The brother of Hayden Panettiere's on-off boyfriend Brian Hickerson has broken his silence to reveal the late actress's final moments. 

Brian administered Narcan to the 36-year-old after he and his brother, Zach, found her unconscious in their Airbnb in Greenville, South Carolina on Sunday, Zach has now revealed in a statement to TMZ.

Zach, who said he wanted to 'clarify my involvement' in the situation, said he was calling 911 while Brian gave Panettiere the medication, which is typically administered to reverse an opioid overdose.

The 38-year-old said he, Brian and the actress were planning to get lunch and go indoor golfing after he attended church.

He called and texted Panettiere and his brother as he was leaving the Sunday service, but didn't hear back from them, he recalled. 

Zach assumed the pair were sleeping so he drove back to the rental, but he didn't have the door code to the unit so he couldn't get in.

'I knocked loudly which apparently woke Brian up from a nap they were both taking on the pull out couch bed in the living room. When we went in and tried to wake Hayden up for lunch, we realized she was unconscious,' he said.

'I immediately called 911 and Brian gave her Narcan. We then jointly performed CPR while 911 was on speaker until Fire and EMS arrived.'

'At that point we watched as they tried every measure possible to save her life,' he added.

Zach says he is 'beyond heartbroken and devastated' by Panettiere's death.

He shared how over the past eight years the actress had become one of my 'dearest and closest friends,' adding that she was someone he loved and cared for deeply.

He urged for an end to the 'assumptions, speculations and accusations' that are swirling around the star's death.

Zach claimed the 'truth' will come out once police have completed their investigation. 

The Heroes star was later declared dead following the suspected overdose.

Details about Panettiere's final moments come just hours after her ex-fiancé Wladimir Klitschko paid a heartbreaking tribute to the actress on what would have been her 37th birthday.

Sharing a photo of the actress, the boxer, 50, wrote: 'Today is Hayden's birthday. Instead of celebrating the way we had planned, today we celebrate her life, her spirit, and the love she brought into this world.

'I want to thank all of Hayden's fans, friends, and everyone who has reached out for the incredible outpouring of love and support. At a time filled with so much sadness, your kindness has reminded me that there is still so much good in humanity.

'If you're reading this, I ask just one thing: Even if only for a moment, think about LOVE. Reach out to a friend. Forgive someone. Be kind. Love is a superpower. Use it.

'Happy Birthday, Hayden. You are loved, you are missed, and you will never be forgotten.' 

The actress had a daughter with the boxer. In 2018, when Kaya was three, she signed full custody to her ex amid her battle with postpartum depression and addiction and Wladimir moved their daughter to his native Ukraine.

On Monday, Greenville County Sheriff's Office said that the post-mortem had been completed and that no signs of trauma were found on the star's body.

Further investigations will now take place to establish her cause of death, with chief deputy coroner Shelton England telling the Daily Mail that it was too early to discuss the drug overdose theory.

On Thursday, Brian, 37, addressed the star's death through his lawyers.

'Hayden's death remains under investigation and it is important to allow that investigation to proceed,' said his attorney Sloan Ellis, from the Ellis Hinton law firm.

'As has been publicly reported, police confirmed there were no signs of foul play.

'Out of respect for Hayden's loved ones and the ongoing investigation, there will be no further comment at this time.'

A Greenville police officer who responded to the residence Sunday afternoon was met by the brothers and then observed paramedics performing CPR on an unresponsive woman, according to a police report. 

'The patient did not seem to have any obvious physical reasons to be unresponsive,' the officer noted. After attempting lifesaving measures, the emergency workers declared Panettiere dead at 2.31pm on Sunday.

Narcan, a nasal spray used to rapidly reverse the effects of an opioid overdose, was found on a mattress inside the South Carolina property, it was revealed on Monday.

Naloxone, the active ingredient in Narcan Nasal Spray, blocks opioids like heroin, fentanyl, morphine and prescription painkillers from binding to brain receptors.

It allows the person to start breathing again within two to three minutes and stays active in the body for 30 to 90 minutes.

Panettiere and Brian were said to have flown from Los Angeles to South Carolina one day before she died.

Hickerson is originally from Greenville, public records show. It is unclear what the purpose of the couple's visit was.

Earlier, a representative for Panettiere confirmed her death, saying her family would remember her as an 'incredible light' who brought 'immeasurable love and joy to all who knew her.'

A statement from her father, Skip, read: '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 also asked that her family be given privacy as they 'take time to process this unimaginable loss'.

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