Los Angeles, California: The iconic comedian Robin Williams made two attempts on his life in his final hours, according to details released by the Marin County Sheriff's Department.

Marin County's assistant chief deputy coroner, Keith Boyd, said Williams' body was discovered by his personal assistant just prior to midday on Monday.

The assistant became concerned when Williams failed to respond to a knock on his bedroom door.

Gaining access to the room, the assistant found Williams' body, Mr Boyd told a news conference.

Williams died by asphyxia, the coroner said on Tuesday, based on the preliminary findings.

A toxicology report will be conducted but will take between two and six weeks to be completed.

Mr Boyd suggested Williams had been dead for some time.

Williams had been seeking treatment for depression, Mr Boyd said. He would not discuss if Williams had left a suicide note.

Funeral arrangements would be decided by the family, Mr Boyd said. Williams' body has been released by the coroner facility in neighbouring Napa County.

Mr Boyd would not say if any drugs or alcohol were involved, stressing that the investigation was continuing.

Comedians, politicians and several generations of fans collectively mourned the death of Williams, famous for his frenetic and freewheeling comedy.

The news of Williams' death rippled across social media, stunning fans young and old and comedians who had been influenced by Williams since he broke out in the 1970s TV comedy Mork & Mindy as a strange and loveable creature from outer space.

Williams had been open about his struggles with alcohol and cocaine and in the past months had entered a rehabilitation centre to help him maintain sobriety.

But many questions remained over his final months and what could have led to his suicide.

On the Hollywood Walk of Fame, dozens of fans congregated around Williams' star early on Tuesday, leaving flowers and candles to honour the versatile actor.

Williams' appeal stretched across generations and genres, from family fare as the voice of Disney's blue Genie in Aladdin to his portrayal of a fatherly therapist in the 1997 drama Good Will Hunting, for which he earned his sole Oscar.

The death of Williams shook Hollywood and colleagues mourned the loss of what many called a big-hearted man and one of the most inventive comedians of his time.

Sarah Michelle Gellar, who played his daughter in the recent CBS television comedy The Crazy Ones, said her life was better for knowing Williams. Their show was cancelled in May after one season.

"To my children he was Uncle Robin, to everyone he worked with, he was the best boss anyone had ever known, and to me he was not just an inspiration but he was the father I had always dreamed of having," Gellar said in a statement. 

Support is available for those who may be distressed by phoning Lifeline 13 11 14; Mensline 1300 789 978; Kids Helpline 1800 551 800.

SMH with Reuters