McALESTER, Okla. — An Oklahoma inmate whose execution was halted Tuesday because the delivery of a new drug combination was botched died of a heart attack, the head of 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.

The execution began at 6:23 p.m. when officials began administering the first drug, and a doctor declared Lockett to be unconscious at 6:33 p.m.

About three minutes later, though, Lockett began breathing heavily, writhing on the gurney, clenching his teeth and straining to lift his head off the pillow. After about three minutes, a doctor lifted the sheet that was covering Lockett to examine the injection site. After that, an official who was inside the death chamber lowered the blinds, preventing those in the viewing room from seeing what was happening.

Patton then made a series of phone calls before calling a halt to the execution. He also issued a 14-day postponement in the execution of inmate Charles Warner, who had been scheduled to die Tuesday, two hours after Lockett was put to death.

“It was extremely difficult to watch,” Lockett’s attorney, David Autry, said afterward. He also questioned the amount of the sedative midazolam that was given to Lockett, saying he thought it was “an overdose quantity.” It was the first time Oklahoma administered midazolam as the first drug in its execution drug combination.

A four-time felon, Lockett, 38, was convicted of shooting 19-year-old Stephanie Neiman with a sawed-off shotgun and watching as two accomplices buried her alive in rural Kay County in 1999 after Neiman and a friend arrived at a home the men were robbing.

Warner had been scheduled to be put to death two hours later in the same room and on the same gurney. The 46-year-old was convicted of raping and killing his roommate’s 11-month-old daughter in 1997. He has maintained his innocence.

Lockett and Warner had sued the state for refusing to disclose details about the execution drugs, including where Oklahoma obtained them.

The case, filed as a civil matter, placed Oklahoma’s two highest courts at odds and prompted calls for the impeachment of state Supreme Court justices after the court last week issued a rare stay of execution. The high court later dissolved its stay and dismissed the inmates’ claim that they were entitled to know the source of the drugs.

By then, Gov. Mary Fallin had weighed into the matter by issuing a stay of execution of her own — a one-week delay in Lockett’s execution that resulted in both men being scheduled to die on the same day.