McALESTER, Oklahoma - The director of the Oklahoma Department of Corrections says an inmate whose execution was halted, because the delivery of a new drug combination was botched, has died of a heart attack.

Director Robert Patton says inmate Clayton Lockett died Tuesday after all three drugs were administered.

Patton halted Lockett's execution about 20 minutes after the first drug was administered. He says there was a vein failure.

Lockett was writhing on the gurney and shaking uncontrollably.

The Oklahoma Department of Corrections provided the following information about the execution process:

The execution process includes the delivery of three drugs, Midazolam, Vecuronium Bromide and Potassium Chloride. Midazolam causes unconsciousness, Vecuronium Bromide stops respiration and Potassium Chloride stops the heart. Two intravenous lines are inserted, one in each arm. The drugs are injected by hand held syringes simultaneously into the two intravenous lines. The sequence is in the order that the drugs are listed above. Three executioners are utilized, with each one injecting one of the drugs.

News 9 Reporter Justin Dougherty was at Lockett's execution, and he said the execution started at 6:23 p.m. and Lockett was unconscious at 6:33 p.m.

Dougherty said Lockett "started to slowly convulse around 6:36 p.m., the curtains closed at 6:39 p.m. and he was pronounced dead from a massive heart attack at 7:06 p.m."

Warner's execution is scheduled for May 13.