A death penalty expert said it was an unprecedented failure, and Tennessee Governor Bill Lee halted the remaining execution for the rest of 2026.
Christa Gail Pike, 50, was alive and loudly snoring after the attempted lethal injection and taken by ambulance from the prison, her lawyers said.
She was being treated at a hospital, but they said they had not been told about her condition.
She had been scheduled for execution at 10am on Wednesday for the killing she committed at age 18 and would have been the first woman executed in Tennessee in more than 200 years.
An appeals court stopped the lethal injection just an hour before it was to start and hours later the US Supreme Court overturned that stay.
Officials gave Pike two doses of pentobarbital, her lawyers said in court filings late on Wednesday.
Governor Bill Lee said in a statement that he had ordered "a comprehensive, third-party review to determine exactly what occurred" and that the remaining scheduled execution would not take place in 2026.
"Carrying out a lawfully imposed sentence is among the state's most serious responsibilities, and the people of Tennessee expect it to be done in a manner that is not only legal and constitutional, but is effective," he said.
The Tennessee Department of Correction said it had "followed every step of the state's lawful, established execution protocol approved by the Attorney-General's Office".
The state's execution protocol calls for a secondary set of drug syringes to be administered "if the inmate is not deceased" after the first set.
It does not spell out what happens if the subject is alive after the second set.
Robin M Maher, executive director of the Death Penalty Information Center, said what Pike experienced was "singular and unparalleled".
Seven other people had survived medical problems due to an execution team's failure to access a vein to administer lethal injection drugs, but no one had stayed alive after receiving the drugs used in such executions, she said.
It was the second time this year that Tennessee has been unable to carry out an execution: in May state officials called off the lethal injection of Tony Carruthers, who was convicted of kidnapping and killing three people in 1994, after executioners tried and failed for over an hour to place an IV to administer pentobarbital.
Pike and her boyfriend were convicted of the fatal stabbing and beating in 1995 of 19-year-old Colleen Slemmer, their classmate at a job training facility in Knoxville.
The case garnered widespread attention in part because of a pentagram that was carved on Slemmer's body and other elements of the crime stoked fears of Satan worship during the "satanic panic" of the 1980s and 1990s.
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