Texas executed a convicted killer using a new batch of drugs on Thursday night after the US supreme court rejected a last-minute appeal from his lawyers, who argued that the state's refusal to disclose details about the supply put him at risk of enduring a painful death that was tantamount to torture.

Tommy Lynn Sells stabbed 13-year-old Kaylene Harris to death at her south-west Texas home in 1999 and claimed to have been responsible for as many as 70 murders nationwide.

The 49-year-old declined to give a final statement at the Texas state penitentiary in Huntsville, near Houston. As he was injected with a lethal dose of the sedative pentobarbital he began to snore and stopped moving after less than a minute. He was pronounced dead 13 minutes later.

Texas prison officials obtained a supply of pentobarbital from a compounding pharmacy in suburban Houston last year. The drugs expired at the end of last month and the state found a fresh source from elsewhere.

Claiming that secrecy is increasingly necessary to protect pharmacies from potential threats and intimidation, officials refused to reveal the origin of the new consignment despite past directions from the state's attorney general's office that the prison system had not established a compelling reason why it should be allowed to deny similar open-records requests.

An Associated Press analysis found no evidence that Texas authorities were investigating threats to pharmacies, though the Oklahoma attorney general said he was examining an alleged bomb threat to a pharmacy in Tulsa.

Lawyers for Sells – the first inmate to die using the new pentobarbital – and Ramiro Hernandez-Llanas, a Mexican national who is scheduled to be lethally injected on 9 April, issued several lawsuits in a bid to compel Texas to disclose information about the drugs.

Last week a district judge in Austin told the state to reveal details to the pair's lawyers but stopped short of ordering the information to be made public. Texas officials appealed and the decision was stayed by the state's supreme court, which is not expected to hold a full hearing until mid-April at the earliest.

On Wednesday this week a federal judge in Houston issued a temporary injunction halting both executions until lawyers for the men had been provided with information about the procurement and quality of the drugs.

Within hours a federal appeals court reversed that ruling, meaning the execution of Sells could proceed. The ruling said the plaintiffs were "speculating" that the new pentobarbital may bring a risk of severe pain and "speculation is not enough".

Lawyers for Sells and Hernandez-Llanas argued that speculation was precisely the problem: that Texas's secretive behaviour caused uncertainty that meant the inmates were unable to make a fair assessment of the quality of the products to be used to kill them. They also accused Texas officials of stalling and thereby making it impossible to mount an adequate defence. Lawyers representing the state said in court documents that tests showed the drugs were of acceptable quality.

Maurie Levin, one of the attorneys for Sells and Hernandez-Llanas, told the Guardian this week that she found it hard to understand why Texas officials pursued appeals against rulings that restricted the release of drug details to the lawyers and their inmates, which seemingly appeased the state's desire not to make the information public.

Opponents of capital punishment claim that many compounding pharmacies are subject to relatively little federal scrutiny so the standard of their drugs could be variable, potentially making for an excessively painful execution that would violate the US constitution's prohibition of "cruel and unusual punishment".

Michael Lee Wilson was executed in Oklahoma in January using a mix of drugs including pentobarbital and his last words were reportedly "I feel my whole body burning."

The level of secrecy surrounding executions has become a controversial issue in several states. As supply shortages prompted by boycotts have forced states to turn to compounding pharmacies and experimental drug combinations, some have sought to reinforce or introduce privacy laws. Death penalty opponents and death row inmates have embarked on litigation in attempts to ensure transparency.

Levin and another attorney for the two inmates, Jonathan Ross, said in a statement after the US supreme court declined to review the case: "It is our belief that how we choose to execute prisoners reflects on us as a society. Without transparency about lethal injections, particularly the source and purity of drugs to be used, it is impossible to ensure that executions are humane and constitutional. It is our hope that the US supreme court and the Texas courts will ultimately agree that we must have transparency about the execution process in order to ensure that prisoners are able to protect their Eighth Amendment rights."

Sells also lost an appeal based on the contention that he had inadequate legal help during his trial. He was the fifth Texas inmate to be put to death in 2014. Another four executions in the nation's busiest death chamber are scheduled between next week and 21 May.