SAN RAFAEL, Calif. (AP) -- Robin Williams committed suicide by hanging himself with a belt at his San Francisco Bay Area home, sheriff's officials said Tuesday. Marin County Sheriff's Lt. Keith Boyd said Williams' personal assistant found the actor Monday in a bedroom at his Tiburon home. The actor also had superficial cuts on his wrist, and a pocketknife was found nearby. Boyd said Williams, star of "Good Will Hunting," ''Mrs. Doubtfire," ''Good Morning, Vietnam" and dozens of other films, was seeking treatment for depression. He would not say whether the actor and comedian left a suicide note. The 63-year-old comedian's wife had last seen him the night before and had left the home that morning thinking he was still asleep. His personal assistant later came to the home and became concerned when he knocked on the door and got no response. Toxicology test results on whether Williams had any drugs or alcohol in his system are weeks away. Boyd said authorities will continue to investigate his death. The actor had periodic bouts of substance abuse and depression for years. Just last month, Williams announced he was returning to a 12-step treatment program. 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 neighbors in Tiburon equally stunned and grief-stricken. Williams had lived in the quiet, waterfront neighborhood for eight years, according to neighbors. Noreen Nieder said Williams was a friendly neighbor 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.