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SAN RAFAEL, Calif. — The tributes to Robin Williams flow from around the world as stunned friends and family search for answers about why the comic legend would take his own life.

Investigators believe Williams, 63, used a belt to hang himself from a bedroom door sometime between late Sunday and when his personal assistant found him just before noon Monday, according to Marin County Assistant Deputy Chief Coroner Lt. Keith Boyd.

The coroner’s investigation “revealed he had been seeking treatment for depression,” Boyd told reporters.

“He has been battling severe depression of late,” his media representative, Mara Buxbaum, said. “This is a tragic and sudden loss.”

The autopsy completed Tuesday morning showed “no indication of a struggle or physical altercation,” which was consistent with the death being a suicide, Boyd said.

A personal assistant found Williams “slightly suspended in a seated position” with a belt around his neck, he said.

Williams’ left wrist had cuts when he was found dead in a bedroom at his home, Boyd said. A pocket knife was found near his body, and a red material consistent with dried blood was found on the knife, Boyd said. He said tests will be conducted to determine whether the substance is blood.

Williams was last seen alive at about 10:30 p.m. Sunday, by his wife, when she went to bed, Boyd said. He apparently went into a bedroom at an unknown time after that, and his wife left the home at about 10:30 a.m. Monday, believing him to be asleep.

Williams’ personal assistant, concerned because he wasn’t responding to knocks on his door, entered the room and found him dead at about 11:45 a.m., Boyd said.

The actor made at least two trips to rehab for drug treatment, including a visit this summer, and he underwent heart surgery in 2009.

Williams married graphic designer Susan Schneider in a Napa Valley, Calif., ceremony in October 2011.

“I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings. I am utterly heartbroken,” Schneider said. “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.”