SAN RAFAEL, Calif. -- Robin Williams died of suicide at his San Francisco Bay Area home, sheriff's officials said Tuesday.

Marin County Sheriff's Lt. Keith Boyd said Williams was found in a bedroom by his personal assistant on Monday at his Tiburon home.

Boyd said toxicology tests will be performed and the investigation is ongoing.

Sheriff's officials said Monday a preliminary investigation determined the cause of death was suicide due to asphyxia. Williams was 63 and had suffered for years from periodic bouts of substance abuse and depression.

Williams' press representative Mara Buxbaum said the actor had been battling severe depression recently. Just last month, Williams announced he was returning to a 12-step treatment program.

Coroner's officials say he was last seen alive at home around 10 p.m. Sunday.

Shortly before noon Monday, the Sheriff's Department received an emergency call from the home, where the star of "Good Will Hunting," "Mrs. Doubtfire," "Good Morning, Vietnam" and dozens of other films was pronounced dead.

Williams made reference to his substance abuse and depression in his comedy routines, including when he sought treatment in 2006 after a relapse that followed 20 years of sobriety.

Williams joked about that fall off the wagon during a comedy tour, saying: "I went to rehab in wine country to keep my options open."

Likewise, when word spread about his struggles with drugs in the early 1980s, Williams responded with a joke that for a time became a catchphrase for his generation's recreational drug use: "Cocaine is God's way of telling you you are making too much money."

Word that he had killed himself left neighbours in Tiburon equally stunned and grief-stricken. Williams had lived in the quiet, waterfront neighbourhood for eight years, according to neighbours.

Noreen Nieder said Williams was a friendly neighbour who always said hello and engaged in small talk. Nieder said she wasn't close to Williams and his family, but she still felt comfortable enough to approach him and ask him about his latest stint in drug and alcohol rehabilitation.

"He was very open about it," Nieder said. "He told me he was doing well."

Fans and friends placed bouquets, candles and personal notes in front of the locked gates of Williams' house.

The Antonio family visited Williams' home Tuesday morning, driving from San Rafael about 10 miles away to drop off flowers.

"He was my favourite actor," said a weeping Brandon Antonio, 13. "He was so funny."

Antonio said Williams' 1995 movie "Jumanji" was his favourite film.