An officer who suspects that a driver is engaged in criminal activity may lack probable cause for an arrest, but it may be enough to engage in what is known as an “investigative stop”.
For example, the police may have received a credible tip that a drug deal is about to occur, or someone is transporting contraband in a vehicle. Even if the officer lacks probable cause to arrest or search the motorist, the officer can stop and temporarily detain the motorist to investigate further if “there is articulable suspicion that a person has committed or is about to commit a crime. Terry v Ohio, 392 US 1 (1968).
This means that an officer can stop and temporarily detain a driver even where no traffic violation has occurred and where the officer does not have probable cause to arrest the driver, provided that the officer has a reasonably articulable suspicion of criminal activity. This applies to anonymous tips as well.
An anonymous tip does not give the right to an officer to go on a fishing expedition and violate one's Fourth Amendment rights against unreasonable search and seizure, but certainly may give rise to reasonable suspicion to temporarily detain someone for investigation and/or to stop a vehicle. And, yes, corroboration of the tip to a reasonable extent is required but will vary from case to case.
You didn't state whether the stop was of a vehicle, a pedestrian, or performed in some other way. The basic requirements are that the police officer must corroborate not only the identity of the accused perpetrator, but must also make independent observations that lead him to believe that the accused is engaged in the type of criminal activity alleged by the anonymous tipster.
For example, if an anonymous tipster calls dispatch and states that a black male at the bus stop, wearing a brown coat, a red shirt, and blue jeans is carrying a gun illegally; and the police observe a man who meets that description but do not observe any activity to lead them to believe that he is carrying a gun illegally, there is not enough reasonable suspicion to justify a Terry stop.
Corroboration of identify alone is not enough.
An anonymous tip never provides enough reasonable suspicion to justify a detention, all by itself. To justify a stop the anonymous tip must be sufficiently corroborated by other evidence the police officer can articulate.
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