(CNN) -- Robin Williams was sober but was struggling with depression, anxiety and the early stages of Parkinson's disease when he died, his widow said Thursday.
The diagnosis of the progressive illness was "an additional fear and burden in his life," a person familiar with Williams' family told CNN on Thursday.
Williams was found dead in his Northern California home Monday from what investigators suspect was a suicide by hanging.
While fans and friends have looked for answers to why the 63-year-old comedy icon would take his own life, his wife, Susan Schneider, issued a written statement about Williams' health that he had kept a secret.
"Since his passing, all of us who loved Robin have found some solace in the tremendous outpouring of affection and admiration for him from the millions of people whose lives he touched," Schneider said. "His greatest legacy, besides his three children, is the joy and happiness he offered to others, particularly to those fighting personal battles.
"Robin's sobriety was intact and he was brave as he struggled with his own battles of depression, anxiety as well as early stages of Parkinson's disease, which he was not yet ready to share publicly.
"It is our hope in the wake of Robin's tragic passing, that others will find the strength to seek the care and support they need to treat whatever battles they are facing so they may feel less afraid."
Williams had been active as an actor in the last year of his life, performing in a CBS sitcom that was canceled this year and acting in four films that have yet to hit theaters.
It is not clear whether the early-stage Parkinson's disease affected his ability to work.
"Friends and family can usually detect changes in the Parkinson's patient including poor posture, loss of balance, and abnormal facial expressions," according to the National Parkinson Foundation. "During this initial phase of the disease, a patient usually experiences mild symptoms. These symptoms may inconvenience the day-to-day tasks the patient would otherwise complete with ease. Typically these symptoms will include the presence of tremors or experiencing shaking in one of the limbs."
Parkinson's disease "causes certain brain cells to die," according to the website of the National Institutes of Health. It is more likely to affect men than women and most often develops after age 50.
Williams used exercise and cycling to manage his stress and depression, and the prospect that the illness would prevent him from doing that was extremely upsetting, adding to the depression, the person familiar with his family said.
Fellow actor Michael J. Fox, who has Parkinson's and established the Michael J. Fox Foundation, said Thursday that he was unaware of his friend's condition.
"Stunned to learn Robin had PD. Pretty sure his support for our Fdn predated his diagnosis. A true friend; I wish him peace," Fox tweeted.
Investigators believe Williams used a belt to hang himself from a bedroom door sometime between late Sunday and when his personal assistant found him just before noon Monday, according to Marin County Assistant Deputy Chief Coroner Lt. Keith Boyd.
Boyd would not confirm or deny whether Williams left behind a letter, saying that investigators would discuss "the note or a note" later.
The coroner's investigation "revealed he had been seeking treatment for depression," Boyd said.
He spent time in a treatment facility in July, a time when his wife and representative have said he was battling depression.
Media reports at the time speculated that Williams had resumed drinking alcohol, but the statement from his wife appears to dispute those reports.
Williams entered rehab because of drug and alcohol addiction at least twice previously.
"Robin spent so much of his life helping others," his wife said. Whether he was entertaining millions on stage, film or television, our troops on the front lines, or comforting a sick child -- Robin wanted us to laugh and to feel less afraid."
CNN's Dan Simon and Carolyn Sung contributed to this report.