Justices opt not to review intellectual disability claim of Robert Campbell, who was sentenced to death for 1991 rape and murder

The US supreme court on Monday declined to review the intellectual-disability claim of a Texas inmate who is also seeking to force the state to disclose information about the drugs it plans to use to kill him.

Robert Campbell was to become the first prisoner put to death in the US since the botched execution of Clayton Lockett in Oklahoma in April, but a federal appeals court issued a stay only around three hours before he was due to be injected with a lethal dose of the sedative pentobarbital.

Campbell was sentenced to die for the 1991 rape and murder of Alexandra Rendon, a 20-year-old Houston bank teller.

The federal court decided that Campbell’s claim that he is intellectually disabled – and so not eligible to be executed –deserves more examination after earlier evidence about his IQ test scores appeared to have been incorrect, incomplete or withheld by officials. His lawyers also argued his trial counsel had been inadequate.

On Monday, the US supreme court opted not to review this aspect of the case, a decision that Rob Owen, Campbell’s lawyer, said was “entirely expected and unsurprising” due to ongoing litigation elsewhere. An earlier wire report inaccurately said the ruling was related to Texas’s refusal to disclose information about its execution drugs.

However, that appeal is still pending, and a decision could be announced within a week – though the supreme court is yet to halt an execution based on a state’s secrecy over its drug supply.

Last month, in a ruling that was a blow to campaigners seeking transparency in the lethal injection process, Texas attorney general Greg Abbott decided that prison officials do not have to publicly reveal details about the state’s supplier of execution drugs.

The ruling issued by Abbott’s office means that the Texas department of criminal justice can continue to withhold information about where it obtained its latest stock of pentobarbital. The decision – which is also being appealed – reversed previous instructions from Abbott in 2012 that state officials could not redact fundamental details about their supply and suppliers from documents requested by the public under Texas’ open-records laws.

Earlier this year the TDCJ began doing precisely that, arguing that facts and circumstances have changed and releasing the information –even privately to lawyers for death row inmates – would put its suppliers at risk of harassment or threats from anti-death penalty groups and jeopardise its ability to source more drugs in the future.

In common with other states, Texas has changed its protocol and resorted to increasingly furtive measures since a Europe-led boycott cut off supplies of its previous drugs of choice. It currently uses a single-drug protocol of pentobarbital bought from a compounding pharmacy in the state.

Attorneys for death row inmates have argued that states’ lack of transparency about drug origins and quality means prisoners are unable to verify that they will not endure unduly painful deaths that would violate the US constitution’s prohibition of “cruel and unusual punishment”.

Texas found a new supplier in March this year but declined to identify it to lawyers for a death row inmate, prompting litigation. After the issue bounced around several courts, a federal appeals court ultimately upheld the state’s secretive approach in April, ruling that lawyers for Tommy Lynn Sells were merely speculating that Texas’s drugs might be unsuitable.

• Editors' note: this story has replaced an Associated Press report that erroneously said the supreme court ruling was related to Texas’s refusal to disclose information about its execution drugs.