Arizona, which is facing a shortage of a lethal injection drug, has obtained supplies from the UK.
The state said it got its sodium thiopental from Britain, the first time a state has acknowledged obtaining the drug from outside the US since the shortage began slowing executions in the spring.
Such a move, experts said, raises questions about the effectiveness of the drug. But it may also further complicate executions in the 35 states that allow them, as inmates challenge the use of drugs not approved by government inspectors for use in the US
"This drug came from a reputable place," chief deputy attorney general Tim Nelson said. "There's all sorts of wild speculation that it came from a third world country and that's not accurate."
Mr Nelson said the state revealed the drug's origins to let the public know that its supply was trustworthy and to dispel rumours. He did not name the company that manufactured it.
Without assurances of the drug's quality, many questions will be raised, including its effectiveness and how it should be handled, and would serve as a basis for lawsuits, said Deborah Denno, a law professor at Fordham University.
"The impact could be huge," she said. "The source of the thiopental is critical."
Later, Arizona executed Jeffrey Landrigan for a 1989 murder in the state's first execution since 2007. Landrigan died by injection at a state prison in Florence on Tuesday after a stay issued by a federal judge was lifted by the US Supreme Court. That stay was based on questions about the effectiveness of the state's supply of an execution drug in short supply. Arizona obtained a supply of the sedative drug sodium thiopental from Britain.
Hospira, of Lake Forest, Illinois, the sole US manufacturer of the drug, has blamed the shortage on unspecified problems with its raw-material suppliers and said new batches would not be available until January at the earliest. There are no FDA-approved overseas manufacturers of the drug.
The limited supply has also directly affected executions in California, Kentucky and Oklahoma, and may delay executions in Missouri, which said its supply of sodium thiopental expires in January.