Death row inmate
Death row inmate Clayton LockettReuters

Clayton Lockett, the Oklahoma prisoner who died in agony in a botched execution, was tasered hours before and had cut his own arm, newly released documents reveal.

Lockett, a convicted murderer, died of a heart attack 43 minutes after being injected with an experimental cocktail of drugs on Tuesday.

In response to international outrage at the execution, Oklahoma authorities have released a detailed timeline of prisoner #206409's final hours.

US president Barack Obama has criticised the execution, which has sparked a nationwide debate on laws which allow states to conceal the identity and source of drugs used for executions.

The UN announced that the execution of the 38-year-old may constitute cruel, inhuman and degrading treatment under international law.

The records show that Lockett resisted being x-rayed as part of execution procedure, and at 5.06 am, and almost an hour later was tasered by prison guards.

Lockett was found to have a self-inflicted laceration on his arm, although it is not known how serious it was or whether it was inflicted in a failed suicide attempt.

At around 6pm, officials settled on an insertion point in the prisoner's groin, because no viable point could be found in his arms or legs. A white sheet was placed over the dying man to preserve his modesty.

At 6.23pm he was given the chance to say final words, but declined. He was injected with Midazolam, a sedative, and ten minutes later declared unconscious. He was then injected with vercorium bromide to stop his breathing and then potassium chloride to stop his heart.

Minutes later, Lockett, who should have been unconscious, began to struggle and writhe and mumble the word "man". The blinds in the death chamber were lowered, so witnesses could not see what was happening.

Officials concluded his vein had collapsed and called Robert Patton, director of the Oklahoma Department of Corrections. Patton asked if Lockett had absorbed enough chemicals to kill him and was told no. He then asked if another vein could be found and if there were enough chemicals to restart the execution.

When he was told no to both questions he ordered an end to the execution at 6.56pm. Within ten minutes, Lockett had died of a massive heart attack.

The execution of a second inmate, Charles Warner, which was scheduled for two hours after Lockett's, was postponed. The state intends to carry out the execution of Warner after a review into the Lockett execution has been completed.

The executions of all of the state's other 54 death row inmates have also been placed on hold.

Lockett, who was convicted of a 1999 robbery in which a woman was shot then buried alive, also twice refused a final meal, records show.