Wladimir Klitschko is mourning his former fiancée Hayden Panettiere, saying the actress remained an “important part” of his life and promising to keep her memory alive for their daughter, Kaya.

“Our family is going through a time of profound shock and grief,” the Ukrainian former heavyweight boxing champion wrote on Instagram after Panettiere’s death at 36. “She left this world far too soon.”

Klitschko, 50, said that although Panettiere was “no longer my partner,” nothing could erase the time they shared or her place in their lives.

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

The words echoed what Panettiere told Fox News Digital in July about a promise she and Klitschko had made: never to speak badly about each other to their daughter.

“Wlad and I have a good relationship,” she said. “We made a promise to each other that we would never say a bad word about each other to our child. That was something that I grew up with. And we’ve stuck to that.”

Panettiere died Sunday in South Carolina. The Greenville County Coroner’s Office and Greenville City Police Department are investigating her death.

A preliminary autopsy found no signs of trauma that contributed to her death. Her cause and manner of death remain pending.

Panettiere transferred custody of Kaya to Klitschko in 2018, saying she knew she was in a bad place at the time and wanted to protect her daughter.

Actress Hayden Panettiere poses with her boyfriend, Ukrainian WBA heavyweight boxer Wladimir Klitschko, at the HBO after party for the 68th annual Golden Globe Awards in Beverly Hills, California, on Jan. 16, 2011. (REUTERS/Lucy Nicholson)

“I just knew that if I wanted to … I knew how bad a place I was in,” Panettiere told Fox News Digital. “I hid it from my daughter as much as I could.”

Panettiere had spoken publicly about her personal struggles, which she detailed in her memoir, “This Is Me,” released in May. She wrote about the lasting effects of growing up in the spotlight, her addiction to alcohol and her battle with postpartum depression after Kaya was born in 2014.

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She said Kaya had a strong support system in Europe with Klitschko and feared a public custody fight between two famous parents would hurt their daughter.

“When I saw her in the environment over there [in Europe] and the support system that she had, I said to myself, ‘If I fight, if I go to war about this, it’s going to permanently damage my child,’” she said.

“It’s going to be incredibly public. She’s got two very famous parents, and it’s going to be ugly.”

Panettiere ultimately decided Kaya should stay with Klitschko.

Ukraine’s Wladimir Klitschko and his wife Hayden Panettiere, right, leave the hall after Klitschko lost to Britain’s Tyson Fury in a world heavyweight title fight for Klitschko’s WBA, IBF, WBO and IBO belts in Duesseldorf, western Germany, on Nov. 29, 2015. (AP Photo/Sebastian Konopka, File)

“I said, ‘You know what? Let her grow up here,’” she recalled. “‘I will travel here as much as I can. I know she’s safe. I know she’s well taken care of — beyond well taken care of. She has an incredible life. And I also knew that there would be a day when she would come back to me.’”

The decision was painful.

“I tried to numb myself,” Panettiere said. “It was very hard to wrap my mind around the pain, the emotional pain of that. But you keep putting one foot in front of the other somehow.”

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In his Instagram post, Klitschko also paid tribute to Panettiere’s career.

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

Wladimir Klitschko waves to fans with his partner Hayden Panettiere after losing to Britain’s Tyson Fury in a world heavyweight title fight for Klitschko’s WBA, IBF, WBO and IBO belts in Duesseldorf, western Germany, on Nov. 29, 2015. (Reuters / Ina Fassbender)

He offered condolences to Panettiere’s parents, friends and fans and asked that the family’s privacy be respected.

“Young and talented people like Hayden should not leave this world so soon,” he wrote. “Rest easy, Hayden. May you find eternal peace.”

Wladimir Klitschko and Hayden Panettiere attend the Giorgio Armani show during Milan Menswear Fashion Week Spring/Summer 2014 on June 25, 2013, in Milan, Italy. (Venturelli/WireImage)

First responders found Panettiere in cardiac arrest, according to the coroner’s office. EMS personnel attempted to resuscitate her, but she was pronounced dead at 2:32 p.m.

“This investigation remains active and ongoing,” the coroner’s office said. “No further details are available for release at this time.”

Panettiere began acting as a baby, appearing in a toy commercial at 11 months old.

THE TONIGHT SHOW WITH JAY LENO — Episode 1007 — Pictured: (l-r) Actress Hayden Panettiere during an interview with host Jay Leno on October 3, 1996 (Photo by Margaret Norton/NBCU Photo Bank/NBCUniversal via Getty Images via Getty Images) (Margaret Norton/NBCU Photo Bank/NBCUniversal via Getty Images via Getty Images)

She voiced Princess Dot in Pixar’s “A Bug’s Life” and played Sheryl Yoast opposite Denzel Washington in the 2000 football drama “Remember the Titans.”

Panettiere later starred in Disney’s “Ice Princess” and “Bring It On: All or Nothing” before breaking out as Claire Bennet, a cheerleader with the ability to rapidly heal from injuries, on NBC’s “Heroes.”

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She went on to star opposite Connie Britton as country singer Juliette Barnes on “Nashville” from 2012 to 2018.

Fox News Digital’s Stephanie Nolasco, Christina Dugan Ramirez and Lauryn Overhultz contributed to this report.