Panettiere was found in cardiac arrest before her death, the Greenville County Coroner's Office said in a press release on Monday. The cause of death is still to be determined for the "Heroes" and "Nashville" actress, but "any signs of foul play or suspicious circumstances" have been ruled out.

Klitschko and Panettiere announced their engagement in 2013 and their daughter Kaya was born the following year. Klitschko met Panettiere at a party in Los Angeles in 2009 and they were together until 2018.

Panettiere transferred custody of Kaya to Klitschko in 2018 due to being in a bad place in life. Panettiere detailed her personal struggles in the memoir "This Is Me," which was released in May.

The 50-year-old former heavyweight champion of the world mourned the death of Panettiere on Tuesday with a tribute on social media.

"Our family is going through a time of profound shock and grief," the tribute read.

"The announcement of Hayden’s tragic death saddens me deeply. She left this world far too soon. Hayden was, although no longer my partner, an important part of my life and the mother of our daughter, Kaya.

"She built an incredible career through immense talent, while also facing the darker sides of a very demanding industry. Nothing will erase the times we shared or the place she had in our lives.

"To our daughter Kaya, I will always speak of her mother with respect and make sure she remembers the person she was.

"I extend my deepest condolences to Hayden’s parents, her friends, and her fans, who are also grieving today. Young and talented people like Hayden should not leave this world so soon.

"I ask everyone to please respect our family’s feelings and privacy during this incredibly difficult time. For now, this is all I wish to say.

"Thank you to everyone who is supporting my family and expressing their sincere condolences.

"Rest easy, Hayden. May you find eternal peace. #WeLoveYouHayden."