In the 1995 U.S. Supreme Court case Wilson v. Arkansas, the court recognized that something called the “Castle Doctrine” and the “knock and announce” rule are embedded in the Fourth Amendment. The idea is that our protections from unreasonable search and seizure mean that police must knock, announce themselves, and give residents time to answer before they enter forcibly.

But here’s the hitch. The Wilson ruling allowed that in cases with “exigent circumstances,” police can enter without knocking if it means a suspect might be able to escape or destroy evidence. That exception prompted many police departments to simply declare in all search warrant affidavits that a no-knock raid was justified, citing the danger of a suspect fleeing, destroying evidence, or assaulting the officers serving the warrant.

That in turn lead to another Supreme Court ruling in 1997, Richards v. Wisconsin, which found that blanket exceptions to the “knock and announce” rule were unconstitutional. The ruling, written by Justice John Paul Stevens, found that, “If a per se exception were allowed for each category of criminal investigation that included a considerable — albeit hypothetical — risk of danger to officers or destruction of evidence, the knock-and-announce element of the Fourth Amendment’s reasonableness requirement would be meaningless.”