The body of an Oklahoma inmate who died after a botched execution of what corrections officials have said was an apparent heart attack was returned from an independent autopsy without the heart or larynx, a state medical official said Monday.

The Dallas County Medical Examiner's Office, which is conducting an independent autopsy on the body of inmate Clayton Lockett, retained the body parts, a practice that is not uncommon, said Amy Elliott, a spokeswoman for the Oklahoma Medical Examiner's Office.

"Oklahoma law reads that the Office of the State Medical Examiner can retain any kind of tissue or samples indefinitely," Elliott said. "And my understanding is it can be the same in Texas."

Dallas County officials did not immediately return messages Monday.

David Autry, Lockett's attorney, said a private doctor is working to complete a second autopsy and has asked Dallas County to preserve all evidence in the case, including the heart and larynx.

"I assume they retained those for additional testing, but we've asked them to preserve all the evidence," Autry said.

Lockett's body has been returned to his family and cremated, Autry said.

Dr. Amy Gruszecki, the medical director of American Forensics, a Dallas facility that conducts independent autopsies, said doctors conducting the autopsy likely found something specific with the heart and larynx that they wanted to further document.

"It's not completely unusual," Gruszecki said. "They might want to do some additional investigation or saw something that was very important to the diagnosis."

Lockett died after his April 29 execution was halted when prison officials noted the lethal injection drugs weren't being administered properly. The doctor inside the death chamber reported a single IV in Lockett's groin became dislodged and the lethal drugs went into his tissue or leaked out of his body.

Oklahoma was using a new three-drug method for the first time, and Lockett writhed on the gurney, gritted his teeth and attempted to lift his head several times before the state's prison director halted the execution. Lockett died anyway, about 43 minutes from what prison officials have said was an apparent heart attack.

Gov. Mary Fallin has ordered an investigation into Lockett's death, and the Oklahoma Court of Criminal Appeals has issued a six-month stay of execution for a second inmate who was scheduled to die on the same night as Lockett.

The Oklahoma Medical Examiner's Office will release the official cause and manner of Lockett's death after it receives the results of the autopsy from Dallas County, Elliott said.

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