ST. LOUIS (AP) — A federal appeals court on Tuesday refused to halt the execution of a Missouri inmate, hours before he was scheduled to be put to death.

A three-judge panel of the 8th U.S. Circuit Court of Appeals denied a stay of execution for Earl Ringo Jr., who faces lethal injection at 12:01 a.m. Wednesday for killing two people while robbing a Columbia restaurant in 1998.

It would be the eighth execution in Missouri this year and the 10th since November.

The appeal questioned Missouri's use of the sedative midazolam before executions, claiming it could dull the inmate's senses, leaving him potentially unable to express any pain. Attorney Richard Sindel said he is appealing to the U.S. Supreme Court.

St. Louis Public Radio reported last week that Missouri used midazolam before each of the last nine executions. Corrections spokesman David Owen said the drug can be administered at the request of the inmate or at the direction of officials with the corrections department. It wasn't clear what circumstances would prompt an inmate to get the sedative if he didn't want it.

"The quantity being administered to these guys, that is a very significant amount of the drug and could have a major effect on their ability to think and recall and formulate any kind of thought," Sindel said.

Owen said midazolam "is used to relieve the offender's level of anxiety" and is not part of the actual execution process.

Midazolam has come under scrutiny after it was used in problematic executions earlier this year in Ohio, Oklahoma and Arizona. In each case, witnesses said the inmates gasped after their executions began and continued to labor for air before being pronounced dead

Meanwhile, a clemency petition to Gov. Jay Nixon cites concerns that Ringo, who is black, was convicted and sentenced by an all-white jury.

The execution is one of two scheduled for Wednesday in the U.S.; Texas was scheduled to execute Willie Trottie for killing his common-law wife and her brother in 1993.

Missouri and Texas use pentobarbital for executions but have declined to disclose where the drug is obtained.

On July 3, 1998, Ringo shared with Quentin Jones his plan to rob the Ruby Tuesday restaurant in Columbia, where he once worked. Jones agreed to join him.

Before sunrise on July 4, Ringo and Jones hid behind a grease pit in the back of the restaurant. Delivery driver Dennis Poyser arrived and was met by manager trainee JoAnna Baysinger. They entered the restaurant. Ringo followed them and shot Poyser, 45, killing him instantly.

He ordered Baysinger to open a safe. She pulled out $1,400 and gave it to him.

Ringo gave the gun to Jones, who stood with the weapon pointed at Baysinger's head for a minute and a half before pulling the trigger.

Ringo admitted to the robbery but claimed the shootings were in self-defense. He was convicted in 1999 and sentenced to death.

Jones pleaded guilty to first-degree murder and was sentenced to life in prison, but he was spared the death penalty when he agreed to testify against Ringo.