HUNTSVILLE, Tex. — A little more than two hours before he was scheduled to be put to death here, a convicted murderer was granted a stay of execution by a federal appeals court on Tuesday so the courts could review his claim that he is mentally disabled — a disability, his lawyers argued, that state agencies had long known and concealed.

The United States Court of Appeals for the Fifth Circuit in New Orleans granted the request from lawyers for Robert James Campbell, 41, who had been set to be the first inmate put to death in America since a botched execution in Oklahoma last month drew attention to the methods, drugs and secrecy surrounding lethal injections.

The court had refused to intervene in the execution based on Mr. Campbell’s lawyers’ contention that the state was withholding crucial evidence on the drug to be used. But it issued the stay on a second argument made by Mr. Campbell’s lawyers on what the law refers to as mental retardation.

“We have been presented with evidence that Campbell, who will soon be executed unless we intervene, may not constitutionally be executed,” wrote Judge James L. Dennis for the unanimous three-judge panel.