The Green Mile star, who suffered a cardiac arrest at his home in Los Angeles in July (12), passed away in hospital on Monday morning (03Sep12).
Duncan never fully recovered from his health scare, and on Tuesday (04Sep12), Assistant Coroner Chief Ed Winter confirmed his death was down to natural causes.
Winter has revealed the actor's personal physician signed off the cause of death and the coroner's office found no reason to contradict that finding, according to E! News.
Duncan's former co-stars Tom Hanks, Gary Sinise and Bruce Willis are among the famous faces who paid tribute to the actor following his death.