(CNN) -- 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 at his home in California, according to Marin County Assistant Deputy Chief Coroner Lt. Keith Boyd.
Boyd would not confirm or deny whether Williams left behind a letter, saying that investigators would discuss "the note or a note" later.
The coroner's investigation "revealed he had been seeking treatment for depression," Boyd told reporters.
"He has been battling severe depression of late," Williams' media representative, Mara Buxbaum, told CNN on Monday. "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.
The personal assistant found Williams "clothed in a seated position, unresponsive, and with a belt secured around his neck with the other end of the belt wedged between the closed closet door and door frame," he said.
Williams' left wrist had cuts, 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. His wife left the home at about 10:30 a.m. Monday, assuming Williams was still 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.
It will take several weeks for toxicology test results to show whether Williams was under the influence of drugs or alcohol when he died, 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 was remembered fondly by actors, comedians, fans, even the president of the United States -- who described him as "an airman, a doctor, a genie, a nanny, a president, a professor, a bangarang Peter Pan (who) was one of a kind."
"He made us laugh," Obama said. "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."
His loss was felt deeply in many circles, but no more so than his own family.
His wife Susan Scheider -- a graphic designer whom he married in October 2011 -- asked the world to focus "not on Robin's death, but on the countless moments of joy and laughter he gave to millions."
Others in his family, including his three children and an ex-wife, praised him as a warm, loving man who always tried "to bring joy to" those around him, as eldest son Zak noted.
"Dad was, is and always will be one of the kindest, most generous, gentlest souls I've ever known," his daughter Zelda said. "... I know ... not just my world, but the the entire world is forever a little darker, less colorful and less full of laughter in his absence. We'll just have to work twice as hard to fill it back up again."
All he needed was an audience of one
On Tuesday, someone who knew Robin Williams well described the man behind the legend.
"There had to be two people in the room" with him, his friend Bob Zmuda told CNN's "New Day." "Then you were an audience, and then he came alive."
But one-on-one, Zmuda said, Williams "had no social skills. He couldn't handle it. ... I knew this man for 35 years and yet it was like I was in an elevator with a stranger."
Zmuda, a comedy writer, created Comic Relief, a program that raised funds for those in need through comedy. Williams hosted it, along with Billy Crystal and Whoopi Goldberg.
Sarah Michelle Gellar played Williams' daughter in the CBS comedy "The Crazy Ones," which was recently canceled after one season.
She told People magazine that "to everyone he worked with, he was the best boss anyone had ever known," and both an inspiration and father figure to her. "And to his family, I thank them for letting us know him and seeing the joy they brought him."
In character, Williams warned against suicide
Williams was known to be generous to fellow stand-up comedians.
"Riffing with Robin Williams was extremely invigorating -- and extremely exhausting," comic Gilbert Gottfried wrote in a column for CNN.com. "I knew I had to be on my toes every second. And when we would actually connect onstage, it was electric for me."
"I've known people who have committed suicide, and my shock always reveals how little I knew about them," Gottfried wrote.
In the 2009 film "World's Greatest Dad," Williams played a man named Lance Clayton -- and delivered a line now making the rounds online: "If you're that depressed, reach out to someone. And remember: Suicide is a permanent solution to temporary problems."
From Julliard to 'Happy Days'
Born in Chicago on July 21, 1951, Williams 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 1978 episode of television's "Happy Days."
"Happy Days" star Henry 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."
It is "unimaginable that this is the reality today, that this incredible human being, incredible, delicate, funny, dramatic human being is gone," Winkler said.
The role led to the spinoff show "Mork & Mindy" which showcased Williams' unusual comic improvisation talents from 1978 through 1982.
He proved his dramatic acting skills in "Good Will Hunting," a 1997 film that earned him a best supporting actor Oscar.
His memorable movies over the past three decades include "Good Morning, Vietnam," "Dead Poets Society," "Mrs. Doubtfire" and "The Birdcage."
Williams credited the influence of Jonathan Winters' comic irreverence and quirky characters as a great influence on his comedy. The connection between the two was completed when Winters was cast as Williams' son on "Mork & Mindy."
When Winters died in 2013, Williams said he was "my idol, then he was my mentor and amazing friend." He tweeted that Winters was his "Comedy Buddha."
Williams and his "Mork & Mindy" co-star Pam Dawber reunited on TV earlier this year on an episode of the CBS comedy "The Crazy Ones."
Williams' fans can look forward to four more movie appearances coming to theaters, including another installment in the "Night at the Museum" franchise.
Dawber summed up the feelings of many in a simple statement: "I am completely and totally devastated. What more can be said?"
Once on the TV program "Inside the Actors Studio," Williams was asked, "If Heaven exists, what would you like to hear God say when you arrive at the pearly gates?"
After a joke about a concert including Mozart and Elvis, he added, "to know that there's laughter."
CNN's Travis Sattiewhite, Rachel Wells and Carolyn Sung contributed to this report.