A lawyer for Brian Laundrie's family released the contents of a notebook found near his body, in which he confesses to killing Gabby Petito.
Petito's body was found in Wyoming after she went missing during a road trip with her fiancé. A local coroner determined she was killed by strangulation.
Laundrie, who was named a person of interest, went missing before Petito's body was found. Police searched for Laundrie for weeks until human remains and "items of interest" confirmed to be Laundrie's were found in a reserve in Florida.
"I ended her life," he wrote in the notebook first reported by Fox News on Friday.
"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," he added.
He further claimed that Petitio was injured and that she was "begging for an end to her pain", reports People.
Laundrie – who died of a self-inflicted gunshot wound – also wrote: "From the moment I decided, took away her pain, I knew I couldn't go on without her."
Petitio's family have brought a civil suit against the Laundrie family and is seeking $30 000 in damages for mental anguish.