Emergency personnel were called to Williams's home in Tiburon, California, at 11:55 a.m. local time, per the Marin County Sheriff's Office.

Williams was found unconscious and pronounced dead at the scene. Authorities are investigating the death, and an autopsy is forthcoming, but initial evidence points to "a suicide due to asphyxia," according to Marin Sheriff's Lt. Keith Boyle.

In a brief statement, publicist Mara Buxbaum said Williams had 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."

Robin and wife Susan Schneider (Paul Zimmerman/WireImage)

"This morning, I lost my husband and my best friend," said Williams's wife, Susan Schneider.

"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."

In July, Williams entered rehab for the second time, but not because of a relapse.

"After working back-to-back projects, Robin is simply taking the opportunity to fine-tune and focus on his continued commitment, of which he remains extremely proud," the actor's publicist said at the time.

Williams reportedly spent a few weeks at Hazelden Addiction Treatment Center in Minnesota.

The actor spoke publicly about his battle with substance abuse. In 2006, he sought treatment for alcoholism after 20 years of sobriety. A rep for the actor said in a statement that Robin "found himself drinking again and has decided to take proactive measures to deal with this for his own well-being and the well-being of his family."

During an interview with Good Morning America months later, Williams explained that falling back into alcohol abuse was "very gradual."

"It's the same voice thought that… you're standing at a precipice and you look down, there's a voice and it's a little quiet voice that goes, 'Jump,'" Williams told Diane Sawyer. "The same voice that goes, 'Just one.' … And the idea of just one for someone who has no tolerance for it, that's not the possibility."

When asked why he relapsed, Robin answered: "It's [addiction] — not caused by anything, it's just there… It waits. It lays in wait for the time when you think, 'It's fine now, I'm OK.' Then, the next thing you know, it's not OK. Then you realize, 'Where am I? I didn't realize I was in Cleveland.'"

The actor admitted he also struggled with cocaine abuse in the early 1980s while starring in the sitcom Mork and Mindy, but quit cold turkey after his friend John Belushi's fatal overdose in 1982.

"Cocaine for me was a place to hide," Williams told People in 1988. "Most people get hyper on coke. It slowed me down. Sometimes it made me paranoid and impotent, but mostly it just made me withdrawn. And I was so crazy back then — working all day, partying most of the night — I needed an excuse not to talk. I needed quiet times and I used coke to get them."

A Chicago native, Williams spent time at Juilliard, where he was briefly classmates with Christopher Reeve.

After an appearance on NBC's Richard Pryor Show, he got his big break, playing the wacky alien Mork in what was supposed to be a one-off guest role on ABC's Happy Days. "We started rehearsing and I realized that I was in the presence of greatness," star Henry Winkler recalled on CNN Monday night. "No matter what you said to him, no matter what line you gave to him, he took it in he processed it and it flew out of his mouth never the same way twice and it was incredibly funny every time."

The character proved so popular that he commanded his own spinoff. Mork & Mindy, costarring Pam Dawber and Jonathan Winters, became a phenomenon, spiking sales of rainbow-colored suspenders and adding "nanu-nanu" and "shazbot" to the lexicon. The sitcom ran from 1978 to 1982. "I am completely and totally devastated," Dawber said in a statement Monday afternoon. "What more can be said?!"

Williams made the leap to film with a string of wildly successful movies, showcasing a range beyond the motor-mouthed improvisation he built his standup career on: The World According to Garp, Moscow on the Hudson, Awakenings, The Birdcage, Mrs. Doubtfire, Patch Adams, A.I. Artificial Intelligence, and Insomnia. He carved out a rich career as a voice-over actor in such animated features as Aladdin, Happy Feet, Robots, and FernGully.

He earned Oscar nods for Dead Poets Society, The Fisher King, and Good Morning, Vietnam, before finally capturing the elusive statuette by upstaging Matt Damon and Ben Affleck with a touching supporting role in Good Will Hunting.

Among the films he completed before his death were May's The Angriest Man in Brooklyn, with Peter Dinklage and Mila Kunis, and the upcoming Night at the Museum: Secret of the Tomb, where he reprised his role of Teddy Roosevelt in the Ben Stiller-led franchise. The latter is slated to open in December.

Despite being one of Hollywood's most bankable movie stars, Williams never forgot his TV roots. Through the years he fronted several standup concerts and joined with Whoopi Goldberg and Billy Crystal to launch the Comic Relief television specials. Williams also made memorable appearances on Friends, Homicide: Life on the Street, Law & Order: SVU, and The Larry Sanders Show.

He returned to television in last season's CBS comedy series The Crazy Ones opposite Sarah Michelle Gellar. The show was canceled in May.

In addition to his Academy Award, Williams won two Emmys, five Grammys, and six Golden Globes (including the career-capping Cecil B. DeMille Award). He also won the admiration of generations of top comedic talent.