COLUMBIA, S.C. (AP) — South Carolina prison officials say they have to delay an execution scheduled for Friday because they won't be able to obtain the necessary lethal injection drugs.

An attorney for the state Department of Corrections wrote in a letter to the South Carolina Supreme Court last week that the agency cannot carry out the execution of Richard Bernard Moore due to the lack of drugs, which it has not had stocked since 2013. The Associated Press obtained a copy of the letter.

The court scheduled Moore's execution after he exhausted his federal appeals this month. Moore, 55, has spent nearly two decades on death row following his conviction for the 1999 killing of a convenience store clerk in Spartanburg County. He would be the first person executed in South Carolina in nearly a decade.

The state’s usual injection protocol calls for three drugs: pentobarbital, pancuronium bromide and potassium chloride. But the corrections agency has said it has not had the drugs in stock since 2013, when its last supplies expired. The agency has previously said it reserves the right to execute Moore with a single lethal dose of the sedative pentobarbital.

Lindsey Vann, one of Moore's attorneys, called the delay “unprecedented” on Monday, adding that she wasn't aware of any other execution in South Carolina history requiring such a delay due to a lack of drugs.

In 2017, corrections officials said they could not carry out the execution order of Bobby Wayne Stone without the appropriate drugs. At the time, however, Stone had not yet exhausted his appeals in court.

Prison officials say that per state law, Moore must be executed by lethal injection by default because he did not choose between lethal injection and electrocution by a deadline earlier this month. Moore's attorneys say he did not make a decision because the agency is not being transparent with its execution protocols.

Moore's legal team is also seeking to block the execution in federal court.

Securing lethal injection drugs has become an increasingly difficult task in the U.S. as drug manufacturers have shied away from selling to states under pressure from anti-death penalty activists. Corrections chief Bryan Stirling, along with the governor and attorney general, have advocated for a bill to shield the identities of manufacturers who provide such drugs.

State lawmakers have also mulled in recent years a bill to require death row prisoners to die by electric chair if lethal injection is not available.

Moore is one of 37 people, all men, currently on South Carolina’s death row. Some prosecutors have sought the death penalty less often in recent years, citing the state’s inability to carry out executions.

South Carolina’s last execution was in 2011.

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This version of the story corrects that the deadline for Moore to choose was this month, not last month.

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Michelle Liu is a corps member for the Associated Press/Report for America Statehouse News Initiative. Report for America is a nonprofit national service program that places journalists in local newsrooms to report on undercovered issues.