The 4th Amendment protects two fundamental liberty interests: the right to privacy and the right to freedom from arbitrary invasion.
A search occurs when a government employee or agent violates a reasonable expectation of privacy. A seizure is the interference with an individual's possessory interest in property. The property's owner must have had a reasonable expectation of privacy in the items seized. A person is seized when law enforcement personnel use physical force to restrain the person if a reasonable person in a similar situation would not feel free to leave.
The prohibition on unreasonable searches and seizures restricts the actions law enforcement personnel may take when performing a criminal investigation; however, the ban also disallows unreasonable searches and seizures in the civil litigation context. Law enforcement may conduct a search only if individualized suspicion motivates the search. The Fourth Amendment prohibits generalized searches, unless extraordinary circumstances place the public in danger.
To sue regarding an alleged Fourth Amendment violation, the plaintiff must have a legitimate expectation of privacy at the searched location. This expectation must meet both the subjective and objective tests of reasonableness. The subjective test requires the plaintiff to genuinely expect privacy, and the objective test requires that, given the circumstances, a reasonable person in a similar situation also would have expected privacy.
Courts ordinarily suppress evidence obtained during an unreasonable search or seizure. This rule, known as the exclusionary rule, applies to both the investigatory and accusatory stages of a criminal prosecution.
If evidence that falls within the scope of the exclusionary rule led law enforcement to other evidence, then the exclusionary rule also applies to this related evidence.
If an officer relies, in good faith, on a defective warrant, then a search made by the officer does not trigger the exclusionary rule.
In order to avoid illegally searching or seizing property, law enforcement officers typically obtain warrants. They must show probable cause, supported by oath or affirmation, and describe in detail the place they will search and the items they will seize. A judge may find probable cause only by examining the totality of the circumstances.
A knock-and-announce warrant requires law enforcement officers to knock on the door of a residence and announce their identity before entering. In 2006, the Supreme Court determined that law enforcement's failure to knock or announce when in possession of a knock-and-announce warrant does not necessitate use of the exclusionary rule.
No-knock warrants allow officers to enter a building without announcing their presence or knocking on the door first. Courts reserve these warrants for situations in which a building's owner or occupier could destroy the sought-after evidence by the time officers wait for someone to open the door.
An anticipatory warrant becomes valid after a certain future condition occurs. Courts reserve these types of warrants for situations in which police have probable cause that, at some time, evidence in a particular location will become available.
The plain-view doctrine allows a government agent to take possession of property not included within a warrant if it was in plain view of the agent. A seizure of evidence in plain view does not compromise any further expectation of privacy than that already compromised by the warrant.
Officers also may search and seize objects on a person if they have placed the person under arrest. This exception extends to situations in which the police, in good faith, arrest the wrong suspect and seize contraband during the search. If a suspect at any time makes a furtive gesture, the gesture justifies a limited warrantless police intrusion.
Officers may make warrantless searches and seizures if they find that exigent circumstances exist and that they have probable cause. An exigent circumstance exists when an officer has a compelling need to take official action but lacks the time needed to get a warrant. Determining probable cause in this context requires a consideration of the totality of the circumstances to determine whether an officer acted in accord with a high probability that the search would turn up contraband or evidence. The decision-maker must examine the facts making up the totality of the circumstances from the viewpoint of an objectively reasonable officer before making the arrest.
An officer entering a residence or building without a warrant to assist in an emergency does not violate the Fourth Amendment. Similarly, an officer who receives voluntary consent to search without a warrant does not violate the Fourth Amendment. Consent must not have resulted from coercion or duress. A third person may give consent for law enforcement to search a premises if that person lawfully owns or occupies the premises. However, if two residents are present and one consents but the other objects, the rights of the objecting party override. Employers may consent to a police search of areas within their building, excluding areas specifically assigned to an employee. Landlords may not permit the search of a leased premises.
The law treats staff and faculty members of public educational institutions as government agents. Therefore, the Fourth Amendment applies to public school employees, but a less stringent standard prevails. In the context of public school searches, employees may perform a search on the condition that they have reasonable suspicion.
Routine searches made at or near U.S. borders also are exempt from warrant requirements.
After 9/11, Congress and the president enacted legislation to strengthen the intelligence-gathering community’s ability to combat domestic terrorism. The USA PATRIOT Act allows law enforcement to search e-mail and telephone communications in addition to medical, financial and library records.
The act also expanded the practice of using National Security Letters, which are administrative subpoenas that require certain people, groups, or companies to provide documents about certain people. These subpoenas carry a gag order, meaning the person or people responsible for complying may not mention the National Security Letter. Under USA PATRIOT Act provisions, law enforcement officers may use these subpoenas when investigating U.S. citizens, even when the officers do not think the individual under investigation has committed a crime. An agency need not obtain a warrant before searching records.