Texas is set to execute its first inmate with a new batch of drugs, but the state prison agency remains determined to keep its supplier a secret, citing threats of violence to pharmacies that sell drugs used in lethal injections.

Since obtaining a new supply of the drug pentobarbital two weeks ago, the Texas Department of Criminal Justice had cited unspecified security concerns in refusing to disclose the source and other details about the sedative it plans to use to put inmates to death.

But in a brief filed Tuesday with the state attorney general's office, Patricia Fleming, an assistant general counsel for the Texas prison system, argued that a supplier in another state received a specific threat of physical violence.

"An individual threatened to blow up a truck full of fertilizer outside a pharmacy supplying substances to be used in executions," Fleming wrote.

As such, she argued, an open-records request filed by an attorney for a condemned inmate seeking the drugmaker's identity should not be granted.

Questions about the source of drugs used by states to carry out lethal injections have arisen in several states in recent months as numerous drugmakers — particularly in Europe, where opposition is strongest to capital punishment — have refused to sell their products if they will be used to carry out executions.

That has led several U.S. prison systems to compounding pharmacies, which are not as heavily regulated by the U.S. Food and Drug Administration as more conventional pharmacies.

A batch of pentobarbital Texas purchased from such a compounding pharmacy in suburban Houston expired at the end of March. That pharmacy refused to sell the state any more drugs, citing threats it received after its name was made public. That led Texas to its new, undisclosed suppler.

An attorney for inmate Ramiro Hernandez-Llanas, set to die later this month, had filed an open-records request with the Department of Criminal Justice March 11 seeking the name of that supplier. The agency had until March 25 to either provide the records, set a specific date to provide them or seek a decision from the attorney general's office that would allow it to withhold the information.

In three past such opinions, the attorney general's office has directed the agency to release records about its lethal injection drugs. Fleming, in the request filed Tuesday, argued that circumstances have changed since 2012, the last time the attorney general's office said the information should be disclosed.

"It is the (prison system's) opinion that release of the information at issue creates a risk of physical harm to pharmacy personnel and customers in and surrounding the pharmacy," she wrote.

Lauren Bean, a spokeswoman for Attorney General Greg Abbott, said before receipt of Fleming's request that the office had 45 business days to reply.

That timing is an issue for both Hernandez-Llanas and Tommy Lynn Sells, who is scheduled for execution on Thursday.

"Even for the fastest court case you could ever imagine under the Public Information Act, it would involve weeks at least, and probably more like months before you ever get to a .... hearing in it," said Bill Aleshire, an Austin attorney experienced in open government issues.

Unwilling to wait, attorneys for both men asked a federal court on Tuesday to either force state prison officials to disclose the drug source or delay the executions while the issue is addressed.

The inmates "are entitled to discover how the state plans to put them to death," said attorneys Jonathan Ross and Maurie Levin.

Last week, they won an order from a state court that directed prison officials to identify the new provider of pentobarbital, but only to attorneys for the two prisoners. The Texas Supreme Court put that order on hold on Friday and set a deadline for briefs that will arrive after the Sells and Hernandez-Llanas' scheduled execution dates.

The federal lawsuit filed Tuesday asked the court to compel the agency to immediately disclose the drug source information so the sedatives can be tested to determine they are "safe and will reliably perform their function, or if they are tainted, counterfeited, expired, or compromised in some other way."

It also seeks a court order halting the two executions so the inmates are "able to litigate their right to be executed in a manner devoid of cruel and unusual pain," the lawyers said.

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State attorneys have argued the new drugs have been tested and fall within acceptable ranges for potency. Last month, the U.S. Supreme Court rejected similar arguments from a Missouri death row inmate who was later put to death.

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