Probable cause is defined as facts sufficient to support a reasonable belief that criminal activity is probably taking place or knowledge of circumstances indicating a fair probability that evidence of crime will be found. It requires more than a mere "hunch," but less than proof beyond a reasonable doubt.
This term was taken from the Fourth Amendment of the Constitution, which states:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
The amount and quality of information police must have before they can arrest or search without a warrant or that a judge must have before she will sign a search warrant allowing the police to conduct a search or arrest a suspect.
Reasonable suspicion would lead a reasonable person to suspect that a crime has been, is being, or will be committed. Then the police may detain the suspect for a brief period and perform a frisk. In some cases, drug-sniffing dogs may be called to the scene, although officers must cite a reason for suspecting the presence of drug evidence in particular. Refusing a search does not create reasonable suspicion, although acting nervous and answering questions inconsistently can. For this reason, it is best not to answer questions if you have to lie in order to do so. Police authority increases if they catch you in a lie, but not if you refuse to answer questions. As a general rule, reasonable suspicion applies to situation in which police have reason to believe you’re up to something, but they don’t know what it is.
