AUSTIN -- For nearly a year and a half, the U.S. Food and Drug Administration has detained a shipment of about 1,000 vials of execution drugs headed for Texas' death chamber. On Tuesday, Texas officials demanded an end to the delays, filing a lawsuit that seeks to force the feds to turn over the drugs.

"My office will not allow the FDA to sit on its hands and thereby impair Texas' responsibility to carry out its law enforcement duties," Texas Attorney General Ken Paxton said in a statement.

Texas and other states that still execute inmates have been hard-pressed to find lethal injection drugs in recent years. American companies have stopped making the drugs, and European makers have stopped selling them to the U.S. Amid the drug shortage in 2012, Texas switched from the three-drug cocktail it used since 1982 to a single overdosing injection of pentobarbital, a barbiturate, but that drug, too, is in short supply.

In July 2015, the FDA intercepted about 1,000 vials of sodium thiopental, also a barbiturate, that Texas was attempting to import from a foreign seller at the George Bush Intercontinental Airport in Houston.

FDA officials said that the drugs lacked the required warnings and directions for use and that they needed federal approval. The state responded to the FDA, explaining that the drugs were legal for importation for law enforcement use. In April 2016, the FDA issued a tentative decision denying admission of the drugs.

But since then, the agency hasn't issued a final decision and has kept the drugs.

In the lawsuit filed Tuesday, Paxton argued the delays are unwarranted and should come to an end.

"Because FDA's delay is unreasonable, TDCJ requests the Court to declare that the delay is unlawful and compel FDA to render a final admissibility decision," the lawsuit states.

As execution drugs have become harder to obtain, the state has turned to compounding pharmacies to make them, has sought drugs from foreign providers and has sought to restrict public access to information about where and how it gets drugs used in lethal injections.

Although Texas executions have slowed in recent years, with 538 lethal injections since the reinstatement of the death penalty in the U.S. in 1976, the state remains the most prolific user of capital punishment.

"The Texas Department of Criminal Justice lawfully ordered and obtained the necessary license to import drugs used in the lethal injection process, yet the Food and Drug Administration stopped the shipment and continues to hold it without justification. This has left the agency with no other recourse than to challenge the unjustified seizure in court," Texas Department of Criminal Justice spokesman Jason Clark said in a statement.

Clark said the TDCJ has enough drugs on hand to complete the nine executions scheduled for the first six months of this year.

"We cannot speculate on the future availability [of] drugs, so the agency continues to explore all options including the continued use of pentobarbital or alternate drugs to use in the lethal injection process," Clark said.