A federal judge called for a hearing to be held hours before a convicted serial killer is scheduled to be executed in Virginia to consider a challenge of the state's lethal injection drugs that it obtained from Texas.

U.S. District Court Judge Anthony Trenga set a hearing for 2 p.m. Thursday after Alfredo Prieto's attorneys asked the court stay his execution until officials disclose more information about the pentobarbital it intends to use — including the name of the supplier, tests confirming its sterility and potency and documents showing that the drugs were properly handled.

The state has set Prieto's execution for 9 p.m. Thursday. The El Salvador native was on death row in California for raping and murdering a 15-year-old girl when DNA evidence linked him to the 1988 slaying of a young couple in Virginia. Authorities say he has been linked to several other murders in both states but he was never charged because he had already been sentenced to death.

The judge on Wednesday granted a temporary restraining order against the state, but the execution could still go forward Thursday evening if the order is vacated after the hearing, said Megan McCracken of the U.C. Berkeley School of Law's Death Penalty Clinic, an expert on challenges to lethal injections.

Prieto's attorneys, Rob Lee and Elizabeth Peiffer, said the lack of information about the execution drugs puts the state at risk of carrying out a cruel and painful execution. Texas allows prison officials to shield where they get execution drugs and Prieto's attorneys say Virginia officials have not provided that information.

The Associated Press filed a public records request for the names of the manufacturers and the suppliers of the drugs, but the documents show only that the drugs were provided by the Texas Department of Criminal Justice.

Texas prisons spokesman Jason Clark told AP last week that the three vials of pentobarbital given to Virginia were legally purchased from a compounding pharmacy, which he declined to name. Virginia will substitute the pentobarbital for midazolam that it intended to use because its supply of that sedative expires Wednesday.

A spokeswoman for the Virginia Department of Corrections didn't immediately respond to a request for comment.

Senior Assistant Attorney General Richard Vorhis said in a letter to Peiffer on Tuesday that the drugs were legally acquired, tested and transported in an appropriate manner. He noted that Texas has successfully used the same compound in 24 executions over the past two years with no problems.

Pentobarbital is the first of three drugs that the state intends to use.

Mylan, the manufacturer of the rocuronium bromide — another drug that will be used — said the company sent several letters to Virginia officials when it learned about the drug's possible use and then demanded that the state return the product when it received no response.

Spokeswoman Nina Devlin said in a statement that the company is contractually restricting its distributors from distributing Mylan products, including rocuronium bromide, for use in lethal injection or for any other use outside of the approved labeling or applicable standards of care.

Pharmaceutical companies, under pressure from death penalty opponents, have stopped selling U.S. prisons drugs for lethal injections. So Texas and other states have turned to less regulated compounding pharmacies for made-to-order drugs, and like in this case, have sometimes turned to other states to acquire the necessary drugs.

Prieto's attorneys in Virginia and California have also both asked the U.S. Supreme Court to stay his execution so that they can prove that he's intellectually disabled and therefore ineligible for the death penalty. A federal appeals court said in June he failed to prove that no reasonable juror would find him eligible for execution and that "absent some new 'smoking gun,'" evidence of his ability to handle everyday tasks was "at best inconclusive."