US Criminal Law/Searches without a warrant
The simplest and most common type of warrantless searches are searches based upon consent. No warrant or probable cause is required to perform a search if a person with the proper authority consents to a search.  A consent search requires the person being searched to freely and voluntarily waive their Fourth Amendment rights, granting the officer permission to perform the search. 
The person has the right to refuse to give consent  and except in limited cases may revoke consent at any point during the search.  In Schneckloth v. Bustamonte the U.S. Supreme Court found that officers were not required to warn people of their right to withhold consent in order for consent to be valid.  However, the prosecution is required to prove that the consent was voluntary and not a result of coercion. 
In cases such as, Lee v. United States, Lopez v. United States, and Hoffa v. United States the courts have ruled evidence found in searches based on consent obtained as an undercover officer or as an informer to be admissible.
A third party can in some limited cases grant consent. The person needs to possess or be believed by the searching officer to possess “common authority over or other sufficient relationship to the premises or effects sought to be inspected”.
Consent can be revoked at almost any time during a consent based search. If consent is revoked the officer or officers performing the search are required to immediately stop searching. The right to revoke consent is not recognized in two cases; airport passenger screening and prison visitation 
Most courts have found the right to revoke consent is removed once a passenger has begun X-ray screening. In United States v. Herzbrun, the U.S. Court of Appeals for the Eleventh Circuit found Herzbrun, “had no constitutional right to revoke his consent to a search of his bag once it entered the X-ray machine and he walked through the magnetometer.” “A rule allowing a passenger to leave without a search after an inconclusive X-ray scan would encourage airline terrorism by providing a secure exit where detection was threatened.,” explained the court in United States v. Pulido-Baquerizo.
A similar argument is also applied to searches of prison visitors in United States v. Spriggs. As long as a prison visitor is warned that all visitors will be searched and consents to the search, consent can not be revoked once the search has begun. Allowing consent to be withdrawn the court reasoned would encourage the smuggling of contraband into prisons by providing a secure escape to the smuggler.
Once consent to search is given an individual may withdraw consent with an “unequivocal act or statement of withdrawal.” Consent may be withdrawn by statements, actions, or a combination of statements and actions. In United States v. Bily, the court found Bily statement to the agents, “That’s enough, I want you to stop,” was a revocation of consent. The revocation of consent must be a clear statement. In the United States v. Gray, the U.S. Court of Appeals for the Eighth Circuit found Gray did not revoke consent with the statements of “[t]his is ridiculous,” and “how long [is] the search going to take.” The district court found that while Gray and his passenger had made “protests to leave,” “there was no specific request to leave, and under the circumstance,... [the officer] was reasonable in continuing the search”. An expression of impatience, is not sufficient to terminate consent.
In United States v. Ho, the U.S. Court of Appeals for the Fifth Circuit, the court found Ho's attempts to retrieve his portfolio from the officer during a search constituted a revocation of his earlier consent to search. In this decision the court recognized his acts constituted a valid revocation of consent. 
During the course of a search an officer may develop reasonable suspicion or probable cause, once this has been developed the individual loses their right to revoke consent. However, in United States v. Fuentes, the court found the “[m]ere refusal to consent to a stop or search does not give rise to reasonable suspicion or probable cause.”
Officers are not required to conduct a search in a way that gives the individual an opportunity to revoke consent. In United States v. Dominguez, the court rejected the idea that, “officials must conduct all searches in plain view of the suspect, and in a manner slowly enough that he may withdraw or delimit his consent at any time during the search.”
The motor vehicle exception
The motor vehicle exception was first established by the United States Supreme Court in Carroll v. United States.  The motor vehicle exception allows an officer to search a vehicle without a warrant as long as he has probable cause to believe that evidence or contraband is located in the vehicle.
The motor vehicle exception is based on the idea of a lower expectation of privacy in motor vehicles due to the regulations they are under. Additionally, the ease of mobility creates an inherent exigency. In Pennsylvania v. Labron the U.S. Supreme Court, stated, “If a car is readily mobile and probable cause exists to believe it contains contraband, the Fourth Amendment thus permits police to search the vehicle without more.”
The scope of the search is limited to only what area the officer has probable cause to search. This area can encompass the entire vehicle including the trunk. The motor vehicle exception in addition to allowing officers to search the vehicle also allows officers to search any containers found inside the vehicle that could contain the evidence or contraband being searched for. The objects searched do not need to belong to the owner of the vehicle. In Wyoming v. Houghton, the U.S. Supreme Court ruled that the ownership of objects searched in the vehicle is irrelevant to the legitimacy of the search.
