UPDATE: Late Tuesday night, the US Supreme Court voted 6-3 to vacate the 11th Circuit Court of Appeal’s stay of Mr. Melson’s execution. Justices Sonia Sotomayor, Stephen Breyer, and Ruth Bader Ginsburg would have left the stay in place. The execution will now go forward as scheduled at 6 p.m. Thursday, June 8, barring further court action, which is unlikely.

Until last week, Alabama had already scheduled its second execution in less than a month, but a stay by the 11th Circuit Court of Appeals has delayed that move and instead brought to the surface many old — and some new — questions about the state’s method of lethal injection.

The execution of Robert Melson, 46, had been scheduled for Thursday, June 8, but the 11th Circuit Court of Appeals — second only to the U.S. Supreme Court — put the pause button on the process until it rules on the merits of Melson’s claims.

Melson’s legal team has challenged the execution on several grounds, primarily arguing that the state’s use of midazolam, a sedative, is not enough to render Melson unconscious for the remaining two injections that stop respiration and circulation. Melson’s attorneys say previous state executions, such as that of Ronald Bert Smith in December, prove the point.

“Mr. Smith’s execution, during which Mr. Smith was not anesthetized, but responded to two consciousness tests and coughed and wheezed for 13 minutes during the execution, illustrates the unreliability of midazolam as the first drug in a three-drug execution protocol,” Melson’s defense team wrote in a brief.

Indeed, according to media witnesses, Smith’s execution was unusual, with the inmate moving, even clenching one hand into a fist, after the sedative had been administered and a consciousness test conducted. However, Jeff Dunn, commissioner of the Alabama Department of Corrections, contradicted that observation, saying “from where I was seated I didn’t see any reaction to the consciousness assessment.”

That disparity between media witnesses and state officials is present in other cases involving midazolam as well. In 2014, the state of Arizona used the drug as part of an execution that took over two hours. One media witness said the inmate gasped for air 640 times before dying. The execution was “very disturbing to watch,” the Fox reporter said, “like a fish on shore gulping for air. At a certain point, you wondered whether he was ever going to die.”

A state official present for that execution, though, had an entirely different interpretation of the same event. “He went to sleep and appeared to be snoring,” the representative of the state’s attorney general said. “This was my first execution, and I was surprised at how peaceful it was.”

According to the Food and Drug Administration, midazolam is a sedative used in minor procedures such as colonoscopies, and experts say the drug may not be effective in high-stress situations.

The state of Alabama hasn’t always used midazolam. Alabama used sodium thiopental until 2011, when its manufacturer stopped production in the U.S. over its use in lethal injections. After that, the state switched to another barbiturate, pentobarbital, but eventually ran out.

In September 2014, the state for the first time acknowledged it would use the newest sedative — midazolam — as the first drug in its three-drug protocol. Given its shelf life, if it has not been able to acquire more of the drug, Alabama may run out of midazolam this September.

The Alabama Attorney General’s office has filed a brief with the U.S. Supreme Court asking it to lift the 11th Circuit’s stay and allow the execution to move forward.

“No one disputes that Alabama will execute Melson using the same lethal injection protocol which this court approved [in 2015],” the Attorney General’s brief says. “The 11th Circuit’s stay was improper and fundamentally misapplied this Court’s long-standing stay principles by completely failing to address, much less find, a ‘likelihood of success on the merits’ of Melson’s method-of-execution challenge.”

In that 2015 case, the Supreme Court approved the use of midazolam in Oklahoma’s three-drug lethal injection protocol, but multiple justices dissented, saying they would have halted the execution for further investigation into midazolam’s effectiveness.

Alabama currently has 183 inmates, including Melson, on death row.