Assessing reasonableness of suspicion

When can a tip-off provide the police with reasonable grounds to suspect that an offence has been committed? 

If the informant is not known to the police, what circumstances can provide sufficient assurance of reliability to establish reasonable grounds for this suspicion?

General rules providing for criteria of reliability may be difficult to apply uniformly in actual cases, and judges may differ in the same case, as is illustrated by the 5-4 split in Navarette v California, USSC No 12-9490, 22April 2014.

Thomas J and Scalia J differed here, the former giving the opinion of the Court and the latter the dissent. The central issue here was the grounds upon which the police stopped a vehicle on a road, which must be borne in mind in jurisdictions where the police can stop without grounds any vehicle for a routine licensing and roadworthiness check.

But more generally, Navarette illustrates how descriptions of requirements for reliability (was the informant an eyewitness, had there been time for fabrication of the report, would a false report carry risks that the informant would be called to account, did the reported facts indicate criminal activity, did subsequent police activity dispel the inference of reasonable suspicion?) can undermine the protection against unreasonable search when the facts are assessed.

Scalia J cogently reasoned that the police activity - following a suspected drunken driver for five minutes without observing anything irregular about the driving - dispelled suspicion. On his evaluation of the evidence there had been time for fabrication - "Plenty of time to dissemble or embellish" - and there was no evidence that the informant knew of any difficulties there may have been in locating her, and there could have been many innocent explanations for the driving behaviour she reported. Further, the driving was observed by the police to have been irreproachable and it gave rise to no suspicion of any criminal activity. A drunk driver cannot (necessarily) decide to drive carefully when he sees that he is under police observation.

The case focuses on the lawfulness of the stopping of the vehicle. A subsequent smelling of marijuana led to search and discovery of 30 pounds of that drug in the vehicle. The case is not concerned with the question of whether the evidence would be admissible if the search had been illegal. Here there was no police misconduct, but on the other hand the police should be deterred from relying on inadequate grounds for a search. Problematically, the distinction between belief and suspicion is difficult to apply in practice, and an erroneous belief that a suspicion is reasonably based can be a slight error compared to the public interest in detecting serious crime. Still, expansion of what is considered to be reasonable grounds for suspicion makes it less necessary to consider the admissibility consequences of police actions in good faith (Illinois v. Rodriguez, 497 U.S. 177 (1990), cited in Findlaw, Annotation 6 - Fourth Amendment, fn 224).