Some state's constitutions require officers to show there was not enough time to obtain a warrant. With the exception of states with this requirement, an officer is not required to obtain a warrant even if it may be possible to do so.
In United States v. Ludwig ,The Tenth Circuit Court of Appeals found that a search warrant is not required even if there is little or no risk of the vehicle being driven off. The court stated, “[i]f police have probable cause to search a car, they need not get a search warrant first even if they have time and opportunity.” In United States v. Johns,the U.S. Supreme Court upheld a search of a vehicle that had been seized and was in police custody for three days prior to the search. The court stated, “A vehicle lawfully in police custody may be searched on the basis of probable cause to believe it contains contraband, and there is no requirement of exigent circumstances to justify such a warrantless search”.
The motor vehicle exception does not only apply to automobiles. The U.S. Supreme Court in California v. Carney found the motor vehicle exception to apply to a motor home. The court did however, make a distinction between readily mobile motor homes and parked mobile homes. A number of factors including, the home being elevated on blocks, whether the vehicle is licensed, and if it is connected to utilities determine if the motor vehicle exception applies. In United States v. Johns, the motor vehicle exception was applied to trucks. In United States v. Forrest it was applied to trailers pulled by trucks. United States v. Forrest applied the exception to boats and in United States v. Hill to house boats. In United States v. Nigro and United States v. Montgomery the motor vehicle exception was found to also include airplanes.
The emergency exception
The emergency exception allows officers to enter and search a residence in certain circumstances when these circumstances prevent them from obtaining a search warrant or consent. In Minnesota v. Olsen, preventing escape was found to be a valid emergency circumstance, In United States v. Santana, the court found that preventing the destruction of evidence and the hot pursuit of a criminal suspect, to also be valid reasons to conduct an emergency search. The right to perform legal warrantless searches to prevent harm to the officers or others was established in Warden v. Hayden. Thompson v. Louisiana and Mincey v. Arizona established the right for an officer to enter and search a residence in order to render immediate aid to a person in need of assistance. 
Permitting officers to enter a residence to render immediate aid was upheld by the U.S. Supreme Court with the comment, “numerous state and federal cases have recognized that the Fourth Amendment does not bar police officers from making warrantless entries and searches when they reasonably believe that a person within is in need of immediate aid.” A “warrantless entry by criminal enforcement officials may be legal when there is compelling need for official action and no time to secure a warrant.” The reason given for allowing these entries is that the police are acting to assist the person in need rather then make an arrest or collect evidence. 
In order for the warrantless, consentless, entry and search to be legal, the officer must “reasonably believe” a emergency situation requiring immediate police intervention exists. The courts have often interpreted this to mean the officer must have probable cause. In Kerman v. City of New York, the court found “probable cause for a forced entry in response to exigent circumstances requires finding a probability that a person is in ‘danger.’” 
The courts commonly apply a test known as the Mitchell Test in determining if an emergency search was valid. This test requires the officer to have had “reasonable grounds to believe” that there was an emergency and an immediate need for police assistance, the search was not motivated primarily by an intent to arrest or seize evidence and there must have been a “reasonable basis approximating probable cause” to believe the area searched was associated with the emergency. 
Emergency searches must be limited to searches for and to aid the injured. Officers may however, seize items in plain view during a lawful emergency search. 
The most common emergency searches are as a result of a 911 call. Many of these calls are anonymous. In most cases the courts have found police response to anonymous calls to be unlawful unless additionally corroborating information is available. This information can include officer observations, such as sounds of violence, fighting, gun shots or cries for help. 
An example of this can be found in Ohio v. Applegate. Officers were sent to the Applegate's home after receiving an anonymous call reporting domestic violence. The officers heard yelling, arguing and sounds of furniture being turned over. The officers entered the house and arrested the defendant. The court found “the movements of the officers were conservative, prudent, and reasonable.” 
Visual observations can also be used to help corroborate an anonymous call. In Colorado v. Thompson, officers responding to an anonymous call found gun shell casings on the driveway and blood on the front door. The officers found a woman covered in blood who claimed her attacker was gone and everything was alright. The police searched the home for further victims and the suspect. No one else was found but police did find several large marijuana plants. The court upheld the search saying “the evidence establishe[d] the existence of an emergency justifying the warrantless entry and limited search of [the] residence.” The court also noted that the officers did no more, “than conduct a quite limited search of the premises to ascertain whether additional injured persons were present.” 
