A divided Supreme Court on Wednesday struck down a Missouri law that allowed police routinely to force drunk-driving suspects to give blood samples without a warrant and without consent.

A 5-4 majority agreed that the inevitable dissipation of alcohol from a suspect’s blood could not be regarded per se as an exigency that would justify a blood draw without a warrant under the Fourth Amendment.

Over the years, the court has endorsed exceptions to the warrant requirement of the Fourth Amendment, including when destruction of evidence is imminent and police cannot wait for a warrant. The issue before the court was whether the natural dissipation of alcohol in blood was the kind of phenomenon that always justified proceeding without a warrant.

Justice Sonia Sotomayor, writing for the majority in Missouri v. McNeely, said no. But she cautioned that exigent circumstances still "may arise" that would allow drawing blood without a warrant, case by case. The court said that with advances in technology and procedures that allow police to obtain warrants quickly, police are not usually hampered by the need for warrants as they combat drunk driving. Most states, she said, now allow prosecutors to obtain warrants by phone, email or teleconferencing.

Chief Justice John Roberts Jr., writing for himself and Justices Samuel Alito and Stephen Breyer in a partial concurrence and dissent, chastised the majority for failing to give clear guidance on what the new rule is. "A police officer reading this Court’s opinion would have no idea – no idea – what the Fourth Amendment requires of him, once he decides to obtain a blood sample from a drunk driving suspect who has refused a breathalyzer." These justices did agree, however, that warrants should be sought when possible.

In a separate concurrence, Justice Anthony Kennedy expressed the hope that "This court, in due course, may find it appropriate and necessary to consider a case permitting it to provide more guidance than it undertakes to give today."

Justice Clarence Thomas was the only court member who said that a per se exigency rule was appropriate, because the natural dissipation of alcohol in the blood in effect results in "rapid destruction of evidence." Thomas wrote, "Even a slight delay may significantly affect probative value in borderline cases of suspects who are moderately intoxicated."

The ruling was a victory for the American Civil Liberties Union and its longtime legal director Stephen Shapiro, who argued for his first time at the high court on behalf of defendant Tyler McNeely. "We know from experience that drunk-driving laws can be strictly enforced without abandoning constitutional rights," Shapiro said in a statement. "Today’s decision appropriately recognizes what half the states have already demonstrated – that maintaining highway safety does not require sacrificing personal privacy."

When a Missouri police officer stopped McNeely for speeding and weaving across the road, he refused to take a breath test. The officer then took McNeely to a hospital for a blood test. McNeely refused again, despite a warning that such a refusal automatically resulted in revocation of his driver’s license The blood sample was taken anyway, revealing an elevated blood alcohol content level. At trial, he sought to suppress the blood results on Fourth Amendment grounds. The trial court agreed that the case did not present circumstances that would have made it impossible to obtain a warrant. The Missouri Supreme Court agreed that dissipation of blood alcohol was not, by itself, enough to justify a warrantless search.

Tony Mauro can be contacted at tmauro@alm.com.