TIBURON — Robin Williams, the former College of Marin student who rose to stardom as a comedian and Oscar-winning actor, was found dead Monday at his home in Tiburon.

The cause of death is pending, but the preliminary investigation suggests he died of suicide by asphyxiation, said sheriff’s Lt. Keith Boyd, assistant county coroner.

Williams had turned 63 on July 21.

An autopsy was planned for Tuesday.

Authorities received a 911 call at about 11:55 a.m. Monday from Williams’ home in unincorporated Tiburon, where he was found unconscious. Williams lived there with his wife, Susan Schneider.

Sheriff’s deputies and Tiburon and Southern Marin firefighters responded, and Williams was pronounced dead at 12:02 p.m.

Williams was last seen alive at about 10 p.m. Sunday at his home.

Boyd declined to release additional details as the investigation was continuing.

Williams, who had spoken publicly about his bouts with alcohol and cocaine use, had checked into a rehab facility in early July to reinforce his sobriety, according to news reports. He also had a heart valve operation in 2009.

“He has been battling severe depression of late,” Williams’ publicist, Mara Buxbaum, told the Hollywood Reporter, a newspaper covering the entertainment industry. “This is a tragic and sudden loss. The family respectfully asks for their privacy as they grieve during this very difficult time.”

The newspaper also published a statement from Williams’ wife: “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.”

Robin McLaurin Williams was born in Chicago and spent part of his childhood in Michigan until the family moved to Marin. He graduated from Redwood High School in Larkspur in 1969 and attended College of Marin before launching his entertainment career.

After achieving fame on the television comedy “Mork & Mindy,” he went on to star in dozens of movies. He was nominated for the Academy Award four times and won for his role in “Good Will Hunting” in 1997.

“Winning the Oscar has a short half-life,” Williams told the Marin Independent Journal in a 1999 interview. “At first people say, ‘Oh, you won an Academy Award.’ Then it’s ‘Didn’t you win an Academy Award? Do you know Matt Damon?'”

Other notable films included “Good Morning, Vietnam” (1987), “Mrs. Doubtfire” (1993) and “Patch Adams” (1998) and “The Fisher King” (1991). He also won or was nominated for numerous awards in the United States and abroad, including the Golden Globe and the Emmy.

News of Williams’ death spread quickly Monday afternoon, leading to an outpouring of tributes on social media.

“He gave so much to so many people,” comedian Ellen DeGeneres posted on Twitter. “I’m heartbroken.” “Robin Williams was like no other,” actor Henry Winkler said on Twitter. “To watch him create on the spot was a privilege to behold..Robin you are an angel now !!! REST IN PEACE.”

According to the Los Angeles Times, Williams’ survivors include three adult children, Zachary, from his first marriage, to Valerie Velardi, and Zelda and Cody, from his second marriage, to Marsha Garces. He married Schneider in 2011.

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