Marin County Sheriff's Office said it suspects the death was a suicide due to asphyxia, but the cause of death is still under investigation.
In a statement, Marin County Sheriff's Office said: "At this time, the Sheriff's Office Coroner Division suspects the death to be a suicide due to asphyxia, but a comprehensive investigation must be completed before a final determination is made."
It added that an investigation into the "cause, manner, and circumstances of the death is currently under way".
A forensic examination is expected to take place today, with toxicology tests to take place later.
Police were called to Williams' home in the town of Tiburon at around 11.55am yesterday after receiving a call that a man was unconscious and not breathing.
Emergency services, including police and firefighters, arrived on the scene five minutes later, but the actor was pronounced dead at 12.02pm, the sheriff's office said.
It added that Williams was last seen alive at his home, where he lived with his wife Susan Schneider, at around 10pm on Sunday.
Mara Buxbaum, Williams' press representative, said: "Robin Williams passed away this morning. He has been battling severe depression of late.
"This is a tragic and sudden loss. The family respectfully asks for their privacy as they grieve during this very difficult time."
In a statement Ms Schneider said: "This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings.
"I am utterly heartbroken. On behalf of Robin's family, we are asking for privacy during our time of profound grief. As he is remembered, it is our hope the focus will not be on Robin's death, but on the countless moments of joy and laughter he gave to millions."
Stars spoke of their shock at Williams' death.
Steve Martin, who appeared alongside Williams in a 1988 theatre production of Waiting for Godot, said: "I could not be more stunned by the loss of Robin Williams, mensch, great talent, acting partner, genuine soul."
Talk show host Ellen DeGeneres tweeted: "I can't believe the news about Robin Williams. He gave so much to so many people. I'm heartbroken."
Police are expected to release more information about his death at a press conference scheduled for 7pm British time today.
The thrice-married father-of-three was known to millions for his performances in films that ranged from the out-and-out manic comedy of Mrs Doubtfire, to more thought-provoking fare like Dead Poets Society and The Fisher King.
In many films, like Good Morning Vietnam, Williams attracted widespread praise by successfully combining his gift for comedy with an exploration of more serious underlying themes.
Nominated for the best actor Oscar three times, Williams, he won the Academy Award for Best Supporting Actor for the 1997 film Good Will Hunting, in which he played the therapist to Matt Damon’s working class maths genius.
Born in Chicago, he started off in stand-up comedy before first achieving widespread fame in the TV sitcom Mork and Mindy. The show ran from 1978 to 1982 and Williams was frequently allowed to indulge his talent for improvised comedy while playing Mork, an alien living on Earth.
As Mork and Mindy became increasingly popular, Williams started to reach an even wider audience with a series of televised stand-up comedy shows.
During the late Seventies and early Eighties, however, Williams developed an addiction to cocaine.
Williams was a close friend of the comedian John Belushi who died of a cocaine and heroin overdose in 1982. Williams had enjoyed wild parties with Belushi and subsequently admitted that the death of his friend and the birth of his son prompted him to quit drugs: "Was it a wake-up call? Oh yeah, on a huge level.”
On August 9, 2006, however, Williams checked himself into a rehab centre, later admitting he was an alcoholic. His publicist announced: “After 20 years of sobriety, Robin Williams found himself drinking again and has decided to take proactive measures to deal with this for his own well-being and the well-being of his family.”
Three years later, in March 2009 Williams was hospitalised by heart problems, and had to undergo surgery to surgery to replace his aortic valve.
A keen charity fundraiser Williams had his second wife, Marsha, founded the Windfall Foundation, with his ex-wife Marsha, to raise money for a wide variety of charities.
He leaves three children and a widow, Susan, his third wife, whom he married in 2011.
If you need someone to listen call the Samaritans in the UK for confidential support on 08457 90 90 90, visit a local Samaritans branch or click here for details
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