California has postponed indefinitely the execution of a convicted killer over ethical issues surrounding the method of death by lethal injection.
Doctors said their involvement in the execution was unethical
Prison authorities failed twice within 24 hours to find qualified medics to administer the drugs to Michael Morales in a jail near San Francisco.
Morales was convicted of the rape and murder of a 17-year-old girl in 1981.
He is now back on Death Row while the state reviews its policy on how to carry out lethal injections.
Lethal injection is the most widely used method in the US, although some offer inmates the choice of alternatives like electrocution or the gas chamber.
Thirty-eight of the 50 US states and the federal government permit capital punishment.
Morales had been due to die at 0001 on Tuesday (0801 GMT) in San Quentin prison.
His lawyers had successfully called for anaesthesiologists to be present, arguing that he would suffer extreme pain because of the mix of drugs in the three-part injection.
But the two court-appointed doctors withdrew, causing it to be delayed.
US METHODS OF EXECUTION
Lethal injection: Authorised in 37 states
Electrocution: In 10 states (sole method in Nebraska)
Gas chamber: In five states (all of which have lethal injection as alternative)
Hanging: Only in New Hampshire and Washington
Firing squad: In Idaho and Oklahoma
They had objected to a court ruling which ordered them to intervene in the event that Morales woke up or appeared to be in pain, calling it "ethically unacceptable".
Prison officials then rescheduled the execution for 1930 on Tuesday (0330 GMT on Wednesday).
They had planned to use a fatal dose of barbiturate that did not require the presence of the two experts, instead of the three-drug cocktail.
A judge approved the plan, but said the drug had to be administered by a medical professional licensed to inject medications intravenously.
But a spokeswoman for the federal courts said the state had told them it was unable to comply with the order, and the execution had been indefinitely postponed.
A spokesman for San Quentin State Prison, Vernell Crittendon, said Morales was relieved at the postponement.
"He smiled," Mr Crittendon said. "He nodded. He thanked me."
But the mother of Terri Witchell, Morales' victim, said she was "totally disillusioned" at the turn of events.
"We've been waiting 25 years with the expectancy that he is gonna pay for his crimes," said Barbara Christian.
"It feels like we just got punched in the stomach."
The lethal injection process will be reviewed in the coming months to examine concerns it might cause such levels of pain that it would be unconstitutional.
"We are really relieved to see we will now have a careful review of the issues," said Natasha Minsker, a lawyer for the American Civil Liberties Union.
Last month, the Supreme Court halted the execution of a man in Florida to consider whether chemicals used in lethal injections violated a constitutional ban on cruel and unusual punishment.