Florida Governor Jeb Bush has halted executions in the US state after a flawed death by lethal injection.
A coroner found chemical burns on Diaz's arms
Mr Bush said he needed to be sure that the method of death did not constitute "cruel and unusual punishment".
The move came after a man convicted of murder, Angel Diaz, took 34 minutes to die - twice as long as normal - and had to be given a second lethal dose.
In California, a judge has ruled death by lethal injection violates a state ban on cruel and unusual punishment.
The judge ruled California's "implementation of lethal injection is broken", but said "it can be fixed".
Lethal injection is the preferred method of execution in 37 US states.
Mr Bush has appointed a panel to look into the practice and evaluate whether it breaches the state's ban as an unnecessarily cruel mode of death, as argued by opponents of the death penalty.
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
He has said no more death warrants will be signed until the commission completes its final report in March.
The move came after the drawn-out death by lethal injection of Puerto Rican-born Diaz on Wednesday.
Diaz was sentenced to death for the 1979 murder of a Miami strip club manager.
Witnesses said his death took more than twice the usual time - 34 minutes rather than the usual 15.
He needed a second dose of the lethal chemicals as the needles were injected straight through his veins and into the flesh of his arms.
Following the autopsy, the medical examiner concluded the injections had been wrongly administered.
He was found to have large chemical burns on both arms and his lawyer reported that the 55-year-old continued to move and mouth words more than 20 minutes after the initial dose.
Inmates are supposed to be rendered unconscious by the chemicals within three to five minutes.
Anti-death penalty activists say lethal injections - introduced in Florida and other states as a replacement for the electric chair and other methods of execution - are just as cruel and should not be considered a more humane substitute.
But the BBC's Emilio San Pedro, in Miami, says that it is too soon to interpret the decisions as a sign that support for the death penalty is waning in the US.
Executions, he says, continue to be seen as the best deterrent of violent crime.