The prisoners' attorneys argued that the combination of drugs, if improperly administered, could cause inmates to suffer an excruciatingly painful death while masking their ability to signal suffering. They suggested executing condemned prisoners with only a fatal dose of anesthetic.
Kentucky countered that the constitutional ban against cruel and unusual punishment does not require executioners to eliminate any risk of suffering.
A majority of the justices sided with that argument.
"Because some risk of pain is inherent in even the most humane execution method, if only from the prospect of error in following the required procedure, the Constitution does not demand the avoidance of all risk of pain," wrote Chief Justice John G. Roberts Jr., in an opinion that garnered the support of Justices Anthony M. Kennedy and Samuel A. Alito Jr.
Four justices - John Paul Stevens, Antonin Scalia, Clarence Thomas and Stephen G. Breyer - agreed with the outcome while writing or signing on to concurring opinions.
Justice Ruth Bader Ginsburg wrote a dissenting opinion, which Justice David H. Souter joined.
Experts and advocates on both sides of the capital punishment debate agreed that the splintered ruling managed one important feat: It resolved the narrow legal issue of what standard lower courts should use to evaluate a state's execution protocols.
In the opinion written by Roberts, the three-justice plurality established that lethal injection procedures are unconstitutional if they present a "substantial risk of serious" pain to the condemned.
Their opinion left open the possibility that lethal injection procedures could be declared unconstitutional if a state refused to adopt a proposed alternative that was "feasible, readily implemented, and [would] in fact significantly reduce a substantial risk of severe pain."
Legal experts - and even the justices themselves in their opinions - offered drastically different predictions about the impact of the high court's new standard on litigation, which has tied up death penalty cases across the nation.
Scheidegger, of the Criminal Justice Legal Foundation, said the decision "comes pretty close to shutting down this chapter of the death penalty litigation."
"People will still make the challenges," he said, "but I think if the lower courts apply this decision properly, they should be dismissed pretty readily."
