Tennessee prison chief resigns following botched execution of Christa Pike

The head of prisons in the US state of Tennessee is resigning after the failed execution of death row inmate Christa Pike on Wednesday.

Governor Bill Lee announced the change on Saturday, sharing the news in a joint statement with Tennessee Department of Correction Commissioner Frank Strada.

Strada said that his stepping down is "in the best interests" of the state, and called the ongoing investigation into the incident "entirely appropriate and necessary".

Pike, who was sentenced to death in 1996 for the murder of Colleen Slemmer, survived two injections and is now in critical condition at a Tennessee hospital as her lawyers seek to commute her sentence.

"Commissioner Strada has served with integrity, and I appreciate his willingness to put the interests of Tennesseans first during this difficult moment," Lee said.

Lee has ordered an independent review into Pike's attempted execution and he halted all executions in the state for the year.

Pike is being treated at a hospital in Nashville, Tennessee where doctor are attempting to clear the drugs used during the execution from her body, according to legal filings.

She was injected with two lethal doses of pentobarbital on Wednesday but remained alive. She was then sent to hospital in an ambulance. While receiving the two doses, "she could be heard crying, whimpering, and breathing loudly throughout the procedure", her lawyers said.

As of Thursday night, she was unconscious, intubated and on a ventilator.

The state's Department of Correction has defended the method it used, saying "the protocol does not allow for additional procedures beyond what was carried out".

Authorities have not yet said whether they will try again to execute Pike. They have also not offered an explanation for what went wrong. Her lawyers filed an emergency motion on Friday to preserve "any and all evidence" related to her failed execution.

Pike's attorneys previously warned that her medical history and anatomy - including thrombocytosis (a blood-clotting condition) and small veins - meant there was a risk she would experience "unnecessary" pain and suffering during a lethal injection.

The attempted execution went ahead after multiple failed, last-minute legal challenges by Pike's lawyers.

The US Supreme Court rejected two separate efforts to stop the lethal injection.

Tennessee's governor also denied a clemency request from Pike, the only woman on the state's death row.

Pike was 18 when she and her then-boyfriend, Tadaryl Shipp, beat, tortured and killed 19-year-old Slemmer in 1995.

She was sentenced to death the following year after a media frenzy around the killing in Knoxville, Tennessee.