TMZ, citing law-enforcement sources, said police responded to an incident involving Panettiere over the weekend and an investigation remained open.
Fellow actor Selma Blair shared her grief in comments on Panettiere’s most recent Instagram post.
“I love you. Don’t be gone. Don’t be gone. Please. Please”, she wrote.
Eleven minutes later, she added: “I love you whale tail my friend.”
Panettiere’s first on-screen appearance was in a commercial in 1990 aged just 11 months old, launching a decades-long Hollywood career.
She was best known for playing Claire Bennet in the hit series Heroes and Juliette Barnes in the musical drama Nashville.
Her film credits included Remember the Titans, Raising Helen, Ice Princess and Scream 4.
The former child actor released her memoir, This is Me: A Reckoning, in May this year, opening up about her experiences of addiction, domestic abuse and the pressures of the spotlight.
In the book, Panettiere detailed several traumatic encounters involving famous men. She recalled being 18 when a friend allegedly led her into a room and told her to get into bed with an undressed older celebrity.
She is survived by her daughter, Kaya, whom she had with former fiance, Ukrainian boxer Wladimir Klitschko.
Kaya has lived with her father since the couple split in 2018.
Panettiere wrote that not living under the same roof as Kaya every day had been “the most gut-wrenching experience of my life”.
Panettiere had previously spoken publicly about her post-partum depression, telling E! News she wished she had known what to look out for.
“I never heard any stories about it at all. I just thought there was something seriously wrong with me, so I thought, ‘[alcohol] will fix this. Duh!’”