Hudson v. Michigan, the recent decision in which the Supreme Court said evidence from a search in which police failed to follow the "knock and announce" rule is admissible in court, ostensibly hinged on how close the connection between a Fourth Amendment violation and the discovery of evidence must be to trigger the exclusionary rule. The dissenters argued that the failure of police to wait more than a few seconds for the suspect, Booker Hudson, to answer the door rendered the whole search invalid, making the evidence police obtained "fruit of the poisonous tree." Writing for the five-justice majority, Antonin Scalia said the exclusionary rule did not apply in this case because the Fourth Amendment violation was not essential to the discovery of the evidence. Had police waited, say, 15 seconds and given Hudson the opportunity to answer the door, Scalia reasoned, they still would have found Hudson's drugs and gun.
Yet as Scalia also noted, if the police believed that wasn't the case, that waiting 15 seconds would have allowed Hudson to get rid of the evidence, the "knock and announce" rule would not have applied. So the nexus between barging in and finding the evidence does not really matter. Whether or not a knock-and-announce violation is necessary to preserve evidence, the evidence can be admitted–an easy rule for police to remember but not one that is likely to encourage respect for the knock-and-announce requirement.
Tim Cavanaugh commented on Hudson earlier this week.