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. 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; 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.
Suicide 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 financial hardship or troubled personal relationships.
Ben Affleck, a co-star and co-writer on the movie "Good Will Hunting" 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." Need help?
The National Suicide Prevention Lifeline is available at 1-800-273- TALK (8255).