(Reuters) - A U.S. regulatory agency told Texas and Arizona that more than a thousand vials of drugs they ordered for executions in their states from India in 2015, and seized by U.S. Customs, will not be released to them, an official said on Thursday.

The Food and Drug Administration notified the Texas Department of Criminal Justice and the Arizona Department of Corrections that their confiscated shipments of sodium thiopental have been refused on the basis that the detained drugs appear to be unapproved new drugs and misbranded drugs, FDA press officer Lyndsay Meyer said.

Officials in Arizona were not immediately available for comment.

“It has taken almost two years for the Food and Drug Administration to reach a decision, which we believe is flawed. TDCJ fully complied with the steps necessary to lawfully import the shipment,” the Texas Department of Criminal Justice said in a statement.

“We are exploring all options to remedy the unjustified seizure,” it said.

Arizona officials did not immediately respond to a request for comment.

Texas sued in January for the drug’s release, saying in its lawsuit that it was importing the sodium thiopental for legal executions.

Sodium thiopental renders a person unconscious and was a staple of lethal injection mixes but has not been made in the United States for several years.

“Texas appears to be trying to carve out an exception for this one purpose (using the drug in a lethal injection),” said Megan McCracken, an expert on lethal injection drugs and a professor at the University of California Berkeley School of Law.

About six years ago, major pharmaceutical companies began imposing bans on sales of their products for use in executions, which left death penalty states scrambling to come up with new mixes and suppliers.

Many have turned to a less powerful, Valium-like sedative called midazolam to render prisoners unconscious. It has been used in troubled executions in Oklahoma and Arizona where inmates who were supposed to be insensate were seen twisting in pain on death chamber gurneys.

Nebraska, South Dakota, Ohio, Arizona and Texas tried to import sodium thiopental from India between 2010 and 2015, according to court records and news media reports, but federal regulators blocked the moves.

Previous attempts to import the drug have also been blocked by federal courts after challenges from death row inmates. The last major case was decided in 2013.