The purpose of the Fourth Amendment to the U.S. Constitution is to protect people from arbitrary arrest, and to protect their homes and personal effects from unreasonable search and seizure.
The Fourth Amendment provides,
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.
Under the Fourth Amendment a search occurs when an agent or employee of the government engages in an action that violates an individual's reasonable expectation of privacy. Pursuant to that principle, a Fourth Amendment search occurs when an individual exhibits a subjective expectation of privacy in the object of the challenged search, and society is willing to recognize that the individual's expectation of privacy is reasonable.
Seizure of a Person
For Fourth Amendment purposes, the seizure of a person occurs when the actions of the police, as interpreted by a reasonable person, would cause the person to conclude that he was being detained by the police and was not free to leave. For a seizure to occur there must be some demonstration of authority by the state actor, usually a police officer, that suggests a detention.
Such a demonstration of authority could range from issuing an oral instruction, to the use of physical contact or display of a weapon, to restraining the person with handcuffs, to detaining the person in the back of a police vehicle or transporting the person to a jail.
The seizure of a person may take the form of an arrest, or may be in the form of an investigatory stop that falls short of being an arrest. The principal distinction is that an investigatory stop is a temporary detention, lasting only as long as necessary to complete the investigation. The most common forms of investigatory stops are:
Terry stops - Investigatory detentions that occur based upon the officer's reasonable suspicion that the detained individual is engaged, or about to be engaged, in criminal conduct. A Terry stop may include a pat-down for weapons in order to ensure the officer's safety. Terry stops are named after the Supreme Court case that defined the authority of the police to temporarily detain a person that they reasonably believe to have been involved in criminal activity, Terry v. Ohio, 392 U.S. 1 (1968).
Traffic stops - the relatively routine process of stopping a motorist, and possibly issuing a ticket for a traffic infraction. Although state laws vary, traffic violations are often non-criminal in nature. It is not necessary that the officer believe that the driver of a vehicle or his passengers are engaged in criminal activity in order to complete a traffic stop, and the stop is valid as long as the officer is legally justified in stopping the car to enforce traffic laws.
Investigatory stops are usually less formal than arrests, with less assertion of control or dominion over the detained individual, although that is not always the case. Investigatory stops should not last longer than necessary to complete the investigation, and the unreasonable detention of a person beyond that time may render the stop unlawful.
Seizure of Property
Under the Fourth Amendment, the seizure of property occurs when the actions of a state actor result in a meaningful interference with an individual’s possessory interest in the property. In most cases the seizure of property will involve its being taken into evidence by the police as part of a criminal investigation. If the police obtain a warrant to seize property, the warrant will indicate the types of property that may be lawfully seized.
Within the context of the lawful execution of that warrant, if the police come across items in their plain view that are obviously illegal in nature or evidence of a crime, the police may seize those items even though they're not specified in the warrant.
As a general rule, the Fourth Amendment requires the police to obtain search and arrest warrants before seizing persons or property.
When a warrant is required, a law enforcement officer must submit a warrant request to a judge or magistrate, supported by a demonstration of probable cause that a search or seizure is justified. The warrant application will typically be accompanied by an affidavit from the officer containing recitations of fact in support of the warrant, although in some circumstances a warrant request may be made by telephone. The warrant requirement has been expanded in the computer age to include searches of cellular phones, smartphones and computers.
There is some disagreement between jurisdictions over the search of employer-provided electronics, and whether an employee has a reasonable expectation of privacy in those employer-provided items such that they would have standing to object to a search. The police are expected to act reasonably when executing a warrant, and to refrain from the use of excessive force.
Where a warrant is required, police action without a warrant or actions that are unreasonable despite their being in possession of a warrant can trigger the exclusionary rule, potentially keeping the fruits of the search or seizure from being used in subsequent criminal proceedings against the person whose rights were violated. Unlawful searches rarely result in liability on the part of the state or the state actor performing the search, as state actors enjoy qualified immunity when performing searches and seizures.
