The company says it determined it was the maker of the potassium chloride because an inventory of drugs kept by the state showed that its stockpile came in vials of 30 milliliters. Fresenius says it is the only manufacturer that packages the drug in vials of that size.

“We made no sales to the Department of Correctional Services, nor have any of our authorized distributors,” Fresenius Kabi wrote in a statement. “So we can only conclude Nebraska may have acquired this product from an unauthorized seller.”

Nebraska is fighting separate legal efforts to force it to disclose where it got the drugs. A statement issued by Attorney General Doug Peterson said the drugs “were purchased lawfully and pursuant to the state of Nebraska’s duty to carry out lawful capital sentences.”

But neither the statement, nor state officials on Thursday, said which company manufactured the drugs, what temperature they are being stored at or whether an injunction would delay the execution. The offices of Mr. Peterson and the Nebraska governor, Pete Ricketts, did not respond to messages on Thursday.

The planned execution of Mr. Moore, 60, is also notable because it would be the first-ever lethal injection in the United States that uses fentanyl, a powerful opioid that is at the heart of the nation’s overdose crisis. Mr. Moore has ceased efforts to prevent his execution.

It is also the second time in a month that pharmaceutical manufacturers have sought to block a state from using their drugs in an execution.

In July, the execution of Scott Dozier in Nevada was delayed after a drug maker, Alvogen, said one of the drugs in the state’s execution protocol had been obtained illicitly. Two other companies that also make drugs that Nevada wants to use in the execution, Sandoz and Hikma Pharmaceuticals, have also sought to block the state from using their products.