Marin County deputies responded to an emergency call from Williams' home in unincorporated Tiburon, California, at 11:55 a.m., reporting "a male adult had been located unconscious and not breathing," the release from the sheriff said.  Williams was pronounced dead at 12:02 p.m., it said.  Williams was last seen alive at his home, where he lives with his wife, at about 10 p.m. Sunday, the sheriff's statement said. "An investigation into the cause, manner and circumstances of the death is currently underway by the Investigations and Coroner Divisions of the Sheriff's Office," the sheriff's statement said. "Coroner Division suspects the death to be a suicide due to asphyxia, but a comprehensive investigation must be completed before a final determination is made." An autopsy is scheduled for Tuesday, the sheriff said.  Williams made at least two trips to rehab for drug treatment, including a visit this summer, and he underwent heart surgery in 2009.

Williams, born in Chicago on July 21, 1951, studied theater at Juilliard School before taking his stand up act to nightclubs. He was cast as Mork, an alien visitor to Earth, for a 1974 episode of television's "Happy Days."  "Happy Days" star Henry Winkler said it was "unimaginable that this is the reality today, that this incredible human being, incredible, delicate, funny, dramatic human being is gone."  Winkler said he "realized I was in the presence of greatness" at Williams' first rehearsal as Mork.  "I just realized my only job is to keep a straight face," said Winkler, who played "The Fonz." "And it was impossible. Because no matter what you said to him, no matter what line you gave to him, he took it in, processed it, and then it flew out of his mouth, never the same way twice. And it was incredibly funny every time."  The role led to the spin-off show "Mork & Mindy," which showcased Williams' usual comic improvisation talents.