The conduct of people at the scene can also be used to corroborate an anonymous call. In United States v. Arcobasso, police responded to a “shots fired within a residence.” They heard the clicking sounds of a gun being dry-fired and observed through an open window a man dry-firing a gun. The officers searched performed a protective sweep and found a shotgun. The court held the search to be legal because “there may have been a shooting victim or another armed person inside…exigent circumstances existed to justify the warrantless search.” In other cases the person running from the police, or “sweating profusely and appearing to be agitated” were found to corroborate an anonymous call. 
Police knowledge of past or ongoing criminal activity may also be used to corroborate an anonymous call. In People v. Love, officers responded to a call of a man with a gun at a hotel. Officers knocked on the room's door which was opened by a woman who upon seeing police attempted to shut the door. The officers pushed the door open and entered the room. The officers found in plain view an automatic handgun, drugs, and drug paraphernalia. The court upheld the police officers actions because, “[t]he nature and specificity of the police radio transmissions, their close temporal proximity, the reputation of the hotel as a locale for drug and prostitution activity, and the hostile reception by [the] woman in the room when the police knocked on the door….”. 
The type of victim may also be a factor in assessing an anonymous report. In Wisconsin v. Boggess, an officer accompanied by a social worker made a warrantless entry to check the welfare of children after receiving an anonymous report of physical abuse. The court found the entry into the house to be reasonable because it, “involved small children inside a home, who are less able to protect themselves from further harm or to independently seek medical attention than are adults. 
Courts generally only except “immediate” emergency searches to be valid. However, a few exceptions do exist. In State v. Kraimer, it was established that a reasonable delay to investigate, and corroborate an anonymous call does not invalidate the application of the emergency exception. Kraimer made three calls to a confidential police helpline stating that “he had shot and believed he had killed his wife 4 days earlier; that his wife’s body was in an upstairs bedroom near a bathroom; and that he had his four children, at home with him, who ranged in age from a 12-year-old male to a 2-year-old female.” Police attempted to locate the caller by checking school absences for children of matching ages. Police identified three families with matching absences. After checking the other two families, police arrived at Kraimer's house approximately four hours after the first call had been received. Neighbors told the police that they had seen the children playing in the yard, conflicting with the explanation of a vacation Kraimer had given the school. The officers called for backup and after a brief delay entered the house and found body of the Kraimer’s wife and other evidence. The court ruled the entry and search to be lawful and all evidence found to be admissible. The delay of the several hours while searching for the caller, did not eliminate the belief an emergency existed, nor did the wait for backup officers as it was a “reasonable precautionary measure.” Additionally the court found the police are not required to accept a layperson's belief that a person is already dead.
The plain view doctrine
The plain view doctrine allows an officer to seize without a warrant, evidence and contraband found in plain view during a lawful observation. In order for the officer to seize the item, the officer must have probable cause to believe the item is evidence of a crime or is contraband. The police may not move objects to get a better view. In Arizona v. Hicks, the officer was found to have acted unlawfully. While investigating a shooting , the officer moved, without probable cause, stereo equipment to record the serial numbers. The plain view doctrine has also been expanded to include the sub doctrines of plain feel, plain smell, and plain hearing. In Horton v. California, the court found eliminated the requirement that the discovery of evidence in plain view be inadvertent. Previously, "inadvertent discovery" was required leading to difficulties in defining "inadvertent discovery". A three-prong test is now used. The test requires the officer to be “engaged in lawful activity at the time”; “ the object’s incriminating character was immediately apparent and not concealed”, and “the officer had lawful access to the object and it was discovered accidentally”.
Another exception to the warrant requirement is open fields. In Hester v. United States, the court found that the Fourth Amendment did not protect open fields and therefore there are no requirements for warrants or probable cause. Police may perform warrantless searches of open fields even if this search would amount to trespassing. In determining if the area can be searched, factors such as the proximity to home, the use of an area, and actions taken to prevent observation and access to the area are used. In Oliver v. United States, the court found abandoned effects, refuse, and public places to also not be protected by the Fourth Amendment.
Searches made along the nation's borders were authorized by the First Congress , in The Act of July 31, 1789. These searches can be made without a warrant, probable cause, or even some degree of suspicion. In United States v. Ramsey, the court stated, “That searches made at the border, pursuant to the longstanding right of the sovereign to protect itself by stopping and examining persons and property crossing into this country, are reasonable simply by virtue of the fact that they occur at the border, should, by now, require no extended demonstration”.
People at immigration points may be searched and be held for up to 16 hours on reasonable suspicion alone. Searches performed near but not at the nation's borders however, are not included.  In Almeida-Sanchez v. United States, the court found a search of an automobile 20 miles from the border to violate the Fourth Amendment as it lacked probable cause.
