NASHVILLE, Tenn. — Tennessee Supreme Court justices peppered lawyers trying to block the state’s lethal injection protocol with questions on Thursday, challenging them to declare some other execution method they would consider acceptable under the law.

Lawyers representing the death row inmates argued that they shouldn’t have to suggest alternatives as part of their legal challenge, which claims the state’s one-drug lethal injection method is unconstitutionally cruel and unusual punishment because it’s likely to inflict extreme pain and can cause a lingering death.

The justices appeared to disagree, citing a 2015 U.S. Supreme Court ruling they said requires challengers of execution methods to suggest alternatives.

“The Supreme Court has said repeatedly that the death penalty is constitutional in this country,” said Justice Roger A. Page. “Doesn’t it follow there has to be some constitutional method to carry it out?”

“The proper test does not involve the showing of an alternate method,” plaintiffs’ attorney Steve Kissinger insisted.

Justice Sharon Lee said that under the plaintiffs’ argument, “it sounds like there would be no protocol which would meet the standards.”

Tennessee has not executed an inmate since 2009 because of legal challenges and problems in obtaining lethal injection drugs. State lawmakers passed a law to allow the state to return to using the electric chair if lethal injections can no longer be administered.

Death row inmates sued after the state moved away from its three-drug cocktail to a one-drug method using a powerful anesthetic called pentobarbital. After pentobarbital’s only commercial producer restricted its distribution to prevent its use in executions, Tennessee decided to have a compound pharmacy mix pentobarbital to order.

Compounded drugs are small, specially mixed batches of drugs that are not subject to the same federal scrutiny as regular doses.

The inmates’ lawyers said this work-around is unethical and illegal.

“It is not a medical procedure; it is not done in the ordinary course of the medical practice; it violates the DEA’s statute; it violates the Tennessee licensing provisions,” said Michael Passino, an attorney for the inmates. “It violates the ethical obligation of the physician.”

State attorney Jennifer Smith said the current lethal injection standards meet constitutional requirements.

“A prisoner is not entitled to a painless death, but to a death that is not cruel, that is not unnecessarily painful,” she said.

Justice Lee asked what Tennessee can do to avoid the problems other states have experienced with lethal injections.

“How do we know our executions won’t be botched?” Lee asked.

“We don’t,” said Smith. “Executions, like any human endeavor are subject to error.”

“We can’t be 100 percent sure that there will not be a problem with an execution. But that’s not what the constitution requires,” she said. “The constitution doesn’t guarantee a perfect process.”