ATMORE, Ala. — The U.S. Supreme Court has decided not to stay the execution of an Alabama inmate after considering his claim that Alabama will use an unreliable drug at the start of the procedure.

Robert Bryant Melson, 46, was scheduled to die at 6 p.m. CDT by lethal injection Thursday evening at a south Alabama prison. The court issued the stay about 15 minutes before the procedure was to begin.

Melson was convicted in the shooting deaths of three fast food restaurant workers during a 1994 robbery. State prosecutors said Melson and another man who used to work at the restaurant, robbed a Popeye’s in Gadsden, 60 miles (96 kilometers) northeast of Birmingham. They said Melson opened fire on four employees that had been ordered into the restaurant’s freezer. Nathaniel Baker, Tamika Collins and Darrell Collier were killed.

The surviving employee, Bryant Archer, crawled for help and was able to identify one of the robbers as the former worker. While he could not identify Melson, prosecutors said Melson told police he had been with the former employee that night. A shoeprint behind the store matched Melson’s shoes and Melson had asked people to provide him with an alibi when he came under investigation, they said.

Melson’s attorneys made a flurry of last-minute court filings seeking to halt the execution, arguing that Alabama planned to use a sedative called midazolam that would not reliably render him unconscious before he is given other drugs that stop his lungs and heart.

States have turned to midazolam, which is a sedative, as the first drug in lethal injection procedures as other drugs became difficult to obtain.

Melson’s attorneys cited midazolam’s use in problematic executions including one in Alabama in December in which an inmate Ronald Bert Smith coughed and heaved for the first 13 minutes of an execution held in December.

“The facts of Mr. Smith’s execution were relevant to Mr. Melson’s claims, because they describe the horrific results of using midazolam in a way it was never intended – as an anesthetic,” Melson’s attorneys wrote in the stay request.

Alabama attorney general’s office asked the courts to let the executions proceed. State attorneys argued midazolam’s use has been upheld by the U.S. Supreme Court and it has allowed multiple executions to proceed using the drug, including the execution of an Alabama inmate just last month.

The state has maintained there was no evidence that inmates experienced pain using midazolam.

“Melson has not presented evidence in the record from which he can show a substantial likelihood of success on the merits of his method-of-execution challenge,” attorneys for the state wrote in their response to the U.S. Supreme Court.