He could have chosen not to respond to the knocking in any fashion, Justice Alito wrote. Or he could have come to the door and declined to let the officers enter without a warrant.

“Occupants who choose not to stand on their constitutional rights but instead elect to attempt to destroy evidence have only themselves to blame,” Justice Alito wrote.

Justice Alito took pains to say that the majority was not deciding whether an emergency justifying an exception to the warrant requirement — an “exigent circumstance,” in legal jargon — actually existed. He said that the Kentucky Supreme Court “expressed doubt on this issue” and that “any question about whether an exigency actually existed is better addressed” by the state court.

All the United States Supreme Court decided, Justice Alito wrote, was when evidence must be suppressed because the police had created the exigency. Lower courts had approached that question in some five different ways.

The standard announced Monday, Justice Alito wrote, had the virtue of simplicity.

“Where, as here, the police did not create the exigency by engaging or threatening to engage in conduct that violates the Fourth Amendment,” he wrote, “warrantless entry to prevent the destruction of evidence is reasonable and thus allowed.”

But “there is a strong argument,” Justice Alito added, that evidence would have to be suppressed where the police did more than knock and announce themselves. In general, he wrote, “the exigent circumstances rule should not apply where the police, without a warrant or any legally sound basis for a warrantless entry, threaten that they will enter without permission unless admitted.”

Justice Ginsburg, dissenting, said the majority had taken a wrong turn.

“The urgency must exist, I would rule,” she wrote, “when the police come on the scene, not subsequent to their arrival, prompted by their own conduct.”

Justice Ginsburg then asked a rhetorical question based on the text of the Fourth Amendment.

“How ‘secure’ do our homes remain if police, armed with no warrant, can pound on doors at will and, on hearing sounds indicative of things moving, forcibly enter and search for evidence of unlawful activity?” she asked.