Search warrants may be invalid if there are problems obtaining the warrant or with the warrant itself.
Under Fourth Amendment law, police may not conduct a search without a warrant. (There are several exceptions to the warrant requirement.) Police officers obtain a warrant by presenting evidence to a court.
When applying for a warrant, a police officer must present evidence to the judge that, based on all information within his knowledge, he reasonably believes that contraband (criminal evidence) will be found on the search premises at the time of search. This evidence is known as probable cause.
A warrant must be "precise on its face." The warrant itself must describe, fairly precisely, what property may be searched and what sort of evidence the police may seize if found. If this information is lacking, the warrant will be invalid.
A warrant can be issued only by a neutral and detached judge---one with no personal interest in issuing or not issuing the warrant.
Even if a search warrant is absolutely valid in itself, police must also execute the warrant properly---with no unreasonable delay and at the time stipulated---or the search itself may be invalid.
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