Brian Laundrie confessed to killing Gabby Petito in a water-logged notebook found near his remains at Myakkahatchee Creek Environmental Park in Florida.
Fox News obtained copies of pages from Laundrie's handwritten journal, which was recovered by the FBI.
"I ended her life," he wrote in the journal. "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."
The notebook does not appear to contain any information about what led Laundrie to murder his fiancée but does make references to her being in pain.
"From the moment I decided, took away her pain, I knew I couldn't go on without her," he added.
Laundrie also apologized to his family for what he did.
"I am sorry to my family. This is a shock to them as well [as] a terrible grief (sic)."
"Please do not make life harder for my family," Laundrie wrote, adding, "they lost a son and a daughter and a daughter – the most wonderful girl in the world. Gabby, I'm sorry."
Attorneys for the Petito and Laundries families were in Tampa on Friday (June 24) to meet with the FBI to receive items from both Laundrie and Petito that were collected during the course of the investigation.
"Today, the Petito family attorney, Patrick Reilly, and myself met with the FBI in Tampa to sort through and take possession of the personal items that belonged to Gabby and Brian," Laundrie family attorney Steve Bertolino told Fox News. "As part of this return of property in FBI custody, I was given Brian's notebook, and I have turned [the] same over to Chris and Roberta Laundrie."