Arizona has already executed one prisoner using a cocktail of drugs including one purchased from a British supplier.
Jeffrey Landrigan, 50, was put to death on 26 October 2010 in the state's first execution for three years. Officials admitted he was injected with sodium thiopental purchased from the UK. Landrigan was convicted of murder in 1989.
He was found guilty of strangling Chester Dyer in Arizona in 1989 during an armed burglary. At the time, he was on the run after escaping from an Oklahoma prison, where he had been serving time for second-degree murder.
A judge agreed to postpone his execution after his defence team raised objections about the safety of the British-sourced drug being used in the lethal injection process. They said the drug had not been approved for use in the United States.
However, that decision was overturned when the Supreme Court judges voted 5-4 in favour of allowing the execution to go ahead. By then, state authorities had revealed the compound had been sourced from Britain, but refused to name the supplier.
The state's attorney general, Terry Goddard, used a law preventing the identification of executioners to ignore an order forcing him to reveal the exact source of the sodium thiopental used as part of the process to end Landrigan's life. The state's chief deputy attorney general, Tim Nelson, would only say the drug came from a "reputable place".
"There's all sorts of wild speculation that it came from a third-world country," he said. "That's not accurate."
Reprieve, the charity that campaigns against the death penalty, believes Arizona has enough of the British-sourced drug for use in several more executions.
The only manufacturer of the drug in Britain is Archimedes Pharma, based in Reading. The company has always denied supplying the drug to the US, insisting it must have been sold on by a middleman.
It has now emerged that just weeks earlier, Arizona had secured all three drugs used in lethal injection executions from the Acton-based firm, Dream Pharma Limited. However, there is no evidence that the drugs allegedly sourced from Dream Pharma were the ones used to execute Landrigan.
Behind the scenes, British diplomats complained to the State Department about Landrigan's execution "given reports that he suffered from severe mental health problems".