UPDATED AT 10am Oct 1:

A US woman who was spared from execution with barely an hour to go has had her death sentence dramatically reinstated just hours later.

Christa Gail Pike, 50, had been scheduled to die by lethal injection at Nashville’s Riverbend Maximum Security Institution on Wednesday morning, local time, for the brutal 1995 murder of 19-year-old Colleen Slemmer.

But with witnesses already gathered at the prison and preparations for the execution under way, the US Court of Appeals for the Sixth Circuit intervened, voting 2-1 to issue a temporary stay shortly before Pike was due to be put to death.

The reprieve, however, proved remarkably short-lived.

Tennessee authorities immediately appealed to the US Supreme Court, which later overturned the stay and cleared the way for Pike’s execution to proceed.

The Tennessee Department of Correction subsequently confirmed the execution would go ahead on Wednesday night.

The extraordinary series of events came after Pike had seemingly exhausted her other avenues for avoiding execution.

Tennessee Governor Bill Lee rejected her request for clemency earlier in the week, saying that after “deliberate consideration” and a thorough review of the case, he would not intervene.

The US Supreme Court had also rejected an earlier request from Pike’s lawyers to halt the execution.

Pike was just 18 when Slemmer, a fellow student at a Job Corps training centre in Knoxville, was murdered in January 1995.

Prosecutors said Pike, her then-boyfriend Tadaryl Shipp and another teenager lured Slemmer to an isolated wooded area after Pike became convinced the 19-year-old was trying to steal her boyfriend.

Slemmer was tortured, beaten and killed in an attack that shocked the US due to its brutality.

Pike was convicted of first-degree murder and sentenced to death in 1996. Shipp, who was 17 at the time of the killing, was later sentenced to life in prison with the possibility of parole.

Pike has never denied her involvement in Slemmer’s murder and has expressed remorse for the crime.

“I was a mentally ill 18-year-old kid,” she said in a statement included in her clemency petition.

“It took me numerous years to even realize the gravity of what I’d done. Even more to accept how many lives I affected. I took the life of someone’s child, sister, friend.”

Her lawyers have argued that Pike’s age, mental health and history of severe childhood trauma and sexual abuse were not adequately considered when she was sentenced to death.

It was that issue which prompted the Sixth Circuit Court of Appeals to step in at the eleventh hour.

The appeals court said more time was required to properly consider whether evidence of Pike’s childhood sexual abuse had been adequately addressed during the sentencing phase of her case.

But Tennessee challenged the ruling almost immediately, arguing the latest legal action represented another attempt to delay a sentence imposed three decades ago.

The US Supreme Court ultimately sided with the state, lifting the stay in an order signed by Justice Brett Kavanaugh.

Justices Sonia Sotomayor, Elena Kagan and Ketanji Brown Jackson dissented.

Sotomayor argued that lifting the stay would allow Tennessee to execute Pike before her remaining legal challenges could be properly considered.

Slemmer’s family, meanwhile, has long supported Pike’s execution.

Her mother, May Martinez, travelled to Tennessee to witness it and has spoken of the pain that has remained with her family for more than three decades.

“Not a day goes by, or a minute goes by that I do not think about Colleen,” she said ahead of the scheduled execution. “Holidays are the worst.”

If the execution proceeds, Pike will become the first woman put to death by Tennessee in around 200 years.

In the space of just a few extraordinary hours, she went from preparing to die, to being granted a last-minute reprieve – and then being told once again that her execution would go ahead.