Brian Laundrie said he wanted to take away Gabby Petito's pain in a journal entry, written before he killed himself.
Laundrie and his fiancée Petito had been driving around national parks in the US in their van last August, all the while documenting their trip on social media.
After Petito's family didn't hear from the 22-year-old for a couple of weeks they reported her missing on 11 September, and her disappearance attracted worldwide media attention.
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Laundrie then disappeared on 14 September and Petito's body was discovered on 19 September on the edge of Wyoming's Grand Teton National Park, which the couple had visited.
A huge manhunt culminated in the unearthing of Laundrie's remains on 20 October in the Carlton Reserve wilderness park, near the Laundries' home.
Along with the 23-year-old's remains were a revolver, a backpack and a notebook. Medical examiners determined Laundrie died of a self-inflicted gunshot wound.
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Photos of the notebook, first published by Fox News, show Laundrie confessing to Gabby's killing, which he said he did out of 'mercy'.
"I ended her life," he wrote.
"I thought it was merciful, that it is what she wanted, but I see now all the mistakes I made. I panicked. I was in shock."
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He went on to describe her death as an 'unexpected tragedy', claiming Gabby got injured one night as they tried to cross a stream.
He said she had a 'small bump on her forehead that eventually got larger' as well as other injuries, and she was 'begging for an end to her pain'.
He added: "From the moment I decided, took away her pain, I knew I couldn't go on without her."
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Laundrie finished the journal entry by expressing his hope that animals in the Carlton Reserve would tear him apart.
He wrote: "I am ending my life not because of a fear of punishment but rather because I cant stand to live another day without her.
"I've lost our whole future together, every moment we could have cherished.
"I'm sorry for everyones loss. Please do not make life harder for my family, they lost a son and a daughter. The most wonderful girl in the world. Gabby I'm sorry.
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"I have killed myself by this creek in the hopes that animals may tear me apart. That it may make some of her family happy.
“Please pick up all of my things. Gabby hated people who litter."
Jake Massey