updated 08/12/2014 AT 1:55 PM ET
•originally published 08/12/2014 AT 2:25 PM ET
Robin Williams’s cause of death was suicide by asphyxiation, authorities in Marin County, California, have confirmed.
In a press conference held Tuesday by local law enforcement and the coroner’s office in the San Francisco Bay area, authorities revealed that the Oscar winner, who died Monday at age 63, hanged himself in his Tiburon, California, home.
He was found in his bedroom on Monday by his personal assistant before a 911 call was placed at 11:55 a.m., according to Lt. Keith Boyd, assistant deputy chief coroner of the Marin County Sheriff’s Office.
Williams was found “clothed, slightly suspended in a seated position” and unresponsive with a belt around his neck, said Boyd, with the other end of the belt closed in the closet. His left wrist had several acute, superficial cuts, and a pocket knife was found nearby.
Williams’s assistant called 911 and “was distraught and indicated that it was an apparent suicide due to a hanging and that rigor mortis had set in,” said Boyd. Williams was pronounced dead by firefighters at 12:02 p.m.
Boyd declined to answer questions regarding whether a note was found or there were any signs of drugs or alcohol. The full forensic examination and toxicology results will be available in approximately two to six weeks, he said, with the final cause of death not available until then.
Boyd also noted that the actor had been undergoing treatment for depression.
Williams was last seen alive by his wife, Susan Schneider, at approximately 10:30 p.m. on Aug. 10. She returned to the home soon after he was found.
The actor, who most recently starred in the CBS sitcom The Crazy Ones, had checked himself into rehab in early July after battling alcohol and drug addiction throughout his life.
The star had been battling severe depression, Williams’s rep said after the death.
Williams had been a consummate pro on the set of The Crazy Ones, which was canceled in May after one season.
“We knew he was struggling,” a source who worked with him on the series tells PEOPLE in this week’s cover story.
The source recalls filming a scene when “his face changed. He looked so exhausted and profoundly, deeply sad. And then one minute later he pulled himself back together and nailed the scene.”
For more on Robin Williams’s tragic death and his legacy as a comic genius, pick up the latest issue of PEOPLE, on newsstands Friday