Kentucky police were following a man who had just sold drugs to an undercover informant. They entered an apartment breezeway, heard a door slam and found they had two choices.

Behind door No. 1 was the dealer. And, unfortunately for him, behind door No. 2 were Hollis King and friends, smoking marijuana.

Smelling the drug, the officers banged loudly on King's apartment door and identified themselves as police. The officers said they heard a noise and feared evidence was being destroyed. They kicked down the door and found King, two friends, some drugs and cash.

King was sentenced to 11 years in prison, but the Kentucky Supreme Court overturned his conviction. It said that the officers had entered the apartment illegally and that the evidence they found should not have been considered in court.

On Wednesday, the case made its way to the U.S. Supreme Court, where it will provide another chance for justices to decide when police may enter a home without permission or a warrant and not violate the Constitution's protections against unreasonable searches.

Justice Elena Kagan spelled out the worry for some on the court.

"One of the points of the Fourth Amendment is to ensure that when people search your home, they have a warrant, and of course there are exceptions to that," she said.

Agreeing with a test proposed by Kentucky prosecutors for when such searches are lawful could mean "essentially eviscerating the warrant requirement in the context of the one place that the Fourth Amendment was most concerned about."

The case before the court was about one of the exceptions Kagan mentioned: so-called exigent circumstances. Those arise when police have reason to suspect criminal activity is underway, but think that if they take the time to get a warrant, a life may be endangered, a suspect may escape or evidence may be destroyed.

In this case, the Kentucky high court said police could not create the emergency they say prevented them from obtaining a warrant.

Kentucky Assistant Attorney General Joshua Farley said the Lexington police officers in this case had probable cause to search the apartment - the smell of marijuana led them to think a crime was being committed.

But there was no time to get a warrant, he said, because they heard noises that led them to think the evidence was being destroyed.