When are police excused from knocking and announcing?

Generally, when police are executing a search warrant, they are required to knock and announce themselves, which puts the inhabitants of the building on notice that police have a lawful authority to enter and, if they are denied that authority, they have the right to forcibly enter. However, under certain circumstances, police are permitted to forcibly enter without first knocking or announcing themselves.

One such scenario is when there are exigent circumstances. For example, if officers are about to execute a search warrant on a house but right before they knock on the door they hear a gunshot inside and someone scream, then it wouldn't make much sense to knock and wait for permission to enter.

A second scenario is when knocking and announcing would be dangerous or counter-productive to the investigation. For example, if police are tracking a highly dangerous suspect who has previously murdered dozens of cops that attempted to arrest them, then it would be pretty dangerous for police to knock and announce themselves before executing a search warrant on the residence where they believe such a suspect is hiding.

Finally, a third scenario is when police believe there is a risk that knocking and announcing themselves will provide enough time for suspects to destroy important evidence. The most common example of this scenario is when a suspect attempts to flush drugs down their toilet. However, there are plenty of crimes that can be obstructed on short notice, such as a computer crime where evidence can be destroyed with only a few keystrokes.