By Carey Gillam and Kevin Murphy
KANSAS CITY, Missouri (Reuters) - The U.S. Supreme Court late Tuesday ordered Missouri to stay the execution of a murderer who had raised a series of objections to his death sentence, including that the drugs used could subject him to cruel and unusual punishment.
Herbert Smulls, 56, was scheduled to be executed with a lethal dose of pentobarbital, a fast-acting barbiturate, shortly after midnight local time (0600 GMT) on Wednesday at a state prison in Missouri.
But the Supreme Court ordered the sentence stayed shortly after 9 p.m. CST (0300 GMT), and offered no explanation as to the court's decision.
Justice Samuel Alito signed the order, which is temporary pending further review by the court.
Smulls was convicted of shooting jewelry store owner Stephen Honickman to death while robbing his store in July 1991. Honickman's wife, Florence Honickman, was also shot during the attack and sustained permanent injuries.
Lawyers for Smulls had argued that the compounded pentobarbital the state planned to use to execute Smulls may not be pure and as potent as it should be and could cause undue suffering.
Missouri and several other states have turned to compounding pharmacies, which are not regulated by the U.S. Food and Drug Administration, to acquire drugs for executions after an increasing number of pharmaceutical manufacturers have objected to their drugs being used in capital punishment.
The U.S. Eighth Circuit Court of Appeals on Friday ruled for Missouri on the use of pentobarbital, finding that Smulls' lawyers did not propose a "feasible or more humane alternative" and have not shown that Missouri sought to cause him unnecessary pain by using the drug.
And on Monday, U.S. District Court Judge Beth Phillips denied Smulls a 60-day stay of execution, citing earlier U.S. appeals court rulings that pentobarbital does not inflict cruel and unusual punishment in violation of the constitution.
Also on Monday, Smulls filed a motion claiming errors were made during his trial, and said he had new evidence that should be considered.
Smulls would be Missouri's third execution since November and the sixth person put to death in the United States in 2014.
(Reporting by Kevin Murphy and Carey Gillam; Editing by David Bailey, Stephen Powell and Lisa Shumaker)