U.S. actor Robin Williams was found dead Monday in his home in Northern California, according to a statement issued by the mayor's office of Marin County, which is investigating what it said is the possible suicide of the Oscar winner. He was 63.

According to his publicist, Williams had "battling severe depression" in the period before his death, though his publicist would not confirm the reports that he took his own life.

The Marin County Sheriff's Office, however, said that it had tentatively concluded that Williams died by suicide, specifically "asphyxia," although it provided no further details.

The star of many Hollywood comedies was found unconscious and not breathing at his home in Tiburon, very near San Francisco, by paramedics called to the house and he was pronounced dead at the scene at 12:02 p.m.

Williams burst into the public eye with his offbeat role as alien Mork on the 1970s television series "Happy Days," a role he parlayed into his own 1978-1982 TV series "Mork & Mindy," and in the years since then he starred in numerous Hollywood films, including "Good Morning, Vietnam," "Dead Poets Society," "Jumanji," "Mrs. Doubtfire" and "Good Will Hunting," winning the Oscar for Best Supporting Actor for the latter in 1997.

He also received two Emmy Awards, four Golden Globes, two Screen Actors Guild Awards and five Grammy Awards in addition to pursuing a successful decades-long and highly acclaimed standup comedy career. In 1986, he co-hosted the 58th Academy Awards.

Williams was well-known for his quirky sense of rapid-fire humor, and also for his improvisational skills and impersonations, but some of his film characters - while often comedic to the extreme - also displayed an emotional depth and poignancy that was surprisingly moving.

Over the years he had battled cocaine and alcohol addiction, as well as depression.

The last time he was seen alive at his home, where he lived with his third wife, Susan Schneider, about 10 a.m. Monday, according to CNN.

Authorities have opened an investigation into the actor's sudden death and an autopsy will be conducted on Tuesday. EFE