A Swiss pharmaceutical company has issued a voluntary recall of a lethal injection drug held by Nebraska, but state officials said Wednesday they won't return the drug and the U.S. Food and Drug Administration says it won't enforce the company's decision.

Naari AG asked Nebraska officials to quarantine its supply of sodium thiopental and return it to the company or the FDA. The company said in a recall letter that the product was "illegally diverted from the company's supply chain" and has been outside of the company's control.

"Therefore, as a precautionary measure, the company is recalling" the batch of drugs in Nebraska's possession, the company said in the letter.

Nebraska's first batch of the drug obtained from an India-based drug company in January 2011 was ruled to have been illegally imported. The state then obtained a new batch from another Indian source late last year, but the Swiss manufacturer of the drug, Naari AG, said that the sodium thiopental that Nebraska bought was a sample intended only to be used for evaluation purposes as an anesthetic in Zambia.

Nebraska state officials said they obtained the supply in a legitimate manner, and would not return it.

"All documents examined indicate our department received the sodium thiopental in a legitimate manner," Nebraska Department of Corrections Director Robert Houston said in a statement. "NAARI has made their position clear; however, we do not feel a response to this recall notice is necessary or appropriate."

FDA spokeswoman Shelly Burgess said her agency doesn't enforce compliance with voluntary recalls initiated by companies.

Sodium thiopental has been in short supply since 2010, when the only U.S. manufacturer, Hospira Inc., ended production because of death penalty opposition from overseas customers. After that, the European Union banned the export of chemicals including sodium thiopental, which further diminished its availability as a lethal injection drug.

In March, a U.S. judge found the FDA wrongly allowed other states to import the drug. U.S. District Judge Richard Leon also ordered the federal agency to immediately notify any state correctional department with foreign-manufactured thiopental that its use is prohibited by law, and that the drug must be returned to the FDA.

The Nebraska attorney general's office has refused a request from the FDA to turn over Nebraska's batch of sodium thiopental. The attorney general instead suggested the FDA appeal the federal judge's ruling.