Plans to introduce cyanide gas as a lethal alternative for death row prisoners in Arizona have been labelled "disgraceful" by Holocaust memorial groups.
The state's Department of Corrections purchased potassium cyanide and other ingredients to make the lethal gas last December, according to newly published records.
"For Auschwitz survivors, the world will finally come apart at the seams if in any place on this Earth the use of Zyklon B in the killing of human beings is considered again," Christoph Heubner, from the International Auschwitz Committee, said.
New documents reportedly show the department's invoices for a solid brick of potassium cyanide, sodium hydroxide pellets and sulphuric acid - the materials required to produce the gas.
The Nazis also used a cyanide-based gas, under the trade name Zyklon B, to kill more than a million people in the chambers of Auschwitz-Birkenau and other death camps during the Holocaust.
"This is a disgraceful act that is unworthy of any democracy and, moreover, insults the victims of the Holocaust," Heubner said.
"Lethal gas, or at least the lethal gas that Arizona is trying to bring back, is the most gruesome of all these methods we've had in this country," Deborah Denno, a law professor at Fordham University in New York, told The Washington Post.
The Arizona documents, obtained via a request from The Guardian newspaper, also disclose the lengths to which prison staff have gone to "refurbish" the state's gas chamber, in its facility in Florence, Arizona.
Officials at the facility assessed the chamber's "operability" last August with a physical inspection of the surfaces, rubber seals, levers and plumbing, according to the documents.
The safety of the chamber, last used more than 20 years ago, was confirmed through tests such as passing a candle over doors and windows to check that no gas was able to escape. The gas chamber was deemed fit for use in early December.
Executions were halted in Arizona after the botched lethal injection of Joseph Wood in 2014, when it took the murderer more than two hours to die.
But the state said this year it was preparing to resume executions.