NASHVILLE, Tenn. (AP) — Tennessee inmate Nicholas Sutton was placed on a death watch early Tuesday ahead of his scheduled execution this week for the decades-old killing of a fellow inmate.

Inmates on death watch are kept under 24-hour surveillance in a cell beside the execution chamber, the Tennessee Department of Correction said. Sutton is scheduled to be put to death Thursday evening in the electric chair.

Sutton, 58, was sentenced to die in 1986 for killing Carl Estep in a conflict over a drug deal while both were incarcerated in an East Tennessee prison. Sutton already was serving time for three murders he committed when he was just 18, including that of his grandmother.

His attorneys have asked Tennessee Gov. Bill Lee to commute his death sentence, arguing that Sutton dramatically reformed himself in prison. Supporters speaking out for him in his clemency petition include seven current or former prison workers and the relatives of some of his victims, including Estep's daughter.

Lee has not yet said whether he will act on the clemency petition.

Sutton has chosen the electric chair, an option in Tennessee for prisoners whose crimes were committed before 1999. If the execution moves forward as planned, Sutton would be the fifth person to die in the state's electric chair in a 16-month span.