SAN RAFAEL, Calif. (AP) — Oscar-winner Robin Williams, whose free-form comedy and adept impressions dazzled audiences for decades, has died in an apparent suicide. He was 63.

The Marin County Sheriff's Office says Williams was pronounced dead at his home in California on Monday. The sheriff's office says a preliminary investigation shows the cause of death to be a suicide due to asphyxia.

His publicist Mara Buxbaum said in a statement: "He has been battling severe depression of late. This is a tragic and sudden loss. The family respectfully asks for their privacy as they grieve during this very difficult time."

Williams shot to fame in the late '70s as the alien in TV's "Mork and Mindy." He conquered the big screen in comic films such as "Good Morning, Vietnam" and "Mrs. Doubtfire." But he won his Academy Award in a serious role — as the therapist in "Good Will Hunting."

His performing style was at its purest in his standup act, as he impersonated a Russian immigrant or parodied anyone from John Wayne to Keith Richards.

Last month, Williams made a brief stay at the well-known Hazelden chemical treatment program in Center City, Minnesota.  At the time, his publicist said it was an opportunity for him to experience some down time after a non-stop work schedule that spanned 18 months.

Williams had

been open about the challenges of maintaining sobriety. He sought treatment in 2006 when he relapsed and returned to drinking after 20 years.