Robin Williams' widow is opening up about the reason behind her husband's 2014 death. In an interview on Tuesday's episode of the Today show, Susan Schneider Williams recalled trying to understand her husband's struggles prior to his death. Williams died by suicide in 2014.

"Robin and I knew there was so much more going on. Robin was right when he said to me, 'I just want to reboot my brain,'" she said. "In that moment, I promised him that we would get to the bottom of this and I just didn't know that would be after he passed."

While reports at the time speculated that depression was to blame for Williams' suicide, his wife learned of the true cause after his death.

"I was called in to sit down to go over the coroner’s report. They sat me down and said he essentially died of diffused Lewy body dementia," she says in a clip from the upcoming documentary Robin's Wish, which aired on the Today show. "They started to talk about the neurodegeneration. He wasn’t in his right mind. It makes sense why he was experiencing what he was experiencing."

Williams was diagnosed with Parkinson's disease prior to his death, a condition that shares symptoms with Lewy body dementia, according to The Mayo Clinic. Other symptoms of Lewy body dementia include problems with thinking, memory and movements. Prior to his death, Williams even became paranoid and lost his grip on reality.

"Lewy body dementia is a devastating illness. It's a killer. It is fast, it's progressive," Dr. Bruce Miller, director of Memory and Aging at the University of California San Francisco, says in the documentary clip. "This was about as devastating a form of Lewy body dementia as I had ever seen. It really amazed me that Robin could walk or move at all."

For Schneider Williams, learning of her husband's condition brought her a sense of relief.

"I was relieved it had a name. Robin and I had gone through this experience together, really being chased by an invisible monster. And it was like whack-a-mole with the symptoms," she told the Today show. "I left there with a name of the disease, the thing that Robin and I had been searching for."

Though that knowledge came only after his death, Schneider Williams is completing her husband's dying wish through her work as the vice chair of the board at the American Brain Foundation.

"I asked him, 'When we get to the end of our lives and we're looking back, what is it we want to have done?' Without missing a beat, he said, 'I want to help people be less afraid,'" she recalled. "I thought it was beautiful. And I said, 'Honey, you're already doing that. That's what you do.' And that is pretty great."

Williams' widow remembered her late husband as "the greatest love I've ever known, my best friend, my partner."

"This was a man who was incredibly rich and deep and versed in so much about humanity and culture. And his humor was like his secret weapon," she said. "There were so many times when he would see someone needed a lift and then he would inject a little bit of humor in just the right way to make a difference."

Robin's Wish is available on demand now.

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