The 47-year-old, who helmed acclaimed documentary Hearts Of Darkness and Sienna Miller's Factory Girl, passed away in Denver, Colorado last month. It was initially thought that the director had died of natural causes.
According to The Guardian, the Denver medical examiner's office said that Hickenlooper suffered a heart attack after taking prescription painkillers and alcohol. The autopsy revealed that Hickenlooper had ethanol and oxymorphone in his system when he passed away.
Kevin Spacey, the star of Hickenlooper's final film Casino Jack, paid tribute to the late director earlier this month.
"I can’t believe he's gone because George was so alive, bubbling with energy, drive, commitment, an open heart and a brilliant sense of humour," Spacey said. "He was one of a kind."