In what circumstances will a court deny a suppression motion against a police officer who violates the knock and announce rule?

Similarly, in United States v. Brown, we affirmed the district court's denial of a suppression motion on the basis of exigent circumstances where officers who violated the knock and announce rule were investigating the underlying offense of trafficking in crack cocaine and heroin and "the use of a firearm incident to that trafficking," and "the suspects were reasonably believed to be armed" in light of a past attempt to collect a drug-related debt from a confidential informant with a "pump-action shotgun." 52 F.3d 415, 421 (2d Cir. 1995).