Before a law enforcement agent may search you or your property, he or she must petition a judge to obtain a search warrant—a legal document that gives authorization to search a specific location for a specific item or items.
In order to obtain a search warrant, an officer must convince a judge that there is sufficient probable cause to believe that criminal activity is taking place at the location to be searched, or that evidence of criminal wrongdoing may be found there.
When petitioning a judge for a search warrant, police officers usually present information in the form of an affidavit, which is a statement written under oath. Affidavits may report the observations of the police officer, police informants, or private citizens. Before granting a search warrant, the judge must be satisfied that the affidavit establishes probable cause.
Because the Fourth Amendment to the U.S. Constitution protects citizens from unreasonable searches and seizures, it is necessary to have a measure by which these actions are lawfully justified. The legal standard by which search warrants are issued and carried out is called probable cause.
Essentially, probable cause exists when there is a reasonable basis to believe that a crime may have been committed or that there is evidence of a crime present at the place to be searched. When determining whether an officer has a reasonable basis for probable cause, a judge will consider all the facts surrounding the circumstance, including any affidavits submitted by police or other witnesses.