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A search warrant is a court order that a magistrate or judge issues to authorize law enforcement officers to conduct a search of a person, location, or vehicle for evidence of a crime and to confiscate any evidence they find. In most countries a search warrant cannot be issued in aid of civil process.
Jurisdictions that respect the rule of law and a right to privacy constrain police powers, and typically require search warrants or an equivalent procedure for searches police conducted in the course of a criminal investigation. The laws usually make an exception for hot pursuit: a police officer following a criminal who has fled the scene of a crime has the right to enter a property where the criminal has sought shelter. This contrasts with authoritarian regimes, where police typically can search property and people without having to provide justification or secure court permission.
In England and Wales, a local magistrate issues search warrants, which require that a constable provide evidence that supports the warrant application. In the majority of cases where police already hold someone in custody, police can search premises[clarification needed] without a search warrant under Section 18 of the Police and Criminal Evidence Act (PACE), which requires only the authority of an inspector.
Under Section 18(5)a of PACE, a constable can conduct a search immediately without an inspector's authorisation. This subsection allows a constable to search the home of a suspect(s) under arrest in their presence before they take the suspect to a police station (or other custody location). Under Section 32 of PACE, a constable who arrests a person who is on their own property or has just left their premises, may immediately search both the suspect and the immediate area.
In Scotland, a country operating on the distinct legal system of Scots law compared to England and Wales - the restrictions governing the use and execution of search warrants are set out under Part XIII under the Criminal Procedure (Scotland) Act 1995. Search warrants must be signed by a Sheriff after a petition from police.
To get a warrant, police must present a judge with an ITO (information to obtain) form that contains reasonable and probable grounds to believe that an offence has been or is being committed, and that the authorization sought will afford evidence of that offence. This hearing is 'ex parte' meaning only the crown is present. This fact obliges the police to include any known facts that hurt their application.
After a search the occupants have a copy of the warrant and may get hold of the ITO through crown disclosure if the occupant(s) are charged. There are numerous different warrant procedures in the Criminal Code, some have specific requirements such as being served during daytime or having a named supervising officer present in the case of a home search. If these(see link) requirements are not met by the police the evidence found may become admissible against the accused at trial.
Under the Fourth Amendment to the United States Constitution, most police searches require a search warrant based on probable cause, although there are exceptions. In the absence of valid consent or an exception to the warrant requirement, whether for purposes of effecting a search or an arrest, police entry in an individual's home always requires a warrant. The probable cause standard for obtaining a search warrant is lower than the quantum of proof required for a later conviction, proof beyond a reasonable doubt.
Under the Fourth Amendment, search warrants must be reasonable and specific. This means that a search warrant must reasonably identify the items to be searched for and the place where law enforcement officials are authorized to search for those items. Unless an exception to the warrant requirement applies, the search of other buildings or areas of a building, persons or vehicles, or the search for additional items that do not reasonably fall under the original warrant, will normally require additional search warrants.
To obtain a search warrant, an officer must prove to a magistrate or judge that probable cause exists for the proposed search, based upon direct information (i.e., the officer's personal observation) or other reliable information. An application for a search warrant will often rely upon hearsay information, such as information obtained from a confidential informant, as long as probable cause exists based on the totality of the circumstances. Police can seize both property and persons under a search warrant. The rationale is that evidence police collect without a search warrant may not be sufficient to convict, but may be sufficient to suggest that a warrant would allow police to find enough evidence to convict.
The issue of federal warrants is determined under Title 18 of the United States Code. The law has been restated and extended under Rule 41 of the Federal Rules of Criminal Procedure. Federal search warrants may be prepared on Form AO 93, Search and Seizure Warrant. Although the laws are broadly similar, each state has its own laws and rules of procedure governing the issuance of warrants.
Search warrants are normally available to the public. On the other hand, they may be sealed if they contain sensitive information. 
Certain searches don't require a search warrant. For example,
In a plain view case, the officer is legitimately on the premises, his observation is from a legitimate vantage point, and it is immediately obvious that the evidence is contraband. The plain view rule applies, for example, when the officer has pulled the suspect over for a seat belt violation and sees a syringe on the passenger seat.
If the subject is arrested in a home or vehicle, police may perform a protective search to make sure that there are no weapons within the vicinity. For example, they may search for weapons the room where they arrested the subject of a warrant, and conduct a "protective sweep" of the premises if they reasonably suspect that other individuals may be hiding.
With rented property, a landlord may refuse to allow law enforcement to search a tenant's apartment without a search warrant, and police must obtain a warrant under the same guidelines as if the tenant were the owner of the property.
People who are occupying rooms at hotels or motels have a reasonable expectation of privacy in their rooms. However, a warrantless search may be possible if the hotel guest has property in their room a considerable period of time after the scheduled check-out time.
As first established by Carroll v. United States, police are allowed to search a vehicle without a search warrant when they have probable cause to believe that evidence or contraband is located in a vehicle. When police arrest an individual shortly after the individual has exited a vehicle, the police may conduct a full search of the suspect's person, any area within that person's immediate reach, and the passenger compartment of the recently occupied vehicle for weapons or any other contraband. However, Arizona v. Gant limits such searches to circumstances where the arrested person could have accessed the vehicle, or when the vehicle could contain evidence of the crime the person is arrested for.
Under The Border Search Exception custom and immigration officers are not required to have a warrant or probable cause to conduct searches and seizures at international borders and their functional equivalents.
This doctrine is not actually an exception to the Fourth Amendment, but rather to the Amendment's requirement for a warrant or probable cause. Balanced against the sovereign's interests at the border are the Fourth Amendment rights of entrants. Not only is the expectation of privacy less at the border than in the interior, the Fourth Amendment balance between the interests of the Government and the privacy right of the individual is also struck much more favorably to the Government at the border. This balance at international borders means that routine searches are "reasonable" there, and therefore do not violate the Fourth Amendment's proscription against "unreasonable searches and seizures".
A sneak and peek search warrant (officially called a delayed notice warrant and also a covert entry search warrant or a surreptitious entry search warrant) is a search warrant authorizing the law enforcement officers executing it to effect physical entry into private premises without the owner's or the occupant's permission or knowledge and to clandestinely search the premises.
In California, the California Electronic Communications Privacy Act mandates that in certain cases concerning electronic search warrants that the court issue gag orders "[...] prohibiting any party providing information from notifying any other party that information has been sought [...]".
British Gas are legally entitled to enter anyone's home if the householder hasn't paid the bill and won't let them in, but they need a warrant from a magistrate, who must be satisfied the information British Gas shows them is correct. In most cases, British Gas also must have issued a prior warning.