Three death row inmates have sued the US state of Texas over a change in the source of a lethal injection drug, saying it creates "a high risk of excruciating pain."
In a suit filed in federal court in Houston Tuesday, Thomas Whitaker, Perry Williams and Michael Yowell called for a halt to executions until Texas can show "the integrity and legality" of the substances it plans to use.
But the Attorney General's office responded Wednesday, saying the state intended to go forward with the drugs it has purchased for the executions.
Yowell is scheduled to be put to death October 9 in Huntsville. No date has been set for the two other plaintiffs.
Texas, which executes more inmates than any other state, announced last week it had succeeded in replenishing its dwindling supply of pentobarbital, an anesthetic used to euthanize animals.
Asked about the source of its supply, Texas justice department spokesman Jason Clark told AFP the state had obtained the drug from a "Texas pharmacy that has the ability to compound."
But this type of pharmacy is "not subject to stringent FDA regulations" and is "one of the leading sources for counterfeit drugs entering the US," the lawsuit alleges.
"There is a significant chance that (the pentobarbital) could be contaminated, creating a grave likelihood that the lethal injection process could be extremely painful, or harm or handicap plaintiffs without actually killing them," the plaintiffs said.
Compounding pharmacies -- which are regulated by local US state authorities and not federal, national ones -- sparked a scandal in November 2012, when one such company was deemed responsible for a deadly meningitis outbreak because of poor hygiene.
The change to the new source of pentobarbital "adds an unacceptable risk of pain, suffering and harm," the complaint alleges.
"This uncertainty and the unnecessary suffering and mental anguish it creates for plaintiffs is an Eighth Amendment violation," it argues.
In his response Wednesday, Texas Attorney General Greg Abbott said the state intents to execute Yowell by administering a five gram (0.18 ounce) dose of compounded pentobarbital that has a potency of 98.8%" and had been tested by an independent lab.
US states that use capital punishment faced a supply problem in 2011 when the United States stopped producing the drug used since the introduction of lethal injections in the 1980s.
Since then, certain states -- including Texas, which has executed more than 500 inmates in three decades -- turned to pentobarbital.
But the Danish producer of the drug refused to provide it to the United States for the purpose of executing humans.