Here are a list of precedents for the case.
Terry v. Ohio (1968): A consensual encounter does not trigger Fourth Amendment scrutiny.
Florida v. Royer (1983): Even when officers have no basis for suspecting a particular individual, they may generally request consent to search luggage provided they do not convey a message that compliance with their request is required.
California v. Hodari (1991): A seizure does not occur simply because a police offcier approaches an individual and asks a few questions. So long as a reasonable person would feel free to disregard the police and go about his business.
Michigan v. Chesternut (1988): A seizure occurs when a reasonable person would believe that he or she is not free to leave.
Boyd v. U.S. (1886): It is one of the primary aims of the Fourth Amendment to protect citizens from the tyranny of being singled out for search and seizure without particularized suspicion.