Oklahoma officials revealed in court papers on Monday that the state has not been able to find two of the drugs it needs to carry out executions in the coming days.
"The state has pursued every feasible option to obtain the necessary execution drugs. This has been nothing short of a Herculean effort," state attorneys wrote.
"Sadly, this effort has (so far) been unsuccessful."
The state then raised the possibility of implementing a new protocol, with just three days to go before the first of the two executions.
"I would have to believe the Oklahoma courts would not allow that to happen," said Madeline Cohen, assistant federal public defender.
Oklahoma's disclosure is the latest sign of a nationwide shortage of the chemicals used in lethal injections, which has sparked legal action and caused execution delays in a number of states.
It came in a brief in a case filed by two death-row inmates, Clayton Lockett and Charles Warner, who are slated to be killed March 20 and March 27, respectively.
They have been fighting to force the state to provide information about the source of the drugs its intends to use to put them to death.
States have been reluctant to identify the specialty pharmacies that compound the lethal doses for them. At least two that have been publicly named then stopped providing them.
In its brief, Oklahoma said authorities believed they had secured a supplier for the drugs, pentobarbital and vecuronium bromide, but found out Friday that the source fell through.
The state said it is considering a last-minute change to its protocol, but conceded that would most likely trigger another round of legal objections.
Cohen noted that the state made no mention of what "open market" drugs it might use or whether they have ever been used in any executions.
When Ohio ran out of pentobarbital last used, it turned to a two-agent cocktail made of the sedative midazolam and the painkiller hydromorphone — both readily available drugs.
It was used in January to execute Dennis McGuire, who took 25 minutes to die and was described by witnesses as gasping for breath.
Ohio has since put one planned execution on hold while it reviews McGuire's death. Louisiana also put off an execution with the same cocktail while the courts evaluate whether it violates the constitutional protection against cruel and unusual punishment.
Warner and Lockett's lawyers had raised questions about the use of pentobarbital and vecuronium bromide after a man executed in Oklahoma in January, Michael Wilson, blurted out, "I feel my whole body burning" after the injection.
They have demanded the state reveal where it gets the drugs from so that it can investigate the source, but the appeal has been bouncing around the courts for several weeks.
Warner is on death row for the murder and rape of his girlfriend's 11-month-old daughter in 1997. Lockett was sentenced to die for the 1999 murder of 19-year-old Stephanie Nieman, who was kidnapped, shot twice and buried alive.
Oklahoma law allows for electrocution or firing squad, but only if lethal injection is found unconstitutional.
The state Corrections Department said it could not comment on what alternatives it was pursuing.
"At this point, it's premature to discuss the next steps in the process," said Aaron Cooper, a spokesman for the Oklahoma attorney general.
"The Attorney General's Office is exhausting all available options to ensure the punishment for this heinous crime is carried out, so that after nearly 15 years the family of Stephanie Nieman finally sees justice served."