Here are a list of precedents for the case.
Griffin v. Wisconsin (1987): A search must be supported, as a general matter, by a warrant issued upon probable cause.
O'Connor v. Ortega (1987): The Fourth Amendment protects individuals from unreasonable searches conducted by the Government, even when the Government acts as an employer.
Colorado v. Bertine (1987): The probable cause standard is peculiarly related to criminal investigations.
Snepp v. U.S. (1980): Those who join the military or intelligence services may be required to give extraordinary assurances of trustworthiness and probity.
Department of Navy v. Egan (1988): The Government has a compelling interest in protecting truly senstive information from those who under compulsion of circumstances or for other reasons might compromise such information.