Supreme Court justices indicated on Monday they are deeply divided over a challenge to the way most US statesexecute prisoners by lethal injection, which critics say creates an avoidable risk of excruciating pain.

With executions in the US halted since late September, the court heard arguments in a case from the state of Kentucky that calls into question the mix of three drugs used in most executions.

Justice Antonin Scalia was among several conservatives on the court, who suggested he would uphold Kentucky’s method of execution and allow capital punishment to resume.

States have been careful to adopt procedures that do not seek to inflict pain and should not be barred from carrying out executions even if prison officials sometimes make mistakes in administering drugs, Scalia said. “There is no painless requirement” in the US Constitution, Scalia said.

But other justices said they are troubled by the procedure in which three drugs are administered in succession to knock out, paralyse and kill prisoners. The argument against the three-drug protocol is that if the initial anesthetic does not take hold, a third drug that stops the heart can cause excruciating pain.

The second drug, meanwhile, paralyses the prisoner, rendering him unable to express his discomfort.

“I’m terribly troubled by the fact that the second drug seems to cause all risk of excruciating pain,” Justice John Paul Stevens said.

Both sides in the case said they are bothered by the seemingly endless series of death penalty cases that come to the court.

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