Carrie Fisher in January 2015 at the Screen Actors Guild Awards in Los Angeles. (Photo: Vince Bucci/Invision/AP)

Tributes for Carrie Fisher continued to mount a day after her shocking death — including from ex-husband Paul Simon — as attention turned from her careers as an iconic movie star and best-selling author to her courage in acknowledging her mental illness.

Funeral arrangements for Fisher, who was 60 when she died Tuesday at UCLA Medical Center four days after she suffered a medical emergency on a flight from London to LAX on Friday, are still pending.

Also pending is the exact cause of her death, whether it was a heart attack or sudden cardiac arrest, and whether it may have been related to her history of substance abuse.

'Star Wars' fan Arielle Hazel places flowers for Carrie Fisher at the movie's hand and footprint cement block in Hollywood on Dec. 27, 2016. (Photo: EUGENE GARCIA, EPA)

By tragic coincidence, British pop icon George Michael was found dead Sunday in his country home outside London, the victim of "heart failure" according to his manager. He was only 53 and also had a long history of substance abuse.

Meanwhile, because Fisher did not have a star on the Hollywood Walk of Fame, fans have taken it upon themselves to give her an unofficial star. In front of Hollywood Boulevard's historic Chinese Theatre, they've taken over a blank sidewalk star, placing candles and flowers and paste-on letters spelling her name and the phrase, "May the force be with you always."

The scene at Carrie Fisher's improvised Walk of Fame star. pic.twitter.com/K865wZG2ft — Ryan Parker (@TheRyanParker) December 28, 2016

The latest:

Simon breaks silence: The equally iconic singer/songwriter tweeted early Wednesday, capturing the sense of the widespread reaction to Fisher's death: She was too young to die.

"Yesterday was a horrible day. Carrie was a special, wonderful girl. It's too soon," Simon tweeted.

"Yesterday was a horrible day. Carrie was a special, wonderful girl. It's too soon."

-Paul Simon — Paul Simon (@PaulSimonMusic) December 28, 2016

Simon, 75, was Fisher's only husband and they were married for just a short period, 1983-84. But they remained close friends, and she kept in touch with his son from an earlier marriage, Harper.

Carrie Fisher and singer Paul Simon stand together at their apartment in New York during their wedding reception in August 1983. (Photo: Mario Suriani, AP)

Cause of death: Why did she die? Was it a "massive heart attack" that felled her on that plane, as TMZ put it? And will there be an autopsy?

These questions are still open but it's worth remembering that sometimes what looks like a "heart attack" isn't. Alan Thicke, 69, who died Dec. 13 after a game of hockey, was initially said to have suffered a heart attack. A later autopsy established he died of a "ruptured aorta," which is not the same thing.

And only on Tuesday, nine months after comedian Garry Shandling died, supposedly of a "massive heart attack," as TMZ put it, the Los Angeles County coroner's office issued its report showing that Shandling actually died from a blood clot that moved from his leg to his heart.

Another question is the role substance abuse may play in the death of someone who is still relatively young. Fisher was open about how she used drugs to "self-medicate" her problems with bipolar disorder. But even recovering addicts can suffer medical consequences years later, says Ben Levenson, founder of Origins Behavioral Health Care treatment centers for addiction.

"The human body was not designed to handle years of chronic addiction," Levenson says. "Every time a person uses cocaine, an addict or even just the recreational user, their heart is likely damaged."

Mental-health hero: After Fisher discovered she suffered from bipolar disorder, she spoke out openly about mental illness, writing about it and joking about it, trying to help reduce the stigma and secrecy around such illnesses.

That earned her kudos from mental health organizations, and after her death, impassioned tributes to her courage emerged on Twitter from others with similar illnesses. Some people even admitted publicly for the first time that they, too, are bipolar.

I’m pretty open about being in recovery; I’ve been more circumspect about mental illness. In honor of Carrie Fisher: I’m bipolar, too. — Ana Marie Cox (@anamariecox) December 27, 2016

@anamariecox Carrie Fisher's openness about her #bipolar was one of the greatest gifts she left our world https://t.co/pUPXvBrs9A#LiveBrave — Jennifer Marshall (@BipolarMomLife) December 28, 2016

The National Alliance on Mental Illness, the largest grassroots support organization for people living with mental illnesses, posted a tribute to Fisher on Twitter and on its Facebook page, highlighting one of her quotes about how living with bipolar disorder requires "stamina and courage" and that functioning is "something to be proud of, not ashamed of."

The organization honored Fisher in 2001 for "making a significant, national contribution to end discrimination" against people with mental illness.

NAMI is sad about Carrie Fisher, an inspiring mental health advocate, who spoke openly to end stigma #stigmafreehttps://t.co/PP4JVvSXPfpic.twitter.com/Af6MurVzwl — NAMI (@NAMICommunicate) December 28, 2016

Ken Duckworth, medical director of NAMI, said Fisher approached dealing with mental illness the same way she approached playing feisty Princess Leia in Star Wars: Pro-active, honest and fearless.

"She is one of my heroes," he said. "She said, 'My liabilities are my strengths,' and that was a beautiful thing to say about living with a condition like bipolar disorder. It does not define you and it does not stop you, and she’s emblematic of that."

He said her box-office legacy was considerable but "pales in comparison to what she did for millions of Americans who live with a treatable illness. Her contribution is quite meaningful...For that she is an American treasure."

Her books are selling: There's been a run on Fisher's most popular books, reports The Associated Press. The Princess Diarist, about the diaries she kept while making Star Wars as a 19-year-old, was on top of Amazon's list of best-selling books on Wednesday.

Wishful Drinking, her funny/sad memoir of her life with substance abuse, and Postcards From the Edge, her semi-autobiographical novel about her relationship with her mother, Debbie Reynolds, were also in Amazon's top 10, with the follow up to Wishful, Shockaholic, ranked as No. 57.

Five of the top 10 books on Amazon's "Movers and Shakers" list, which measures titles that show the greatest upward movement in sales over a 24-hour period, were by Fisher. That list also included Courtney Carbone's book, I Am a Princess, about Fisher's Princess Leia character, at No. 11.

What about Gary Fisher: In her later years, Fisher never went anywhere without her therapy dog, a 4-year-old French bulldog with a lolling tongue she named Gary Fisher. As the multitude of pictures of him over the years showed, he was a ham for any camera, a star on any red carpet.

Saddest tweets to tweet. Mommy is gone. I love you @carrieffisher — Carrie Fisher's Dog (@Gary_TheDog) December 27, 2016

He was with her on the United flight carrying her home from promoting her new book in London on Friday, and later was spotted with her daughter, Billie Lourd, 24, at the hospital. After her death, fans were worried about his fate.

But TMZ reported Lourd is the likely next "Mommy" for Gary; she already has a Frenchie named Tina who is friendly with Gary.

But who will post the funny-touching tweets on the Carrie Fisher's Dog Twitter account?

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Contributing: The Associated Press

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