There are a number of important exceptions to the requirement for a search warrant, including:
Administrative Searches - Routine administrative searches do not require a warrant. A common example of an administrative search is the health inspection, with restaurants and commercial food facilities being subject to periodic inspection by the state. In some states, administrative searches include sobriety and immigration check lanes, where drivers are stopped and asked to verify that they are sober or that they are lawfully in the nation.
Exigent Circumstances - Due to the nature of the situation, it is important to take prompt action to ensure that evidence is not lost while the police wait for a warrant.
Consent - If a person consents to being searched, no warrant is required.
Brief Investigatory Stops - Terry stops and traffic stops, described in this article under the definition of seizure.
Searches Incident to Arrest - When a person is taken into custody, the police may perform a complete search of the person, up to and including a strip search, for weapons and contraband.
Inventory Searches - The police may establish and follow procedures to inventory items taken into evidence, such as the contents of an impounded motor vehicle.
Plain View - The item seized was observed from a lawful vantage point, with the officer able to determine from that vantage point that there exists probable cause to believe that the item is connected with criminal activities.
Plain Feel - During a lawful pat down an item is detected that, by characteristics the officer can feel, gives rise to probable cause that the item is contraband or evidence of criminal activity.
Safety Checks - Due to circumstances that suggest that a person may be injured or in danger, the officer enters the premises to verify that there is no need for emergency assistance or medical attention. Items within the officer's plain view during such a safety check are subject to possible seizure.
Border Searches - When you go through customs, customs officers have broad authority to search your person and possessions without probable cause.
Probation Searches - People on probation are subject to a significant level of observation, potentially including visits by probation officers to their homes and the search of their premises, as well as potentially being required to submit to drug testing.
The Fourth Amendment provides protection against action by the state, not against the actions of private individuals. By way of example, if you are suspected of shoplifting in a store and the police are called, the police are restricted by the Fourth Amendment in relation to whether and how they detain and search you. In contrast, members of the store's private loss prevention staff are not restricted by the Fourth Amendment.
If a state actor violates your Fourth Amendment rights, you may be able to have the evidence that is obtained in violation of your rights excluded from your criminal case. That is not true for a search by a loss prevention officer or by a private investigator, and the evidence they obtain will not be excluded from legal proceedings against you, although a private individual's unreasonable detention or search of another person can result in civil and even criminal liability.
When a private individual is acting at the instruction of the state, it is possible for the private individual to be treated as a state actor for Fourth Amendment purposes. If you hire a private investigator to follow your spouse, the investigator is a private actor whose actions are not subject to the Fourth Amendment. If the police hire a private investigator to follow you, they cannot avoid the application of the Fourth Amendment by contending that the person they hired is not an agent of the state.
The exclusionary rule prevents the government from using most evidence gathered in violation of the Constitution. In order to assert the exclusionary rule the person making the claim of an unlawful search or seizure must have standing, meaning the legal right to make the claim based upon the violation of their own Fourth Amendment rights.
If the police unlawfully search your home, your rights were violated and you may attempt to invoke the exclusionary rule. If the police unlawfully search your friend's home and find an item that you accidentally left in that home during a social visit, your rights were not violated by the search. If charged with a crime your friend may be able to invoke the exclusionary rule, but the evidence found in his home may still potentially be used against you.
When you attempt to invoke the exclusionary rule, the state may argue that an exception to the rule applies and that the evidence should be admitted over your objection. Common exceptions include:
The Good Faith Exception - The evidence was seized by police who were acting in reasonable reliance upon a warrant later found to be invalid.
Inevitable Discovery - Under the facts the evidence would eventually have been discovered even had the police not acted improperly.
Independent Source Doctrine - Although initially obtained through an unlawful search, the evidence is later independently obtained through a lawful search.
Attenuation - The alleged connection between the wrongful search or seizure and the evidence is too remote or attenuated to justify the application of the exclusionary rule.
Impeachment - Although not admissible as part of the prosecution's case in chief, the excluded evidence may be admissible in order to impeach the testimony of the defendant at trial. The use of the evidence would be to challenge the credibility of the witness, and not as substantive evidence of guilt.