· an informant whose information appears to be correct after at least partial verification by the police
· a victim of a crime related to the search
· a witness to the crime related to the search or
· another police officer.
Case Example 1: Hoping to obtain a warrant to search Olive Martini's backyard, a police officer submits an affidavit to a magistrate. The affidavit states that "the undersigned is informed that Olive operates an illegal still（A distillery.蒸馏室，酒厂） in her backyard."
Question: Should the magistrate issue a search warrant?
Answer: No. The affidavit is too vague, and does not identify the source of the information so that the magistrate can properly judge its reliability. "Probable cause" therefore does not exist.
Case Example 2: Same case. The affidavit states that "I am a social acquaintance of Olive Martini. On three occasions in the past two weeks, I have attended parties at Martini's house. On each occasion, I have personally observed Martini serving alcohol from a still in Martini's backyard. I have personally tasted the drink and know it to be alcoholic with an impertinent aftertaste. I had no connection to the police when I attended these parties."
Question: Should the magistrate issue a warrant authorizing the police to search Martini's backyard?
Answer: Yes. The affidavit provides detailed, firsthand information from an ordinary witness （without police connections） that indicates criminal activity. The affidavit is reliable enough to establish probable cause for issuance of a warrant.
14. What If a Police Officer Makes a Search Under a Warrant That Shouldn't Have Been Issued in the First Place?
In most situations the search will be valid. In U.S. v. Leon （1984）, the U.S. Supreme Court ruled that if the police conduct a search in good-faith reliance on the warrant, the search is valid and the evidence admissible even if the warrant was in fact invalid through no fault of the police. The Court's reasoning is that:
· it makes no sense to condemn the results of a search when police officers have done everything reasonable to comply with Fourth Amendment requirements, and
· the purpose of the rule excluding the results of an invalid search as evidence is to curb the police, not a judge, and that if a judge makes a mistake, this should not, therefore, be grounds to exclude evidence.
For example, assume that a judge decides that an affidavit submitted by a police officer establishes probable cause for the issuance of a warrant. Even if a reviewing court later disagrees and decides that the warrant shouldn't have been issued in the first place, the officer's search in good-faith reliance on the validity of the warrant will be considered valid, and whatever the search turns up will be admissible in evidence. If, however, the warrant is issued on the basis of statements in the affidavit that the police knew to be untrue or which were recklessly made without proper regard for their truth, the evidence from a search based on the warrant may later be excluded upon the proper motion being made by the defendant.