The police must have probable cause for a search. A warrant is not always necessary. For example, in the context of a DUI, if the officer has probable cause he can have blood drawn due to the rapidly changing blood-alcohol ratio. This is called exigent circumstances. Search warrants require a sworn affidavit. If the affidavit is based on information obtained from a CI (also called a confidential informer, cooperating individual, criminal informer, citizen witness or snitch), the CI must have proven reliable in the past.