Williams' widow says he was 'chased by an invisible monster' in final months

Robin Williams' widow, Susan Schneider Williams, is opening up about the troubles her late husband endured before he died by suicide in 2014 and how the beloved star didn't fully understand why he was suffering.

After news of Williams' death broke, speculation emerged about how money, drugs or depression may have played a part, but Schneider Williams said there were other factors at play.

"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 told TODAY's Hoda Kotb. "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."

Schneider Williams, who shares her husband's story in the new documentary "Robin's Wish," says her husband died of Lewy body dementia, a neurological disease that "can lead to problems with thinking, movement, behavior, and mood," according to the National Institute on Aging. It's also the second-most common form of degenerative dementia after Alzheimer's disease, reports the Lewy Body Dementia Association.

"I was called in to sit down to go over the coroner's report. They sat me and down and said he essentially Robin died of diffused Lewy body dementia. They started to talk about the neurodegeneration. He wasn't in his right mind," Schneider Williams said.

"Lewy body dementia is a devastating illness. It's a killer. It is fast, it's progressive," said Dr. Bruce Miller, director Memory and Aging at the University of California San Francisco. "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."

Schneider Williams said finding out what led to her husband's death helped put some pieces of a puzzle together.

"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. I left there with a name of the disease, the thing that Robin and I had been searching for," she said.

The "Good Will Hunting" star had been diagnosed with Parkinson's disease, but feared he had dementia or schizophrenia, and turned paranoid while losing his grip on reality.

"I saw a guy who wasn't himself and he thought that was unforgivable," "Night at the Museum" director Shawn Levy said in the documentary.

As his condition progressed, Williams and his wife were told by doctors to sleep in separate beds, which confused the Oscar winner.

"He said to me, 'Does this mean we're separated?' And that was a really shocking moment," Schneider Williams said. "When your best friend, your partner, your love, you realize that there's a giant chasm somewhere, and you can't see where it is. But that's just not based in reality. That was a hard moment."

Schneider Williams, who wrote candidly about her husband's struggle in an essay for the American Academy of Neurology in 2016, hopes to shine a light on brain disease through "Robin's Wish." She says it was always her husband's desire to help others.

"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 said. "I thought it was beautiful. And I said, 'Honey, you're already doing that. That's what you do.' And that is pretty great."

If you or someone you know is at risk of suicide please call the U.S. National Suicide Prevention Lifeline at 800-273-8255, text TALK to 741741 or go to SpeakingOfSuicide.com/resources for additional resources.