The Supreme Court today said police officers cannot use drug-sniffing dogs along the perimeter of a house without first obtaining a search warrant. In a 5-4 ruling handed down Tuesday, the high court said the use of police-trained dogs to investigate a home's surroundings constitutes a "search," as defined by the Fourth Amendment.

"A police officer not armed with a warrant may approach a home and knock, precisely because that is no more than any private citizen might do," Justice Antonin Scalia wrote in the majority opinion. That allowance changes, he argued, once police dogs are introduced, because they signal an intent to uncover evidence.

"[I]ntroducing a trained police dog to explore the area around the home in hopes of discovering incriminating evidence is something else," Scalia wrote. "There is no customary invitation to do that."

Today's ruling upholds a 2011 decision from the Florida Supreme Court, which earlier determined that evidence obtained by drug dogs could not be used in court against defendant Joelis Jardines. In that case, a chocolate labrador named Franky was deployed along the perimeter of Jardines' Miami-area home, after police received an anonymous tip that Jardines was growing marijuana in his home. According to police reports, the dog sniffed along the base of Jardines' front door, where he detected odors of marijuana. Miami-Dade police then obtained a warrant based on this evidence, and pressed charges.

Writing in dissent, Justice Samuel Alito argued that homeowners should have no reasonable expectation that odors won't emanate from their homes, where they could be detected by passing drug dogs. He also argued that police didn't violate the defendant's privacy by simply walking up to his front door with their labrador in tow, pointing to the long history of using dogs to investigate crime scenes.

"Dogs have been domesticated for about 12,000 years," Justice Alito wrote. "They were ubiquitous in both this country and Britain at the time of the adoption of the Fourth Amendment and their acute sense of smell has been used in law enforcement for centuries.

"Yet the Court has been unable to find a single case — from the United States or any other common-law nation — that supports the rule on which its decision is based. Thus, trespass law provides no support for the Court’s holding today."