Hayden Panettiere Death: Boyfriend's Brother Says Brian Used Narcan on Her

Hayden Panettiere's boyfriend Brian Hickerson gave her Narcan after he and his brother Zach Hickerson found her unconscious the day she died ... at least according to Zach ... and that's huge on many levels.

Zach tells TMZ ... "Brian gave her Narcan."

The admission is telling ... Hayden and Brian traveled to South Carolina the day before she died, so it seems they either traveled with Narcan or bought some in South Carolina shortly after arriving. Whatever the case, it suggests Hayden and/or Brian were really struggling with substance abuse.

Zach says he went to their Greenville Airbnb after not hearing from them, knocked, woke Brian up from a nap, and then the brothers discovered an unconscious Hayden ... with Zach calling 911 as Brian gave her Narcan, a medication administered nasally to reverse the effects of an opioid overdose.

In a statement to TMZ on Friday, Zach says ... "There are a lot of assumptions, speculations and accusations currently swirling. The truth will come out as the authorities complete and release additional details from the investigation. And that is important because the people who loved and adored Hayden deserve the truth."

He continued ... "At this time, I want to clarify my involvement in effort to keep the focus on the fact that we lost one of the most precious individuals on this earth and to allow those that cared about her to mourn without additional noise, chaos, and misinformation being created and propagated by the media as well as the public."

Zach tells us what happened the day Hayden died, saying ... "On Sunday morning, I woke up at my Airbnb, got dressed and went to Church. I exchanged messages that morning with Brian on my way. We planned on us all getting lunch and playing indoor golf after the service."

He continued ... "When I left church I called and texted both of them. When I didn’t reach Brian or Hayden, I just assumed maybe they fell back asleep so I drove over and went to the Airbnb. When I got there I knew the code to the building but not the door code to the AirBnb."

Zach then detailed how he and Brian both found Hayden, saying ... "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 recalled ... "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."

Zach tells us he's distraught over Hayden's death, saying ... "I am beyond heartbroken and devastated by this tragic and traumatic loss. Hayden was one of my dearest and closest friends that I loved and cared about for the last 8 years."

He concluded ... "Out of respect for her loved ones, the speculation and sensationalism of her death needs to end until the facts can be released from the investigation."

We first reported Narcan was found in the room ... a device used to dispense it was left behind on a mattress. But we didn't know who administered the dose, and who received it.

The dispatch audio also mentioned an "overdose."

Hayden's death is under investigation ... and we first reported the DEA is involved.