Williams' wife likely was home at the time he killed himself but was unaware, Marin County Sheriff's Lt. Keith Boyd said.
Williams was last seen alive by his wife Sunday night when she went to bed, Boyd said. She woke up the next morning and left, thinking he was still asleep elsewhere in the house.
Shortly after that, Williams' personal assistant came to the Tiburon home and became concerned when Williams failed to respond to knocks at a door. The assistant found the 63-year-old actor clothed and dead in a bedroom.
Boyd said all evidence indicates Williams, star of ''Good Will Hunting,'' `'Mrs. Doubtfire,'' `'Good Morning, Vietnam'' and dozens of other films, committed suicide by hanging himself with a belt. But he said a final ruling will be made once toxicology reports and interviews with witnesses are complete.
The condition of the body indicated Williams had been dead for at least a few hours, Boyd said. Williams also had superficial cuts on his wrist, and a pocketknife was found nearby.
Williams had been seeking treatment for depression, Boyd said. He would not say whether the actor left a suicide note.
''We still have people we want to speak with so there is some information we're going to withhold,'' Boyd said. ''We're not discussing the note or a note at this point as the investigation is ongoing.''
The Oscar-winning actor for years dealt with bouts of substance abuse and depression and referenced his struggles in his comedy routines. Just last month, Williams announced he was returning to a 12-step treatment program.
The circumstances of the death do not help explain what motivated him, suicide experts said. Understanding that would require a detailed ''psychological autopsy'' that includes the review of medical and other records, and interviews with family and friends.
These experts stressed that suicide rarely is triggered by a single factor, such as depression or substance abuse. Typically there are at least two such influences, often compounded by acute stress, such as from financial hardship or troubled personal relationships.
''We know from decades of research that there are numerous factors that contribute to suicide risk,'' said Michelle Cornette, executive director of the American Association of Suicidology.
Word that the actor had killed himself left neighbors in the quiet, waterfront neighborhood of Tiburon stunned and sparked an outpouring of grief and praise from his family and Hollywood colleagues.
''Yesterday, I lost my father and a best friend and the world got a little grayer,'' his son Zak Williams said in a statement Tuesday. ''I will carry his heart with me every day. I would ask those that loved him to remember him by being as gentle, kind, and generous as he would be. Seek to bring joy to the world as he sought.''
Makeshift memorials of flowers and notes popped up around the country including on his star on the Hollywood Walk of Fame, at his home and outside the house where the `80s sitcom ''Mork & Mindy'' was set in Boulder, Colorado. People also gathered to remember Williams at a bench in Boston's Public Garden where he filmed a scene for ''Good Will Hunting.''
Ben Affleck, a co-star and co-writer on that movie, was among the legions of friends and fans who shared tributes online.
''Robin had a ton of love & did so much for so many,'' Affleck tweeted. ''He made Matt & my dreams come true. What do you owe a guy who does that Everything.''
Actor and comedian Ricky Gervais wrote: ''I am deeply saddened. He was a lovely man who would keep everyone laughing even if he wasn't feeling good himself.''