It found Laundrie was solely responsible for the “tragic death of Gabby Petito” as it announced it was wrapping up its four and a half month long investigation.
A notebook recovered near Laundrie’s remains in Florida’s Myakkahatchee Creek Environmental Park contained a written confession, the FBI said.
“A review of the notebook revealed written statements by Mr Laundrie claiming responsibility for Ms Petito’s death,” the FBI said in a statement.
FBI Denver Division Special Agent in Charge Michael Schneider said “the investigation quickly focused on the last person believed to see her alive — Mr Laundrie”.
He said the public had played an “invaluable” role in the investigation.
“The FBI’s primary focus throughout the investigation was to bring justice to Gabby and her family.”
In a statement, Petito family attorney Richard Stafford thanked the FBI for its “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.”
Ms Petito’s remains were found in the Grand Teton National Park in Wyoming on 19 September, three weeks after she last spoke to her family.
Laundrie, 23, was reported missing on 17 September after telling his parents he was going hiking in the Carlton Reserve near their home in North Port, Florida.
Chris and Roberta Laundrie led authorities to personal items belonging to their son Brian in the Myakkahatchee Creek Environmental Park on 20 October, and law enforcement agents found his remains nearby.