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A report into a botched execution in Oklahoma in April has found medical staff failed to place and monitor an intravenous line properly.

Clayton Lockett, 38, died 43 minutes after he was given a lethal drug.

The Oklahoma governor ordered an independent review into the case after witnesses said they saw Lockett writhing and gasping for air.

There had been concerns about the effectiveness of a new cocktail of drugs used in the execution.

Lockett was sentenced to death on 20 April for the 1999 shooting of Stephanie Nieman, 19, and for watching as two accomplices buried her alive.

His execution was stopped after 20 minutes after witnesses saw him writhing and mumbling words.

A ruptured vein was discovered and Lockett subsequently died of a heart attack soon afterwards.

The 32-page report by by the Oklahoma Department of Public Safety found the actions of experienced physicians and a prison warden had contributed to Lockett's drawn-out death.

It said new drugs used in the execution - Midazolam, Vercuronium Bromide and Potassium Chloride - were not at fault.

In recent years, US states have found it difficult to obtains drugs previously used for lethal injections because of an embargo by European pharmaceutical firms.

Lockett's killing was the first time Oklahoma had used Midazolam in an execution.

The drug was also used in lengthy attempts to execute an Ohio inmate in January and an Arizona prisoner last month.

Each time, witnesses said the inmates appeared to gasp after their executions began and continued to struggle for air before being pronounced dead.

Although the report said troubles with the IV line made it difficult to determine the drug's effectiveness, it did not blame them for the botched execution.

"The drugs worked. At the end of the day ... the drugs did what they were designed to do," said Department of Public Safety Commissioner, Michael Thompson.

Investigators said the medical team had inserted the intravenous line in Lockett's groin after failing to find suitable veins in his arms, legs, neck and feet.

"This investigation concluded the viability of the IV access point was the single greatest factor that contributed to the difficulty in administering the execution drugs," the report said.

Investigators also faulted the decision by a prison warden to cover up the insertion point because it was close to the prisoner's genitals.

It is normal practice to monitor an insertion point during an execution but the report found in Locket's case, the sheet covering his genitals was only removed when it became apparent the execution was not progressing as normal.

It was only then the team noticed a swelling larger than a golf ball near the injection site, the report said.

Investigators listed a series of recommendations for future executions, including proper monitoring of IV insertion points and the formal and continuous training for staff.

They also called for for executions not to be scheduled within seven days of each other.

Another inmate initially scheduled for execution the same day as Lockett had been granted a stay until the matter was investigated.