Oklahoma investigators looking into the execution of an inmate who writhed, moaned and clenched his teeth on a gurney after being injected with three lethal drugs are set to release their findings.

Gov. Mary Fallin ordered the state Department of Public Safety to investigate the April 29 execution of Clayton Lockett and recommend possible procedural changes. The agency has said it would release its report Thursday morning.

State prisons director Robert Patton halted Lockett's execution as the man struggled against restraints in the death chamber. Lockett died behind a shuttered curtain 43 minutes after his execution began, and Patton said the cause was a heart attack. But autopsy results released last week said he died from the drugs: midazolam, vercuronium bromide and potassium chloride. The report did not explain why the execution took so long or why Lockett writhed.

Lockett, 38, had been convicted of shooting Stephanie Nieman, 19, with a sawed-off shotgun and watching as two accomplices buried her alive in 1999.

His execution was the first time Oklahoma had used midazolam, a sedative. An Ohio inmate given the same drug also gasped and appeared in distress before dying in January, and an Arizona inmate who received it in July struggled to breathe for nearly two hours before dying.

Drugmakers in recent years have objected to states using their products in executions, so Oklahoma and others have had to find alternative sources. Many states won't say where they obtain the drugs, saying they fear their suppliers may be subjected to harassment. Some states turn to loosely regulated compounding pharmacies that custom-make medications.

A June lawsuit against the Department of Corrections on behalf of 21 Oklahoma prisoners alleged that prison officials are experimenting on death row inmates and violating the U.S. Constitution's ban on cruel and unusual punishment by tinkering with the state's lethal injection procedures. The state says those claims are false.

After Lockett's death and pending the Department of Public Safety's report, all Oklahoma executions were postponed for six months. Three executions have been set for November and December.