Greenville, South Carolina - The Drug Enforcement Administration has reportedly joined the probe into the unexpected death of actor Hayden Panettiere.

TMZ reports that DEA agents will be assisting the parallel investigations currently underway by the Greenville County Coroner's Office and the Greenville Police Department.

The Heroes star was found dead at the age of 36 on Sunday at an apartment complex in the South Carolina city.

A cause of death has yet to be confirmed, though a 911 dispatch audio revealed that first responders spoke of a "cardiac arrest" and a possible overdose.

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On Wednesday, authorities were seen at the residence of Hayden's on-and-off boyfriend, Brian Hickerson, after it was disclosed that he and his brother Zach were present when she passed away.

The Nashville actor is said to have secretly rekindled her romance with Brian but was advised by her team to keep the reunion private due to multiple incidents of domestic violence.

Hayden had previously opened up about her struggles with opioid and alcohol addiction, which led to her giving up custody of her daughter Kaya, whom she shares with ex-fiancé Wladimir Klitschko.

Emotional tributes to the Scream 4 star from her co-stars and friends continue, while her estranged mother, Lesley Vogel, called Hayden an "amazingly talented person" who "lost her way."