LOS ANGELES, Aug. 12 -- U.S. Oscar-winning comedian Robin Williams committed suicide by hanging at his home in North California, the local coroner's office said Tuesday.

Lt. Keith Boyd of the Marin County coroner's office said Williams was last seen by his wife, Susan Schneider, when she went to bed around 10:30 p.m. Sunday. She left their home around 10:30 a.m. Monday, believing her husband was still asleep in a different room.

"Mr. Williams' personal assistant became concerned at approximately 11:45 a.m. when he failed to respond to knocks on his bedroom door," Boyd said.

"At that time, the personal assistant was able to gain access to Mr. Williams' bedroom and entered the bedroom to find Mr. Williams clothed, in a seated position, unresponsive, with a belt secured around his neck, with the other end of the belt wedged between the closed closet door and the door frame," he added.

"His right shoulder area was touching the door with his body perpendicular to the door and slightly suspended," Boyd said. "Mr.Williams at that time was cold to the touch with rigor mortis present in his body.

The inside of Mr. Williams' left wrist had several acute superficial transverse cuts. A pocketknife, with red material on its blade, was in close proximity to Mr. Williams.

Boyd said the pocketknife was also examined, but it is unknown at this time if the dried red material is in fact blood or if it is Mr. Williams' blood.

The coroner's office announced Monday that physical signs showed Mr. Williams died from asphyxia due to hanging, while further forensic examination was conducted Tuesday morning.

Boyd said toxicology tests will be conducted to determine if Williams had any drugs or alcohol in his system, but those tests would take two to six weeks to complete.

Williams' media representative, Mara Buxbaum, told media Monday that Williams, 63, had been battling severe depression.

Williams' star on the Hollywood Walk of Fame was turned into a memorial after the news of his death was released on Monday. Fans paid respects to the talented comedic actor by placing flowers around his star.

U.S. President Barack Obama on Monday offered condolences to Williams' family and friends. "He made us laugh. He made us cry. He gave his immeasurable talent freely and generously to those who needed it most -- from our troops stationed abroad to the marginalized on our own streets," he said.

On the Sunset Strip in Los Angeles, the Laugh Factory changed its marquee to read: "Robin Williams. Rest in Peace. Make God Laugh."

The TCL Chinese Theater on Hollywood Boulevard will dim the lights in its forecourt for about one minute at 8:30 p.m. Tuesday local time (0330 GMT Wednesday) in tribute to Williams. It is the eighth time in the theater's 87-year-old history to do so.

The actor's first film, "Popeye," debuted at the Chinese Theater, and he left his hand and foot prints in the forecourt in 1998.

A Chicago native, Williams studied theater at Julliard before being a stand-up comedian, which led to his casting as the offbeat alien Mork from Ork on a 1974 episode of "Happy Days." The character was so popular that it led to the spinoff "Mork & Mindy."

His film career earned him four Oscar nominations, including a win for his supporting role opposite Matt Damon and Ben Affleck in "Good Will Hunting." He was nominated as lead actor for his roles in "Good Morning, Vietnam," "Dead Poets Society" and "The Fisher King."

Williams most recently starred in the CBS series "The Crazy Ones." Williams' other films include "Patch Adams," "The Birdcage," "Mrs. Doubtfire," "Jumanji," "Hook," "Toys" and "Awakenings."