The head of the prison system in the US state of Tennessee is resigning after the execution of convicted murderer Christa Pike went wrong, the state governor said Saturday.

Witnesses said Pike was still breathing and moaning after two doses of the anesthetic pentobarbital failed to kill her during Wednesday's procedure.

The attempt was halted and she was transferred to hospital in critical condition.

It was the second time this year that an execution in the southern state had to be halted.

What do we know about the resignation?
Governor Bill Lee said in a statement that prison commissioner Frank Strada would leave his post sometime this month.

Lee said he thanked Strada for "his willingness to put the interests of Tennesseans first during this difficult moment."

In the same statement, Strada acknowledged that: "Stepping aside is in the best interest of Tennesseans as the independent review moves forward."

He said the department had carried out its responsibilities in accordance with established protocol, but called the review "entirely appropriate and necessary."

The state has halted further executions until the review has been completed.

Strada took on the role in January 2023. Nearly two years later, he announced that Tennessee would, from then on, only use pentobarbital for lethal injections of death-row inmates.

What went wrong?
Tennessee authorities have yet to explain why Pike survived.

Her attorneys said they believed problems with the IV lines caused the pentobarbital to flow into her body instead of her bloodstream.

Pike's lawyers said Friday that she is unconscious and on a ventilator in hospital.

While they welcomed Strada's departure, her legal team said, “It does nothing to help Christa now.”

"What happened to her Wednesday reflects a systemic failure that goes far beyond any one person," the four-lawyer team said.

"We hope the governor's call for a full, independent review will expose the profound problems within Tennessee’s entire death penalty system," it added.

Had the drugs killed Pike, she would have been the first woman executed by the state since 1820.

Pike was sentenced to death in 1996, aged 20, for the murder of Colleen Slemmer, whom Pike was convinced was trying to steal her boyfriend.

Slemmer, who was 19, was tortured and killed in a remote area near the University of Tennessee in Knoxville.