TUCSON, Ariz. -- Compounding the nation's severe shortage of execution drugs, federal authorities have seized shipments of a lethal-injection chemical that Arizona and Texas tried to bring in from abroad, saying the imports were illegal.

The Food and Drug Administration said Friday that it impounded orders of sodium thiopental, an anesthetic that has been used in past executions in combination with drugs that paralyze the muscles and stop the heart. The anesthetic currently has no legal uses in the U.S.

"Courts have concluded that sodium thiopental for the injection in humans is an unapproved drug and may not be imported into the country for this purpose," Food and Drug Administration spokesman Jeff Ventura said in a statement.

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In addition, the FDA bars importation of drugs from manufacturers that are not approved by the agency, a rule intended to protect Americans from impure or otherwise dangerous pharmaceuticals. Sodium thiopental is no longer made by any FDA-approved companies.

Arizona paid nearly $27,000 for sodium thiopental, an anesthetic that has long been used in executions, according to documents obtained by The Associated Press. Federal agents intercepted the shipment when it arrived via British Airways at the Phoenix airport in July, the documents show.

The documents do not reveal what country or company Arizona tried to import the drugs from.

Texas and FDA authorities gave even fewer details about the seizure there.

Death penalty states have been struggling to obtain execution chemicals for several years after European companies refused to sell the drugs. States have had to change drug combinations or put executions on hold while they look for other options. Tennessee brought back the electric chair as a backup method of execution, and Utah did the same with the firing squad.

Other states have also looked into buying drugs from international pharmacies. Ohio, which has halted executions until at least 2017 because of a lack of drugs, sent a letter earlier this month to the FDA asserting that the state believes it can obtain a lethal-injection drug overseas without violating any laws.

Nebraska ran afoul of the FDA earlier this year when the agency said it could not legally import sodium thiopental and a second lethal-injection chemical it had bought for $54,400 from Harris Pharma, a distributor in India. That shipment apparently never made it to the United States.

"Just wanted to let you know have a few states who have already ordered sodium thiopental. Would Nebraska be interested as I will have a few thousand vials extra," Chris Harris, CEO of Harris Pharma, wrote in April to Nebraska officials, who released the correspondence under a public records request.

Harris did not name those states, and no one answered the door at the residential address in Kolkata, India, that is listed as the firm's office.

Key details are blacked out of the Arizona documents, which were released as part of a lawsuit against the corrections department over transparency in executions, and it is not clear what country or company the state was doing business with.

But, down to the font and formatting, the paperwork for the purchase resembles the Nebraska paperwork involving Harris Pharma.

A lawyer who has challenged Texas' death row practices questioned why any state would want to run the risk of a botched execution by buying drugs from an overseas supplier whose manufacturing standards are not well known.

"We have no idea what we're getting. None. It could be water. It could be witch hazel. It could be white vinegar," Maurie Levin said. "We literally have no idea."

"The department is contesting FDA's legal authority to continue to withhold the state's execution chemicals," Arizona Corrections Department spokesman Andrew Wilder said.

In Texas, the state Department of Criminal Justice is "addressing the lawful status of imports with the Food and Drug Administration and is awaiting their decision," spokesman Jason Clark said in a statement.

Texas Department of Criminal Justice officials say they legally obtained a federal import license.

Other states have also looked into buying drugs from international pharmacies. Ohio, which has halted executions until at least 2017 because of a lack of drugs, sent a letter earlier this month to the FDA asserting that the state believes it can obtain a lethal-injection drug overseas without violating any laws.

The AP obtained the Arizona documents as part of a lawsuit against the corrections department over transparency in executions.

Executions have been put on hold in Arizona following the drawn-out death of Joseph Rudolph Wood in July 2014. The state has said it doesn't plan on seeking death warrants for inmates until it resolves a lawsuit originally filed by Wood and other death row inmates seeking information about the drugs used in executions.

Wood, convicted of killing his ex-girlfriend and her father, snorted repeatedly throughout the 90 minutes it took for him to die. Authorities later revealed he was given 15 doses of midazolam and a painkiller. He was supposed to die with one dose.

Arizona announced Friday it is adding another drug combination and making executions more transparent for reporters and inmate attorneys.

The current protocols, last updated in March 2014, eliminated the midazolam and hydromorphone combination used in Wood's execution. It added pentobarbital, sodium thiopental and midazolam with potassium chloride and saline as three options. Now, a fourth option would allow the state to use a mix that includes vecuronium bromide.

"Once again, the Arizona Department of Corrections is trying to skirt the law in order to get execution drugs. Nobody is above the law, and that includes the Arizona Department of Corrections," said Wood's attorney, Dale Baich.