The lawyer of a death row inmate watched as his body "turned purple" and "head snapped back" while he was being put to death by gas chamber.
Attorney Jim Belanger represented Don Harding, a triple murderer responsible for a string of killings in Arizona, the US, in 1980.
Don Harding tied up businessmen Robert Wise and Martin Concannon at a hotel before beating and shooting them, the Daily Star reported.
The 43-year-old was convicted of killing in another man in a similar way, and was put to death in the state’s gas chamber in 1992.
Mr Belanger recalled the grim execution for azcentral, as Arizona plans to reintroduce lethal gas in state executions.
He and other witnesses were initially taken to a chapel next to the gas chamber.
Harding had been stripped “virtually naked” and was wearing white underwear and an electronic monitor attached to his chest.
Cyanide pellets were dropped into a bowl of sulphuric acid, which then released the gas that slowly killed him.
Mr Belanger wrote: "His face was red and contorted as if he were attempting to fight through tremendous pain. His mouth was pursed shut and his jaw was clenched tight. Don then took several quick gulps of the fumes.
"Don’s body started convulsing violently and his arms strained against the straps. His face and body turned a deep red and the veins in his temple and neck began to bulge until I thought they might explode.
"Every few seconds he gulped for air. He was shuddering uncontrollably and his body was wracked with spasms. His head continued to snap back and forth. His hands were tightly clenched.”
These convulsions continued for several minutes and spittle began to fall from Harding’s mouth.
In total, it took 10 minutes and 31 seconds for him to die – at least eight of which he spent “writhing in agony”.
Arizona Daily Star reporter Carla McClain said his body “turned bright red, almost purple”.
Cameron Harper, who was an anchorman for the local ABC affiliate, afterwards said: “It was an ugly event.
“We put animals to death more humanely than this guy.”
Steve Twist, Arizona’s former assistant attorney general, called it “an awful, solemn way” of punishment, the Los Angeles Times reported.
Arizona is now preparing to kill inmates with hydrogen cyanide, the same gas used by the Nazis at Auschwitz.
According to documents obtained by The Guardian, the state’s department of corrections has already spent more than $2,000 [£1,409) on the ingredients and $1,530 [£1,078] on a brick of potassium cyanide.
