Pettigrew, 42, was an assistant track and field coach at the University of North Carolina and a 2000 Olympian who was stripped of his 4x400 metres relay gold medal after admitting to doping. He also was the 1991 400 metres world champion.

He was found unresponsive by friends in the back seat of his vehicle in rural central North Carolina on August 10.

An empty bottle of a sleep aid was found in the car, the investigation report said.

Diphenhydramine, an antihistamine, is often used as a sleep aid and for the treatment of allergies.

In addition to the Olympic gold, Pettigrew helped the United States win three world titles in the 4x400 meters relay.

He relinquished the Olympic gold and two of the world relay medals after admitting to using performance-enhancing substances.

"He told his wife to pray for him," the investigation report said of a telephone call from Pettigrew the day before his body was found.