The world is mourning the unexpected loss of Hayden Panettiere after her "tragic" death at 36.

The Heroes alum passed away on Sunday (August 16) after she was found "unresponsive" by first responders in Greenville, South Carolina, around 2 p.m. following a 911 call from an "acquaintance" of the actress, per E! News. Though medical assistance was administered, she was pronounced dead at the scene. Her death remains under investigation but authorities report there are no signs of foul play.

Following news of her death, tributes from family, friends and colleagues began to pour out mourning Panettiere's sudden passing. Her father Skip Panettiere mourned the heartbreaking loss of his daughter in a statement shared by her representative on his behalf to People.

"It is with profound sadness that we share the tragic passing of our beloved Hayden," the statement said. "She was an incredible light and a force of nature who brought immeasurable love and joy to all who knew her — and to the millions who watched her onscreen."

Many of Panettiere's former costars and other celebrities also reacted to her death and shared their own tributes on social media, E! reports. Selma Blair left a comment under Panettiere's final Instagram post begging for the news not to be real, saying, "I love you. Don't be gone. Don't be gone. Please. Please," while her Scream costar Melissa Barrera shared and Instagram Story reading, "Rest in peace sweet Hayden."

Jay Shetty, who spoke with Panettiere on a May episode of his On Purpose podcast, said his was in "disbelief" over her passing in an Instagram Story, adding, "You were so special. May you always be protected and peaceful."

Viola Davis said that the "gifted" Panettiere will "be remembered" in her own Instagram post, sharing a throwback photo of the two actresses. They starred opposite each other in the 2016 film Custody.

"... an old, wise soul with such a deep heart. I wish the world had more time with you because I had a burning curiosity about who you were becoming," she wrote. "I saw this beautiful Phoenix rising from the ashes. Your tests became your testimony, and your testimony became your legacy."

Her Nashville costar Derek Hough was also "heartbroken" to her of her death, writing, "The little time I got to spend with her, she was so sweet, so kind. I've always admired her work and the joy she brought people."

Bethany Joy Lenz, who starred in the soap opera Guiding Light with Panettiere when they were younger, said she is "crushed" and shared photos of the late actress as a child on the show while remembering the "wild little blonde" who would run around set and knew everyone's lines.

"We'd connect on and off over the years, like the cousin in the family reunion who got famous and life moved her away — but I always felt such joy seeing her when we were lucky enough to cross paths," she said, adding, "I don't understand how this is real."