Oklahoma prison officials halted the execution of a death row inmate on Tuesday night after botching the delivery of the lethal three-drug combination, the Associated Press reports.

Here’s more from the AP:

An Oklahoma inmate whose execution was halted Tuesday because the delivery of a new drug combination was botched died of a heart attack, the state Department of Corrections said. Director Robert Patton said 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 said there was a vein failure. Lockett was writhing on the gurney and shaking uncontrollably.

Oklahoma was scheduled to execute two inmates on Tuesday after the carrying out of the death penalty sentence was delayed by the inmates’ legal challenges to the secrecy behind the state’s lethal injection procedure. The execution of the second inmate, Charles Warner, was postponed, the AP reports.

*Correction, Oct. 8, 2015: This post originally misidentified the man to be executed in Oklahoma as James Warner; his name is Charles Warner.