ST. LOUIS — Dating to the days when the guillotine operator or the hangman wore a mask, a certain amount of anonymity has always surrounded executions. But that secrecy is increasingly coming under fire, with judges, death penalty opponents and lawyers questioning why so little can be known about a state’s solemn responsibility.

An Associated Press survey of the 32 death penalty states found that most refuse to disclose the source of their execution drugs. The states cloaked in secrecy include some with the most active death chambers – among them Texas, Florida, Oklahoma and Missouri.

Most states have recently begun relying on loosely regulated “compounding pharmacies” for execution drugs but refuse to name them, citing concerns about backlash that could endanger the supplier’s safety. But many states refuse to provide even more basic information – how much of the drug is on hand, the expiration date, how it is tested.

Those who question the secrecy wonder how an inmate’s constitutional right against cruel and unusual punishment can be guaranteed if nothing is known about the drug being used to kill him.

“As far as we know, it could be coming from a veterinary source, it could be coming from some dark corner of the Internet,” said Cheryl Pilate, a Kansas City, Mo., attorney who handles death row appeals.

The most prolific death penalty states have successfully deflected most challenges to secretive protocols. But momentum is building toward unlocking details.

Following a Missouri execution in December, a federal appeals judge wrote in a dissenting opinion that the state was using “shadow pharmacies hidden behind the hangman’s hood.” The state has executed three other men since then.

Last week, an Oklahoma judge voided the state’s execution law, agreeing with two inmates who claimed a “veil of secrecy” that prevents them from obtaining information about lethal injection drugs violated their constitutional rights.

And on Wednesday, a federal judge in Texas halted the execution of a serial killer, ordering the state to disclose the supplier of a new batch of drugs and information on how they are tested. A federal appeals court threw out that ruling hours later, and Tommy Lynn Sells was put to death Thursday.

Kent Scheidegger of the California-based Criminal Justice Legal Foundation, which supports capital punishment, said forcing states to reveal their drug source can lead to obstruction of justice.

“People who have already waited 20 years for justice, to be told, after the case has been thoroughly reviewed, ‘You still can’t have justice because of a restriction on the lethal injection chemicals,’ that’s preposterous,” Scheidegger said.

The concern, critics say, is that a poorly made drug could cause suffering. Convicted killers may not generate much sympathy, but lawyers note the Constitution applies to them, too.