*Correction appended

Editor's note: This story has been updated to include extended comment from the Texas Department of Criminal Justice.

The drug used in Tuesday night’s botched execution in Oklahoma – midazolam – is stored by the Texas Department of Criminal Justice and can be used at any time in the state's death penalty protocol, raising concerns among defense lawyers and others about the state’s secretive lethal injection process.

Documents obtained by defense attorneys and shared with The Texas Tribune show that TDCJ obtained midazolam last June, has approximately 30 vials of the drug and the expiration date for them is 2015.

Oklahoma authorities quickly shielded reporters attending the Tuesday night execution of Oklahoma inmate Clayton Lockett — the first of two scheduled executions there that night; the second was stayed — after the inmate was given the drug midazolam, the first of a new three-drug execution cocktail used in that state. A writhing Lockett died of a heart attack 43 minutes after the execution began. It is uncertain whether the drug or the way it was administered caused the problems. Earlier this year, in Ohio, an execution using the same drug took more than 20 minutes.

“I think the whole method is problematic,” said Kenneth Williams, a criminal law professor who teaches a class on capital punishment at South Texas College of Law in Houston. “I think it was ironic that the inmates there were fighting the process because they thought the new drugs were problematic, and they turned out to be right."

Texas currently uses a single massive dose of pentobarbital in its execution protocol and has not used the midazolam yet. But, Texas is one of several death penalty states across the nation struggling to maintain an adequate supply of pentobarbital because European producers of the drug have stopped selling it in the United States for use in executions. Texas used a three-drug cocktail similar to the protocol in Oklahoma from 1982 until 2012, when the state changed its process because it could not obtain another type of drug that was used in the in execution protocol.

TDCJ officials had little to say Wednesday about midazolam or whether they plan to use it.

"We are not commenting on the specific type or quantity of drugs we have on hand. Our protocol calls for a single lethal dose of pentobarbital which we have used since 2012. We have no immediate plans to change the execution protocol," Robert Hurst, a TDCJ spokesman, said in an email.

A call to the man most knowledgeable about the process in Texas — William Stephens, director of TDCJ's correctional institutions division — was not immediately returned. Stephens, who was out of town at a conference, could not be immediately reached.

For the past few months, Texas and other states have successfully fought legal challenges from death row inmates to reveal anything about the type of drugs they use.

On April 3, Tommy Lynn Sells was executed in Huntsville after his appeals based on finding out what type of drugs were to be used on him, were denied. On April 9, Texas executed death row inmate Ramiro Hernandez, and on April 16, Texas executed Jose Villegas.

In all three executions, the drug pentobarbital was used. The state of Texas turned to compounding pharmacies to make the execution drug after a European manufacturing source for pentobarbital dried up. After the name of the Houston-area pharmacy that was producing the pentobarbital was released, it stopped providing the drug and TDCJ has turned to other sources, which officials refuse to name.

Oklahoma injected Lockett with midazolam, a sedative that is also used as an anti-seizure drug, followed by vecuronium bromide and potassium chloride. Lockett, along with Charles Warner, whose execution was stayed Tuesday night, had challenged their executions on the grounds they had a right to know the source of the drugs to be used. Initially, a lower Oklahoma court ordered a delay of both men's executions because of the secrecy about the execution drugs. Oklahoma's supreme court also issued a stay but then reversed itself. Oklahoma Gov. Mary Fallin also issued a stay of execution, but only for one week. Fallin, who on Wednesday called for an investigation into the execution Tuesday night, has supported the executions because neither man contested his guilt.

Florida, which was first to use midozolam in executions, uses a three-drug method similar to the one in Oklahoma, but it administers a much larger dose of the drug than Oklahoma did. Ohio used midazolam alongside a different drug, hydromorphone, in the January execution of Dennis McGuire, who was gasping and snorting during the 26 minutes before his death.

“What happened in Oklahoma, what happened in Ohio is relevant to what is happening in Texas,” said Maurie Levin, an attorney who represents Texas death row inmates and is fighting the secrecy surrounding the state’s lethal injection process. “Not only because Texas looks to both those states for execution drugs and protocol, but Texas is in possession of the drug that was used in last night's botched execution and in the botched execution of Mcguire in Ohio.”

David Dow, a University of Houston professor who is now the Rorschach visiting professor at Rice University, said the lack of transparency about the execution process means TDCJ could use midazolam at any time.

“The way the Texas Administrative Act works is that it gives those officials the authority to do essentially anything they want to without any governmental input,” Dow said. “That doesn’t mean they are able to do something that is unconstitutional. But the problem is that in order to argue that they are doing something is unconstitutional, you have to know what they’re doing.”

Correction: The original version of this story reported that Texas changed its death penalty protocol in 2011. It changed in 2012.

Reference TDCJ Execution Drugs