The US Supreme Court granted a last-minute stay to a convicted murderer who is challenging the drugs prison officials had planned to use on Wednesday for his execution.
Herbert Smulls was to be executed overnight by the state of Missouri, but Justice Samuel Alito granted him a reprieve late Tuesday, amid ongoing controversy over the use of lethal pentobarbital manufactured by a compounding pharmacy whose identity has not been revealed.
A central US appeals court also suspended the execution but has yet to issue its final decision.
Alito, the justice assigned to cases from the region of the United States that includes Missouri, is expected to rule soon on a similar petition from Louisiana inmate Christopher Sepulvado, due to be put to death next week.
Sepulvado is scheduled to die for fatally beating and scalding his six-year-old stepson in 1992.
Smulls is on death row for killing a jeweler during an armed robbery. The governor of Missouri overnight rejected his petition for a stay of execution.
Attorneys for both men maintain that the failure to reveal the identity of the pharmacy makes it impossible to know if the execution would constitute "cruel and inhumane punishment" under the Eighth Amendment of the US Constitution.
They suggested that the barbiturate used to put the inmates to death might have been tested and approved by the same lab that tested drugs used in a controversial recent execution in Oklahoma.
Michael Lee Wilson, who was executed on January 9, said he could feel his whole body "burning" as he was put to death.
An expert cited by lawyers for Smulls said the same Oklahoma lab is believed to have approved drugs produced in Massachusetts responsible for an outbreak of meningitis in November 2012.
After the Missouri governor refused late Tuesday to stay the execution, lawyer Cheryl Pilate asked the Supreme Court to demand that the state divulge the name of the pharmacy that provided the drug it intends to use for the lethal injection.
Missouri, along with some other US states, faces a shortage of drugs previously used in lethal injections after European drugmakers refused to supply substances to be used in executions.
And, like others, Missouri has turned to so-called compounding pharmacies not subject to federal drug safety regulations to procure the drugs.
Sepulvado's lawyers have also filed an appeal with the Supreme Court.