There is no crime scene exception to the fourth amendment. An officer responding to an emergency situation can use exigent circumstances to justify entry to a private residence or business to render aid, etc, however, once the exigency is gone, the requirement of the fourth amendment reestablishes itself (See Mincey v. Arizona - 1978). An officer could use the plain view doctrine to justify seizing items of contraband or other items readily recognizable as evidence of a crime. But to open drawers or manipulate anything to view something not in plain view would create an illegal seizure and render the evidence inadmissible. It is always better to err on the side of obtaining a search warrant. The scene can and must be preserved while the warrant is being sought and everything seen while in the residence is admissible as probable cause.
Prison administrators are permitted to search prison cells and are not even bound by a reasonableness standard. In Hudson v. Palmer the Court stated “the Fourth Amendment proscription against unreasonable searches does not apply within the confines of the prison cell”. Some unreasonable searches may however be in violation of the Eighth Amendment or state laws against harassment, or malicious property destruction.
In Griffin v. Wisconsin, the court found a warrant or probable cause is not required to search the home of a person on probation provided “reasonable grounds” for a search exists. The court explained, “A State's operation of a probation system, like its operation of a school, government office or prison, or its supervision of a regulated industry, . . . presents 'special needs' beyond normal law enforcement that may justify departures from the usual warrant and probable cause requirements” “Probation, like incarceration, is a form of criminal sanction”. The Court also noted a requirement of a warrant or probable cause would interfere with the proper functioning of the ongoing [non- adversarial] supervisory relationship.
Searches incident to a lawful arrest
An exception to the warrant requirement is searches incident to a lawful arrest. This is also known as the Chimel Rule after the case that established it, Chimel v. California. This rule permits an officer to perform a warrantless search during or immediately after a lawful arrest. This search is limited to only the person arrested and the area immediately surrounding the person in which the person may gain possession of a weapon or destroy or hide evidence.
Searches of public school students
The warrantless searches of public school students during school is one of the more controversial types of warrantless search. The Court defined many of the aspects of the warrantless search of students in New Jersey v. T.L.O.. The court found the Fourth Amendment to apply to searches conducted by public school officials because “school officials act as representatives of the State, not merely as surrogates for the parents.” The court did, however, also state, “the school setting requires some easing of the restrictions to which searches by public authorities are ordinarily subject.” The court ruled that neither the warrant requirement or the probable cause standard applied, rather a reasonableness standard is used. In order for the search to be valid “reasonable grounds for suspecting that the search will turn up evidence that the student has violated or is violating either the law or the rules of the school.” Additionally searches must, “not [be] excessively intrusive in light of the age and sex of the student and the nature of the infraction” or discriminatory on the basis of age, sex, or race. 
Warrantless interceptions of communications
Communications between informants and suspects are often used. These recordings can be some of the most valuable evidence in a case, providing evidence that would otherwise be unobtainable. If the communications are intercepted lawfully officers are permitted to record these conversations and use them as evidence 
The U.S. Supreme Court has found the Fourth Amendment to not apply when the individual speaks in the presence of law enforcement or to those cooperating with law enforcement. In Hoffa v. United States, the U.S. Supreme Court ruled the use of informants to capture the contents of conversations, does not violate the other party's Fourth Amendment rights. The court found the Fourth Amendment protections do not apply to a “wrongdoer’s misplaced belief that a person to whom he voluntarily confides his wrongdoing will not reveal it.”
In a similar case, Lopez v. United States, Lopez had attempted to bribe an Internal Revenue Service agent. The agent has made recordings of the conversations in which Lopez attempted to bribe him. The court found these tapes to be admissible because the agent was one of the parties in the recorded conversations. 
In United States v. White, the U.S. Supreme Court affirmed the earlier decisions with the statement, “[i]f the conduct and revelations of an agent operating without electronic equipment do not invade the defendant’s constitutionally justifiable expectations of privacy, neither does a simultaneous recording of the same conversations made by the agent”.
Law enforcement may use individuals to record conversations they are a party to, or overhear, even if they do not understand the conversation. In United States v. Longoria, an informant working with FBI agents recorded conversations involving Longoria. Longoria contested the admissibility of conversations recorded while the informant was present but the conversations were in Spanish, a language the informant did not understand. The U.S. Court of Appeals for the Tenth Circuit found the conversations to be admissible because, “the informant may very well have concealed his ability to speak Spanish, the same as he concealed the recording equipment and his allegiance with law enforcement.”
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