The Constitution of the United States provides certain rights and protections for all U.S. citizens. This important document was designed to ensure that the government does not invade the privacy of its citizens or try to establish a police state. The Constitution also serves as the basis for the legal standard of reasonable suspicion.
Under the Fourth Amendment of the Constitution, the government and all law enforcement agencies are prohibited from conducting unreasonable searches and seizures. This is why, generally speaking, police officers obtain warrants before conducting searches. In order to obtain a search warrant, an officer must convince a judge that sufficient probable cause—a higher standard of justification than reasonable suspicion— exists to justify the search. The officer must swear and affirm to the validity of the probable cause and also name the specific places to be searched and items to be searched for.
There are, however, exceptions to the search warrant requirement. For instance, a police officer who has reasonable suspicion may detain a suspect and even conduct a pat-down for weapons. If a weapon is found, this could serve as sufficient probable cause to conduct a full search, without requiring a search warrant.
When determining reasonable suspicion, a judge will look at the facts surrounding the incident to determine whether those facts would raise suspicion in the mind of an objectively reasonable police officer. For example, an officer who observes a person drop a suspicious object and flee at the sight of a police vehicle would have reasonable suspicion to detain that person for questioning.