The notebook was found next to his body in the Florida swamp where he was found dead in October after shooting himself in the head. Along with a notebook, officers also found a backpack and a revolver.
He had been on the run since September 13 after strangling Petito in Wyoming during their cross-country RV trip earlier this summer.
In an announcement on Friday, the FBI said it was closing its investigation into Petito’s death after concluding that Laundrie did admit to it.
The bureau did not say what specifically he wrote in to take responsibility.
The FBI opened its investigation into Petito’s disappearance after she was reported missing by her parents on September 12. At the time, she and Laundrie has been on a cross-country road trip in her white van.