"A review of the notebook revealed written statements by Mr. Laundrie claiming responsibility for Ms. Petito's death," the FBI's Denver Field Office said in a statement released Friday.
The notebook was discovered in October, in the same area as Laundrie's remains, in Florida's Myakkahatchee Creek Environmental Park, authorities said.
Petito's and Laundrie's families have reached an agreement on how the couple's belongings, including a notebook, will be distributed, Laundrie family attorney Steve Bertolino said Friday, about four months after Petito's remains were found in Wyoming.
Laundrie had been the subject of a weekslong manhunt when his body was found. His family was informed he shot himself in the head, and his death was ruled a suicide, Bertolino said in November.
Bertolino would not say which family will receive the notebook. The Petito family met with the FBI in Tampa, Florida, this week and reached an agreement on distributing her and Laundrie's belongings, he said.
The Petitos thanked the FBI for its support, according to a statement from the family's lawyer.
"We truly appreciate the FBI's diligent and painstaking efforts in this extremely complicated case. The quality and quantity of the facts and information collected by the FBI leave no doubt (that) Brian Laundrie murdered Gabby," it said.
The news comes the week after an independent investigator determined two police officers who pulled over Petito's van in Moab, Utah, on August 12 should be placed on probation for "several unintentional mistakes" during the traffic stop.
The officers agreed not to charge Petito, who had acknowledged striking Laundrie first, as long as she and Laundrie spent the night apart. The independent investigator found the officers neglected their duty by not pressing charges.
Laundrie, 23, and Petito, 22, set out on a road trip in Petito's converted Ford Transit van in June and documented their travels on social media. Laundrie returned alone, driving the van to his parents' Florida home September 1.
As police searched for Petito in mid-September, Laundrie went missing. Petito's remains were found a few days later. Her death was ruled a homicide by manual strangulation.
The specific evening leading up to Petito's death remain a mystery. The FBI described Laundrie as a "person of interest" in her murder, but he was not charged in her killing. His remains were found October 20 in an area that had been underwater during previous searches of a 25,000-acre reserve in North Port.
Laundrie's father, who was searching with authorities when his son's remains were found, discovered the notebook near Laundrie's body. Experts said at the time the notebook could be key in providing answers. The notebook was wet, and officials did not immediately say whether they gleaned any information from it.
Bertolino told CNN, "Gabby and Brian are no longer with their families and this tragedy has caused enormous emotional pain and suffering to all who loved either or both of them. We can only hope that with today's closure of the case each family can begin to heal and move forward and find peace in and with the memories of their children. May Gabby and Brian both rest in peace."
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