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Fourth Amendment to the United States Constitution
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.
Reasonable expectation of privacy
Not every incident where an officer ascertains information is considered a "search." An officer who views something which is publicly viewable (for instance, by looking through the window of a house from the street) is not conducting a "search" of the house. In United States v. Katz (1967), the Supreme Court ruled that there is no search unless an individual has an "expectation of privacy" and the expectation is "reasonable"—that is, it is one that society is prepared to recognize. So, for example, there is generally no search when officers look through garbage because there is no expectation that garbage is private (see California v. Greenwood (1988)). Similarly, there is no search where officers monitor what phone numbers an individual dials (although Congress has placed statutory restrictions on such monitoring). This doctrine sometimes leads to somewhat unexpected results; in Florida v. Riley (1989), the Supreme Court ruled that there was no expectation of privacy (and thus no search) where officers hovered in a helicopter 400 feet above a suspect's house and conducted surveillance.
The Supreme Court has also ruled that there can be no expectation of privacy in illegal activity. Therefore, investigations that reveal only illegal activity (such as some use of drug sniffing dogs) are not searches.
Searches and seizures without warrants
A warrant is not necessary for a search or seizure under certain circumstances. Officers may search and seize objects that are in "plain view." Before the search and seizure, however, the officers must have probable cause to believe that the objects are contraband.
Similarly, "open fields"—pastures, open water, woods and other such areas—may be searched without warrant, on the basis that the individuals conducting activities therein had no reasonable expectations of privacy. Contrary to its apparent meaning, the "open fields" doctrine has been expanded to include almost any open space other than the land immediately surrounding a domicile (for instance, in Oliver v. United States (1984), the police ignored a "no trespassing" sign, trespassed onto the suspect's land without a warrant, followed a path several hundred yards, and discovered a field of marijuana. The Supreme Court ruled that no search had taken place. Some commentators have been critical of the Open fields doctrine.
There are also "exigent circumstances" exceptions to the warrant requirement—for instance, if an officer reasonably believes that a suspect may destroy evidence, he might be permitted to seize the evidence without a warrant.
The Supreme Court has also held that individuals in automobiles have a reduced expectation of privacy, because vehicles generally do not serve as residences or repositories of personal effects. Vehicles may not be randomly stopped and searched; there must be probable cause or reasonable suspicion of criminal activity. Items in "plain view" may be seized; areas that could potentially hide weapons may also be searched. With probable cause, police officers may search any area in the vehicle. They may not, however, extend the search to the vehicle's passengers without probable cause to search those passengers.
Under common law, a police officer could arrest an individual (arrests constitute seizures, at least for the purpose of the Fourth Amendment) if that individual committed a misdemeanor in the officer's presence, or if the officer had probable cause to believe that the individual committed a felony. The Supreme Court has applied the common law rule in American jurisprudence. The officer in question must have had probable cause before making the arrest; evidence discovered after the arrest may not be retroactively used to justify the arrest.
Another common law rule—that permitting searches incident to an arrest without warrant—has been applied in American law. The justification for such a search is that the arrested individual must be prevented from destroying evidence or using a weapon against the arresting officer. In Trupiano v. United States (1948), the Supreme Court held that "a search or seizure without a warrant as an incident to a lawful arrest has always been considered to be a strictly limited right. It grows out of the inherent necessities of the situation at the time of the arrest. But there must be something more in the way of necessity than merely a lawful arrest." In United States v. Rabinowitz (1950), the Court reversed its previous ruling, holding that the officers' opportunity to obtain a warrant was not germane to the reasonableness of a search incident to an arrest. The decision suggested that any area within the "immediate control" of the arrestee could be searched, but it did not define the term. In 1969, deciding Chimel v. California , the Supreme Court elucidated its previous decisions. It held that when an arrest is made, it is reasonable for the officer to search the arrestee for weapons and evidence. Similarly, it was held that it is reasonable for the officer to search the area within the arrestee's immediate control, that is, the area from which the defendant may gain access to a weapon or evidence. A search of the room in which the arrest is made is therefore permissible, but the same is not true of a search of other rooms, as the arrestee would not probably be able to access weapons or evidence in those rooms at the time of arrest.
The reasonable grounds standard is further applied to searches of homes of individuals on probation.
It has been held that searches in public schools require neither warrants nor probable cause. It is merely necessary that the searching officers have reasonable grounds for believing that the search will result in the finding of evidence of illegal activity. Government offices may be searched for evidence of work-related misconduct by government employees on similar grounds. Searches of prison cells are subject to no restraints relating to reasonableness or probable cause; neither are searches conducted at the border. Finally, searches may be conducted if the target thereof gives consent.
At common law, all evidence, no matter how seized, could be admitted in court. In Weeks v. United States (1914), however, the Supreme Court adopted the "exclusionary rule," whereby evidence seized unlawfully was declared inadmissible in court. The rule mainly serves as a deterrent to police officers seeking to conduct unlawful searches and seizures; it has, however, a number of exceptions. In United States v. Leon (1984), the Supreme Court applied the "good faith" rule: evidence seized by officers objectively and in good faith relying on a warrant that was later found to be defective was still deemed admissible. If an officer dishonestly or recklessly prepares an affidavit forming the basis of the warrant, if the issuing magistrate abandons his neutrality or if the warrant lacks particularity, however, evidence seized pursuant to the warrant would still be excluded. It is unclear if the "good faith" exception applies to warrantless seizures. On January 8, 1974, the Supreme Court ruled that grand juries may use illegally obtained evidence in questioning witnesses.
A defendant may ask for evidence to be excluded only if its seizure violated his own Fourth Amendment rights. The defendant may not assert the rights of a third party.
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