Oklahoma inmate Clayton Lockett died during a botched execution Tuesday, minutes after a doctor had called a halt to the procedure, raising more questions about new death penalty cocktails used by the state and others.

Thirteen minutes after a doctor administered a lethal injection at the state's death chamber in McAlester, Lockett lifted his head and started mumbling. The doctor halted the execution, said state corrections department spokesman Jerry Massie.

Lockett died of an apparent massive heart attack about 40 minutes after the procedure started, he said.