The only dissenter in the 3-2 ruling was Judge David Tatel, an appointee of former President Bill Clinton. The judges were reviewing a lower court's injunction that had blocked the scheduled executions.

The decision was seen as a win for Trump's Justice Department, which issued new guidelines last July that would have allowed the federal government to carry out its first executions in 16 years.

The fates of the four men remain unresolved because their death sentences were sent back to the lower court for further proceedings.

In December, the U.S. Supreme Court declined the Justice Department's request to vacate the lower court's injunction that scuttled the planned executions.

At issue is the question of whether the condemned men should be put to death by the injection of only one barbiturate — pentobarbital — as called for in the Justice Department's July 2019 memo.

Many of the 28 states where the death penalty is still legal require a lethal injection cocktail containing not one but three barbiturates. Those states include Indiana, where the scheduled executions were to take place.

Pharmaceutical companies have stopped producing at least one of the three drugs used in that lethal mixture, and several botched executions have resulted from some states using untested formulas.

The 1994 Federal Death Penalty Act calls for executions to be carried out "in the manner prescribed by the law of the State in which the sentence is imposed."

Judge Gregory Katsas argues in his majority opinion that the "manner prescribed" simply refers to the method of execution rather than the protocols each state follows in carrying out each kind of execution.

"The government says that 'manner' here means 'method'," Katsas writes, "such that the FDPA regulates only the top-line choice among execution methods such as hanging, electrocution, or lethal injection. In my view, the government is correct."

Judge Neomi Rao, in a concurring opinion, argues that while the word "manner" refers not only to the method of execution, it cannot be interpreted in isolation. "It is a broad, flexible term," she says, "whose specificity depends on context."

In his dissent, Tatel says the best understanding of the 1994 statute is that it "requires federal executions to be carried out using the same procedures that states use to execute their own prisoners.

"Had Congress intended to authorize the Attorney General to adopt a uniform execution protocol," Tatel argues, "it knew exactly how to do so."