Robin Williams was reportedly suffering from Lewy body dementia around the time of his death.
According to TMZ, sources connected to the actor's family say the condition was a "key factor" in Williams' suicide. The website says it obtained documents confirming that Williams had the disease, described by the Mayo Clinic as the second most common type of progressive dementia (after Alzheimer's disease). It can cause a "progressive decline in mental abilities, as well as visual hallucinations, which generally take the form of objects, people or animals that aren't there."
According to the Lewy Body Dementia Association, the disease affects an estimated 1.3 million people in the United States.
Williams was found dead on the morning of Aug. 11 in a bedroom of his Northern California home. Sheriff's officials said he committed suicide by hanging himself with a belt.
Williams' wife, Susan Schneider, confirmed that the comedian was in the early stages of Parkinson's disease and also suffered from depression and anxiety.
The actor, who was 63 when he died, had battled periods of substance abuse throughout his life. He entered a substance abuse rehabilitation program shortly before his death. An autopsy found no alcohol or illegal drugs.