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The Drug Enforcement Administration (DEA) has reportedly joined the investigation into actress Hayden Panettiere's death.

TMZ is reporting that the DEA has agents assisting parallel death investigations that are already underway.

Panettiere died Sunday at the age of 36. She was found unresponsive and in cardiac arrest at an apartment in Greenville, South Carolina. Resuscitation efforts were unsuccessful and the "Nashville" and "Remember the Titans" actress was pronounced dead.

Preliminary autopsy results found no signs of trauma that contributed to her death, Fox News Digital confirmed.

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Hayden Panettiere in a red dress at the amfAR Gala in Los Angeles in November 2022.

The DEA is reportedly joining the investigation into the death of actress Hayden Panettiere. She died Sunday at the age of 36. (Emma McIntyre/Getty Images for amfAR)

A police report obtained by Fox News Digital revealed Panettiere's on-and-off boyfriend, Brian Hickerson, and his brother, Zach, called 911 after finding the actress unresponsive. Hickerson later handed authorities a bag of medication Panettiere had allegedly been taking.

Hickerson was among those at the Greenville apartment when Panettiere was found, and authorities executed a search warrant at the residence, according to the report. Police also conducted a welfare check Wednesday at a South Carolina home linked to the Hickerson brothers, but no one answered the door.

Panettiere’s death came just weeks after the actress spoke publicly about her past struggles with addiction and the difficult decisions she made while seeking help.

The New York native began acting as a child, landing a role on the soap opera "One Life to Live" at just 4 years old. She later appeared alongside Denzel Washington in the 2000 football drama "Remember the Titans" before becoming widely known for her role as Claire Bennet on NBC's "Heroes."

Hayden Panettiere

Hayden Panettiere has died. She was 36.  (Getty Images)

Panettiere went on to star as country singer Juliette Barnes on "Nashville," a role that earned her two Golden Globe nominations.

The actress had also been candid about her history with alcohol and opioids. In a 2022 interview with People, Panettiere said she was first given "happy pills" at age 15 ahead of interviews and later began drinking and occasionally using opioids as her career accelerated.

"As I got older, the drugs and alcohol became something I almost couldn’t live without," she told the outlet at the time.

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Hayden Panettiere holding her onscreen child in Sleepwalker

Hayden Panettiere holding her onscreen child in Sleepwalker. (Brainstorm Media)

Panettiere revisited some of those struggles in her memoir, "This Is Me: A Reckoning," and recently spoke with Fox News Digital about her decision to allow her daughter, Kaya, to live with her father, former heavyweight boxing champion Wladimir Klitschko, in Europe.

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

She said she ultimately decided against fighting Klitschko over custody because she believed doing so would hurt their daughter.

"I said, ‘You know what? Let her grow up here,’" Panettiere said. "I know she's safe. I know she's well taken care of — beyond well taken care of. She has an incredible life."

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Just weeks before her death, Panettiere also filmed what would become one of her final public campaigns, partnering with People for the Ethical Treatment of Animals to condemn the captivity of dolphins and other marine mammals.

PETA released the video following her death as a tribute to the actress, who had advocated for marine mammals for nearly two decades. Her activism gained international attention in 2007 when, at 18, she paddled into waters near Taiji, Japan, to confront dolphin hunters during an annual hunt, an episode later featured in the Oscar-winning documentary "The Cove."

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In the recently recorded campaign, Panettiere urged the public not to patronize marine parks that keep dolphins in captivity.

"Vow today to never go to a marine park that keeps dolphins in extreme captivity," Panettiere said in the video. "Together, we can end this cruelty."

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Panettiere is survived by her daughter, Kaya.

Fox News Digital's Tracy Wright and Lauryn Overhultz contributed to this report.