The Supreme Court of the southern US state of Tennessee has temporarily suspended executions pending its ruling on the constitutionality of carrying out the death sentence by a different lethal injection and the electric chair.
In a ruling on Friday, the judges suspended three executions scheduled for later this year and another for March 2016.
The court recently suspended half a dozen executions to hear individual appeals, but the ruling, announced on Monday, paralyses all protocol for executions pending revision, Efe news agency reported.
Tennessee last executed a prisoner in 2009, but since then has had problems sourcing the ingredients for its lethal injections.
Responding to these problems, in 2014 the state reinstated the electric chair as an approved alternative method of execution.
This approval came ahead of other states including Utah and Oklahoma, which legalised the firing squad and nitrogen suffocation respectively as execution methods.
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Tennessee, where 69 prisoners remain on death row, last year scheduled 11 executions between mid-2014 and 2016, only to have the schedule suspended in the courts.
Two of these prisoners have since died natural deaths, authorities said.
Tennessee has joined other states in suspending executions pending a Supreme Court ruling on the constitutionality of an alternative drug for lethal injections, which failed to work as planned on three occasions in 2014, causing suffering to prisoners.
Oklahoma, Florida and Alabama have suspended death sentences pending the Supreme Court's decision on the use of midazolam, the sedative that caused the failures in 2014.
Ohio has suspended executions while preparing a new protocol for injections, while Georgia has halted execution by lethal injection after warnings of further problems with the drugs used.
Tennessee judges must now rule on whether, as the death row inmates allege, the alternative lethal injection and the electric chair violate the US Constitution's eighth amendment which protects citizens from cruel or unusual punishment.
Since the US Supreme Court reinstated the death penalty in 1976, Tennessee has executed only six prisoners, making it one of the states least inclined to this punishment.