HOUSTON - Texas prison officials are helping their counterparts in Virginia prepare for a scheduled execution next week by providing the state with pentobarbital, a lethal drug that corrections agencies nationwide have had difficulty obtaining.

The disclosure, which surfaced in a court filing in an Oklahoma death penalty case, was confirmed Friday by the Texas Department of Criminal Justice. Virginia prison officials also confirmed the trade, saying they needed pentobarbital to replace a dose of another drug they intended to use, midazolam, that will soon expire.

Texas prisons spokesman Jason Clark said the three vials of pentobarbital given to Virginia were legally purchased from a compounding pharmacy, which he declined to name. Texas and Oklahoma are among a handful of states with laws — being challenged by death penalty opponents — that allow prison officials not to disclose where they get execution drugs.

Virginia prison officials gave Texas pentobarbital to use as a backup in 2013, and when asked for help by Virginia earlier this year, "we reciprocated," Clark said. "The agency has not provided compounded drugs to any other state."

Robert Dunham, executive director of the Washington-based Death Penalty Information Center, an anti-death penalty organization, said the drug exchange raised concerns about government transparency.

"It puts a whole new spin on the efforts by state departments of corrections for secrecy in the execution process," Dunham said Friday.

Lawyers for condemned Oklahoma inmate Richard Glossip mentioned the Texas-Virginia drug exchange when challenging Oklahoma's plan to use midazolam during Glossip's lethal injection next week. The powerful sedative achieved notoriety after it was used in executions that took longer than expected last year in Arizona, Ohio and Oklahoma.

In the Thursday court filing, which is part of a multi-tiered effort to stop Glossip's execution, his attorneys argue that pentobarbital is one of Oklahoma's preferred execution drugs.

Glossip's attorneys cite the Virginia-Texas exchange as proof that the drug is available, and included a photo of what they said were three bottles of pentobarbital with April 2016 expiration dates in Texas' possession. The lawyers said they obtained the photo from the Virginia Department of Corrections through a Freedom of Information Act request.

Glossip's lawyers also assert in their Thursday federal court filing that Texas was "compounding or producing pentobarbital within its department for use in executions." Clark denied the allegation, saying the state agency has no authority to manufacture its own drugs.

"We do not have a pharmacy license," he said Friday.

The Texas-provided pentobarbital is set to be used during the Oct. 1 execution of 49-year-old Alfredo Rolando Prieto for the 1988 slaying of a young couple in Virginia. Prieto already was on death row in California for raping and killing a 15-year-old girl.

Virginia Department of Corrections spokeswoman Lisa Kinney said the agency intended to use midazolam as part of Prieto's three-drug lethal injection, but that the state's supply of midazolam is set to expire a day before Prieto's execution.

The department didn't respond to multiple requests for comment about why the state sought the drugs from Texas and whether it was having difficulty obtaining them elsewhere.

Texas has said it buys its pentobarbital from a compounding pharmacy because traditional drug manufacturers have balked at providing drugs for use in executions. Unlike traditional pharmacies, compounding pharmacies aren't as strictly regulated by the U.S. Food and Drug Administration.

Prieto's attorney, Rob Lee, said Friday that the Texas-supplied drugs didn't appear to be the product of an FDA-approved process. He said he asked Virginia prison officials for more information, but hasn't heard back.

"Due to secrecy laws, we have been unable to determine how to assess the quality or effectiveness of the proposed sedative, and whether it was provided and obtained legally," Lee said in an email to The Associated Press.

Clark said the drugs had been vetted and tested, but didn't say by whom.

Virginia, where lawmakers unsuccessfully tried to pass an execution-drug secrecy law earlier this year, has identified its drug suppliers in the past.

Texas has carried out 528 executions since 1982, far more than any other state. The last 24, going back to 2013, have used pentobarbital from a compounding pharmacy as the lone drug for lethal injections.

Glossip was convicted of ordering the 1997 killing of his boss in Oklahoma City. Another man, Justin Sneed, admitted to the killing and said Glossip offered him $10,000 to do it, which Glossip denies.

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Alanna Durkin reported from Richmond, Virginia. Associated Press writers Kelly Kissel in Little Rock, Arkansas, and Tim Talley in Oklahoma City also contributed to this report.

News from © The Associated Press, 2015