US death row inmate Christa Pike awake and speaking after failed execution, lawyers say

Her lawyers called her progress "medically unprecedented", and while her prognosis remains unclear, "at a minimum, we expect a long recovery".

Deborah Denno, a criminologist and professor at Fordham Law School, said she was "absolutely astonished" that Pike is conscious and speaking.

There were seven other cases where prisoners survived slated executions, but in those cases, executioners were not able to inject anything into the inmates' veins, Denno told the BBC.

Pike "is the only person for whom some kind of toxin has been injected," she said.

"They did everything they were supposed to do as part of the execution process, and it simply didn't work," Denno said. "This has never happened before."

Authorities have not yet said whether they will try again to execute Pike. They have also not offered an explanation for what went wrong.

Tennessee Governor Bill Lee suspended all executions for the rest of the year - and the head of prisons in the state said over the weekend that he plans to resign.

Pike was 18 when she and her then-boyfriend, Tadaryl Shipp, beat, tortured and killed 19-year-old Slemmer, whom they had met at a job training camp for troubled teens.

Pike's attorneys are scheduled to appear in court in Nashville on Wednesday. They also plan to hold a news conference outside the court.

Dr Joel Zivot, an anaesthesiologist advising Pike's defence team, said on Tuesday that the likelihood of recovering from lethal injection is low.

"Most people who get lethal injection die from it," he told the BBC. "In fact, pretty well everyone, except Christa. So, the fact that she survived it ... was unusual."

But there remains a lot of uncertainty and many unknowns, he said, including how much of the pentobarbital got into her system, what her blood levels were after the injection and what her recovery will look like.

"I would not, in any way, say at this point that all the big risks are behind her," he noted.

Zivot had previously said the two doses of pentobarbital she received last week would have been enough that, had it entered her bloodstream in normal circulation, "she would have surely died".

Instead, Zivot said a possible explanation is that instead of entering Pike's bloodstream, the pentobarbital was more likely to have spread across the tissue under her skin.

Zivot said last week he thought her ending up with a brain injury was "very possible", due to the delays in resuscitating her. He also believed it would take about a week to truly understand her condition and do a neurological exam. It's now been six days since the failed attempt to execute her.

Pike's lawyers 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.

They said on Tuesday that she has endured "severe damage" to her arms.