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It was on their second wedding anniversary that Susan Schneider Williams noticed a change in her husband Robin Williams.

On that day, Oct. 22, 2013, Robin was experiencing pain in his stomach and the incident was "alarming," she said during an interview on CBS This Morning.

"Robin and I had been together six years to that point, and I knew my husband's normal baseline of fear and anxiety. And his fear and anxiety spiked and sustained at a level that was very scary," she told the morning hosts. "So that was kind of the beginning, really the way I see it."

Over the next 10 months, she noticed more things that just seemed off about the legendary actor, including just speaking to friends and colleagues.

"It got difficult for him to even — even interacting with people became very difficult," she said. "He would question things afterward or during ... in the realms of 'Did I do OK?' Things that focused around insecurity and fear."

Robin, she said, suffered from Lewy body dementia, a disease that affects the brain, which can often lead to behavioral changes. People who suffer from Parkinson's disease and Alzheimer's often suffer from similar symptoms.

On Aug. 11, 2014, Robin committed suicide at the age of 63. While many have speculated that his depression and mental health drove him to end his life, Susan believes there were other factors.

In fact, she recently detailed her late husband's struggles in an essay for the medical journal Neurology titled "The Terrorist Inside My Husband's Brain." In it, she said he had issues with paranoia, delusions, insomnia, heartburn and a slight tremor in his left hand.

"[Parkinson's disease] is actually an accurate diagnosis; however, that was the clinical side. The pathology was that he had diffuse Lewy body disease, which is what took him," she said. "I can tell you in his autopsy, the coroner's report was clear that he had Lewy body throughout all of his brain and brain stem — nearly every region."