Hilary Duff has shared a heartfelt tribute to her late friend and former child star Hayden Panettiere, who died Sunday at the age of 36. Panettiere was found in cardiac arrest at her South Carolina apartment.

Duff took to Instagram on Tuesday to express her grief, writing, "Sweet Hayden. Really tough hearing this news. We lost touch through the years but I will always remember you as so hardworking and effervescent, with that big, gorgeous smile."

Recalling their shared experiences, Duff added, "I remember staying up late, chatting and gossiping, and connecting because we were part of such a small sliver of people who actually understood the strange lot in life we were navigating.. so young, with no real idea how to make sense of any of it."

First responders were dispatched to Panettiere's residence after a 911 call placed shortly before 2 p.m. Sunday. They found the "Heroes" actor unresponsive and initiated advanced life support measures, but resuscitation efforts were unsuccessful, according to a statement from the Greenville County Coroner’s Office. Preliminary investigations have not indicated any signs of foul play.

Panettiere, born in 1989, began her professional acting career in 1996. She went on to appear in films such as "A Bug's Life" and "Remember the Titans," as well as the popular ABC series "Nashville," for which she also recorded soundtrack songs. She recently released a memoir detailing struggles with substance abuse, domestic abuse, and the birth of her daughter.

Duff and Panettiere, both Disney Channel fixtures, navigated their early careers in parallel. Duff concluded her run on "Lizzie McGuire" in 2004, the same year Panettiere starred in the Disney Channel movie "Tiger Cruise." The two were often seen together during their careers, attending events like the 2007 Teen Vogue Young Hollywood party. Panettiere also supported Duff at the release party for her album "Dignity" that same year.

Panettiere is survived by her parents, Skip Panettiere and Lesley Vogel, and her daughter, Kaya, with whom she shared with ex-fiancé Wladimir Klitschko. Tributes have poured in from fellow actors, friends, and family members.

Duff concluded her tribute by writing, "I so deeply wish this wasn’t the end for you. Sending a heart full of love to your family, the many people you impacted and everyone who loved you."

The cause and manner of death remain under investigation. A person familiar with the situation told The New York Times that the cardiac arrest was reportedly linked to an apparent overdose. For those seeking help with substance use disorder or mental health issues, the SAMHSA National Helpline can be reached at 800-662-HELP (4357).