The Heroes star died on Sunday at the age of 36 after being found unresponsive by a friend at her apartment in South Carolina.
The TMZ website has obtained dispatch audio of the call to emergency services, which appears to pick up officials discussing an "overdose".
The short recording includes one officer saying: "Responding. Overdose. CPR is in progress."
After receiving the emergency call, paramedics rushed to the apartment and tried to resuscitate Hayden using "advanced cardiac life support".
But after an hour, Nashville star Hayden was pronounced dead at 2.32pm.
An official statement from the Greenville Police Department read: "(Officers), accompanied by EMS personnel, responded to a report of an unresponsive female" about 1.50pm on Sunday.
"Upon arrival, medical assistance was administered; however, the individual, later identified as Hayden Panettiere, was pronounced deceased at the scene.
"The case is under investigation by the Greenville Police Department in conjunction with the Greenville County Coroner's Office. The preliminary investigation has not indicated any signs of foul play or suspicious circumstances. An acquaintance of Ms Panettiere placed the 911 call."
Panettiere's representative Kasey Kitchen also confirmed officials are looking into what happened and more details will be revealed in due course.
Kitchen told NBC News: "There's an investigation ongoing. We should know more tomorrow."
A coroner is due to perform an autopsy to determine an official cause of death.
Panettiere's death was confirmed by her father, Alan Panettiere, who hailed the actress as an "incredible light and a force of nature".
Alan said in his statement: "It is with profound sadness that we share the tragic passing of our beloved Hayden.
"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."
Her father asked for privacy "as our family takes time to process this unimaginable loss".
Panettiere recently released her memoir, This Is Me: A Reckoning, in which she detailed her troubled personal life, including domestic abuse, battles with alcoholism, stints in rehab and a battle with undiagnosed postpartum depression after the birth of her daughter Kaya - who she had with her ex-fiancé former unified world heavyweight champion boxer Wladimir Klitschko.
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