Probable cause is the threshold police must meet to search your property, arrest you, or seize evidence. The Fourth Amendment requires that every warrant be supported by probable cause and describe with specificity what's being searched for. Probable cause means an officer has enough facts to lead a reasonable person to believe a crime has been committed or that evidence exists in the place to be searched. It's not a hunch; it's not a gut feeling.
In Illinois v. Gates (1983), the Supreme Court established that judges evaluate probable cause by looking at the totality of circumstances—all the facts together, not any single piece of evidence in isolation. An anonymous tip alone won't establish probable cause. Neither will a slight bulge in a suspect's pocket. But reliable informant tips combined with police observation and known criminal history might.
This standard took on new urgency in Carpenter v. United States (2018), where the Supreme Court ruled 5-4 that police need a warrant backed by probable cause to access historical cell phone location data. The FBI had obtained 12,898 location points tracking Timothy Carpenter's movements over 127 days without a warrant. Chief Justice Roberts wrote that this kind of "detailed, encyclopedic, and effortlessly compiled" surveillance demands Fourth Amendment protection. Before Carpenter, prosecutors could get location history with a lower legal standard. Now, probable cause stands as the barrier between your phone and the government.
Probable cause protects against arbitrary police searches. Without it, officers could stop, search, and arrest anyone for any reason. With it, they must point to specific facts suggesting crime.
People often confuse probable cause with reasonable suspicion. Reasonable suspicion justifies a brief stop and pat-down for weapons. Probable cause is needed for arrest and searches.
Probable cause protects against arbitrary police searches. Without it, officers could stop, search, and arrest anyone for any reason. With it, they must point to specific facts suggesting crime.
People often confuse probable cause with reasonable suspicion. Reasonable suspicion justifies a brief stop and pat-down for weapons. Probable cause is needed for arrest and searches.