St Louis - Lawyers for a convicted US murderer were making final pleas for his life on Tuesday, just hours before his scheduled execution, saying the use of a lethal drug of undisclosed origin "terrifies" him.
Herbert Smulls was convicted and sentenced to death for killing a Missouri jeweller and badly injuring his wife during a 1991 robbery. He was scheduled for lethal injection at (0501 GMT) on Wednesday.
His defence attorney, Cheryl Pilate, had an appeal pending before the US Supreme Court and was seeking clemency from Governor Jay Nixon, who said Tuesday that he was still weighing the request.
Pilate's arguments are mostly focused on the secrecy around the execution drug.
State prison officials have refused to reveal from which pharmacy they obtained the drug, pentobarbital. Pilate contends that the secrecy makes it impossible to know whether the drug could cause pain and suffering during the execution process.
St Louis County prosecutor Bob McCulloch said talk about the drug is a distraction aimed at sparing the life of a cold-blooded killer.
Missouri had used a three-drug execution process until the drug makers stopped selling those drugs for executions. The state switched late last year to pentobarbital. The state argues that the compounding pharmacy providing the drug is part of the execution team, and therefore its name cannot be released to the public.
Compounding pharmacies custom-mix drugs for clients and are not subject to oversight by the US Food and Drug Administration, though they are regulated by states.
The drug was used to execute two Missouri inmates late last year.
Also on Tuesday, Missouri Senate Democratic Leader Jolie Justus introduced legislation that would create a commission responsible for setting the state's execution procedure. She said ongoing lawsuits and secrecy about the current lethal injection method should drive a change in protocol.