An officer must comply with all of the elements of the "knock and announce" rule, unless there are exigent circumstances or it would otherwise be considered a useless gesture.  The courts look to certain factors to determine if there were exigent circumstances, including:  (1) the gravity or violent nature of the offense with which the suspect is to be charged; (2) whether the suspect is reasonably believed to be armed; (3) whether there is reasonably trustworthy information that the suspect is guilty; (4) whether there is strong reason to believe that the suspect is on the premises; (5) the likelihood that the suspect will escape if not swiftly apprehended; and (6) whether the entry is made peaceably.
 
A "no-knock warrant" is a legal term referring to a warrant where the court has provided advance authorization to enter without complying with the "knock and announce" rule.  State statute neither authorizes nor prohibits "no-knock warrants."