The botched execution of a murderer in Oklahoma last year resembled a scene from "a horror movie" according to witness interviews.

The execution of Clayton Lockett made world headlines after the killer took 43 minutes - instead of the usual 10 - to die from a lethal injection using an untested cocktail of drugs that had not been previously used in the United States.

Around 5,000 of pages of transcripts, based on around 100 interviews with witnesses carried out by the Department of Public Safety, were released after The Tulsa World filed a lawsuit against state authorities.

The testimony paints a gruesome portrait of Lockett's death on April 29 last year, with medical officials making multiple failed attempts to find a vein in the prisoner's body to carry out the procedure as they raced to complete it before a second execution scheduled on the same day.

Lockett's prolonged death occurred after the decision to pump the drugs into his body via femoral artery backfired and the product, midazolam, began leaking into his body tissue rather than into his veins.

According to witness statements published by The Tulsa World, the execution chamber was a "bloody mess".

One witness said the scene "was like a horror movie" as Lockett bucked and writhed up from the gurney when he was supposed to be unconscious.

The US Court of Appeal described Lockett's execution as a "procedural disaster" and a case involving other Oklahoma death row inmates is to be heard in the US Supreme Court in April.