Corrections Director Robert Patton had to halt the execution of Clayton Lockett due to a 'vein failure' which prevented the deadly chemicals from reaching the inmate.
Witnesses to the execution said Clayton convulsed violently in the gurney and proceedings were halted after 20 minutes.
A spokesperson for the Oklahoma Department of Corrections told US media that Lockett, 38, died of a massive heart attack following three lethal injections.
'We believe that a vein was blown and the drugs weren't working as they were designed to. The director ordered a halt to the execution,' said Jerry Massie.
A reporter from the Associated Press tweeted from the scene that Lockett was 'conscious and blinking, licking his lips even after the process began. He then began to seize.'
Lockett was sentenced to death for the 1999 shooting of 19-year-old Stephanie Nieman, who he shot twice and then watched as two accomplices buried her alive.
The family of Nieman released this statement:
'We are thankful this day has finally arrived and justice will finally be served.'
Another inmate who was meant to be put to death in the same room two hours later, had to be sent back to the cells after the debacle, with his execution put back 2 weeks.