A family photo of Candice and Kristin Hermeler.
A Melbourne woman who killed herself at a US shooting range in a suicide pact with her twin sister had no drugs or alcohol in her system at the time, an autopsy report has found.
Kristin Hermeler was known to have a history of depression before she turned the gun on herself at the Family Shooting Centre near Denver, Colorado last November.
Her identical twin sister Candice, who also suffered a self-inflicted gunshot wound to the head, survived.
According to an autopsy report released today by the Arapahoe Coroner's Office, Kristin had experienced periods of despondency in the past, the Denver Post reported.
The 29-year-old died from a gunshot fired into her mouth, which damaged her spinal cord, brain stem and brain.
Toxicology tests found no trace of alcohol, significant medications or "drugs of abuse" in her body, the report found. The sisters had "virtually simultaneously suffered gunshot wounds of the head".
American investigators have conceded they will probably never know why the twins shot themselves, after deciding not to re-interview the sole survivor.
Candice, who underwent brain surgery after the shooting, has been interviewed by police but refused to say why the pair entered the suicide pact. It is not known if she has returned to Melbourne with her parents.
The twins were in the Denver area for more than a month before the November 15 shooting and had taken gun lessons in the lead-up to the tragedy.
CCTV footage showed them smiling and walking casually into the shooting range before they turned their rented .22 pistols on themselves.
The footage does not show the moment they fell backwards, having shot themselves.
After the shooting, police found a letter in the sisters' luggage from the family of one of the killers in the 1999 Columbine High School massacre, in which 13 people died.
A copy of a Time magazine cover showing Columbine killers Eric Harris and Dylan Klebold was also in their hotel room, while it emerged that Kristin had written to a Columbine student who had been bullied by Harris.
In an an online tribute to Kristin after the shooting, her devastated parents Kelsay and Ernest Hermeler, from Surrey Hills, said they were broken-hearted at their loss.
"Our beautiful, kind and sweet Angel has left our world for a gentler place where we hope she will find peace," they wrote.
"She touched everyone she met with her generosity of spirit and desire to help people less fortunate than herself.
"Her love of animals knew no bounds and she had an unfailing desire to rid the world of animal cruelty. Our lives were enriched by her presence and we were so proud of everything she achieved.
"We love her with all our hearts."
The tribute is signed: "Her broken-hearted Mum and Dad, Kelsay and Ern Hermeler, and her twin sister Candice".
The twins attended Girton Grammar School in Bendigo before graduating from Methodist Ladies College in Kew.
For help or information visit beyondblue.org.au, call Suicide Helpline Victoria on 1300 651 251, or Lifeline on 131 114.
