Convicted killer Christa Pike suffered the worst botched execution after she survived two doses of lethal injection last night, a death penalty expert says.

Pike, 50, who was sentenced to death for the 1995 murder of her classmate Colleen Slemmer, was administered two syringes of pentobarbital at the Riverbend Maximum Security Institution in Nashville, Tennessee late Wednesday.

But she was still alive and loudly snoring after the attempted lethal injection and was rushed to a hospital by ambulance, her attorneys said. 

She remains hospitalized, but her legal team claims they have not been informed about her condition and no updates have been given. 

Pike's case marks the second botched execution in Tennessee this year, with Robin Maher, the executive director of the Death Penalty Information Center, claiming it is the worst in modern history.

'This event is the worst we've seen and unlike any other botched execution in the modern era,' Maher told the New York Times. 'There is no precedent.' 

Seven other people have survived medical problems due to an execution team's failure to access a vein to administer lethal injection drugs, but no one has stayed alive after receiving the drugs used in such executions, she said. 

Maher added that what Pike experienced is 'singular and unparalleled.' 

Officials raised curtains to the execution chamber at 7.27pm, showing Pike strapped to a gurney.

'I'm going to leave this world the way I spent most of my life and that is in love,' the killer said in her 'last' words. She described herself as at peace.

But Pike remained awake and at one point raised her head and asked prison officials if her arm was supposed to feel that way. It was not clear what she was referencing.

By 8.26pm, execution witnesses reported that the second dose of pentobarbital had been administered.

Pike continued to be heard snoring behind the closed curtain until the microphone was cut off at 8.53pm. Witnesses were then escorted out of the area.

Lawyers for Pike filed an emergency motion late Wednesday to the Tennessee Supreme Court seeking to immediately halt and stay the execution, saying she was in 'unnecessary agony' and it was violating her right to be executed free of cruel and unusual punishment. 

They also filed motions in the Sixth Circuit court and a district court.

The Tennessee Department of Correction has said in a statement that it '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.

This 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. 

Pike was initially scheduled to be put to death Wednesday morning, and witnesses had gathered at the Riverbend Maximum Security Institution in Nashville when the 6th Circuit US Court of Appeals voted 2-1 to issue a stay. 

That court said a delay was needed to consider whether Pike's allegations of childhood sexual abuse were fully considered at her sentencing.

The attorney general's office appealed to the Supreme Court, saying that halting the execution at the last moment traumatized Slemmer's family and rewarded 'abusive delay tactics' by Pike's attorneys. 

Later in the day, the high court cleared the way for the execution to proceed. The conservative majority did not explain their order, while the three liberal members signed a dissent that said Pike's claims deserved a closer look.

Pike did not deny killing Slemmer, but her supporters argued that the state should consider her age at the time, her mental illness and her allegations of severe sexual abuse.

After the murder, Pike was diagnosed with bipolar disorder and post-traumatic stress disorder. In her clemency petition, she said she only wanted to fight Slemmer but killed her in a frenzied attack when she couldn't 'put the brakes on.'

'I was a mentally ill 18-year-old kid. It took me numerous years to even realize the gravity of what I'd done,' Pike said in a statement.

The killing rocked the city of Knoxville. Prosecutors said Pike, fearing Slemmer was trying to steal her boyfriend, lured her to a wooded area on January 12, 1995. 

Pike cut Slemmer with a box cutter and beat her with a large chunk of asphalt. Tadaryl Shipp, Pike's boyfriend, admitted that he carved the pentagram into her body.

Shipp was 17 when the crime happened and received a sentence of life in prison with the possibility of parole. Pike was the only one sentenced to death.

Pike would have been the 30th person executed in the US this year after Florida carried out an execution on Tuesday.