a reasonable person to reasonably believe that an offense has been, or is being, committed.[7] The probable cause standard cannot be quantified into exact percentages because it deals with probabilities and depends on the circumstances. The Court has even stopped state courts from adding more precise quantifications of probable cause.[8]

Anonymous Informants and Probable Cause

            Many search warrants are based on information provided by confidential informants,[9] one study finding that between 80 to 92 percent of search warrant applications are based on anonymous tips.[10]

            Courts analyze “the totality of the circumstances” to determine whether a tip is sufficient to establish probable cause. In Illinois v. Gates, the Bloomingdale, Illinois police received an anonymous letter that a local couple had been selling illegal drugs and had over $100,000 worth of drugs in their basement.[11] The letter also described the couple’s travels to Florida to procure illegal drugs and then sell them in Chicago. A search warrant for their residence and automobile was then obtained from an Illinois judge, based on the Bloomingdale police officer's affidavit setting forth the information in the anonymous letter.

            The Illinois State Supreme Court concluded that the anonymous tip could not justify probable cause because the letter lacked important details and police weren’t able to independently corroborate the letter’s details.[12] The Supreme Court, however, disagreed, holding that the magistrate who issued the warrant had a “substantial basis” to conclude that probable cause existed.[13] Justice Rehnquist wrote for the Court that analyzing probable cause under the totality of circumstances is not rigid, but is fluid and flexible. Probable cause may be established even if a tip appears to be deficient.[14] Anonymous accusations of illegal behavior may justify a search or seizure if police could corroborate some, even if not all, aspects of the anonymous informant's predictions of a suspect's illegal behavior.[15]

            It appears that establishing probable cause based on the totality of circumstances favors law enforcement because it is so malleable. Police have a great deal of leeway to demonstrate probable cause because police have many ways to show that the circumstances demonstrate probable cause. Still, police may not use an anonymous tip alone to establish probable cause, because an anonymous tip alone seldom demonstrates the informant's basis of knowledge or an appropriate level of verification. Accordingly, police must take the time to corroborate an anonymous tip for it to lead to probable cause.[16] 

Requiring probable cause for a search or arrest warrant has two aims. First, it protects people from unreasonable searches and seizures, and second, it promotes police transparency and accountability, as law enforcement officials must explain to a magistrate why they want to search or arrest someone.[17] Probable cause is not a legal concept frozen in time, as it has proven to be flexible and has been recast throughout our history to ensure that its aims are achieved.