“After deliberate consideration of Christa Gail Pike’s request for clemency, and after a thorough review of the case, I am upholding the sentence of the State of Tennessee and do not plan to intervene,” Governor Lee said in a statement.

Pike’s legal team had filed the clemency petition earlier this month, requesting that her death sentence be commuted to life in prison.

Moving ahead with Pike’s execution puts Tennessee “on singular footing”, her attorneys said, as she would be the first person the state “executes in the modern death penalty era who was a teenager at the time of the offence”.

Attorneys Stephen Ferrell, Kelly Gleason and Randy Spivey said the governor’s decision “leaves unanswered vital questions about Tennessee’s execution protocol”.

“With this decision, the State of Tennessee is ignoring what both science and our legal system have decided: an 18-year-old brain is not an adult brain, and 18 is too young to face execution,” they said in a statement on Monday.

The murder of Colleen Slemmer created a media frenzy in Knoxville, Tennessee in 1995, when the 19-year-old was found with a pentagram – a symbol of Satanic ritual – carved into her chest.

Pike had accused Colleen of insulting her and trying to steal her boyfriend. Two other residents at the camp later testified that Pike had bragged about the murder before and after, and showed them a piece of Colleen’s skull.

A year later, she was convicted of first-degree murder and sentenced to death, while then-boyfriend Tadaryl Shipp- who was 17 at the time – was given a life sentence with the possibility of parole.

At the time of her incarceration, the 21-year-old was the youngest person on death row, according to the Tennessee Department of Corrections.

Pike is scheduled to be executed on 30 September after years of failed appeals.

Colleen’s mother has insisted she wants Pike’s execution to move forward so that her daughter “finally can be resting”.