It was the first time authorities squarely pinned the blame for Ms Petito’s death on Brian Laundrie, though he was the prime suspect all along.
Investigators meanwhile announced that Laundrie, 23, had sent text messages to intentionally deceive people that Ms Petito, 22, was still alive after he beat and strangled her in late August, according to a statement released by the FBI in Denver.
“All logical investigative steps have been concluded in this case,” FBI Denver Division Special Agent in Charge Michael Schneider said in the statement.
“The investigation did not identify any other individuals other than Brian Laundrie directly involved in the tragic death of Gabby Petito.”
Ms Petito’s body was found on September 19 at a campground near Grand Teton National Park in north-western Wyoming.
A coroner determined she had died about three weeks earlier of “blunt-force injuries to the head and neck, with manual strangulation”.
Laundrie returned home to Florida in early September but went missing soon after. In October, his parents found an item belonging to him in a nature preserve near their house.
Further searching turned up Laundrie’s remains, a revolver, backpack and the notebook. Medical examiners determined Laundrie died of a self-inflicted gunshot wound.
The FBI did not specify what, exactly, Laundrie had written in the notebook and in the texts he had sent to mislead. The texts were sent between Laundrie’s phone and Ms Petito’s phone, according to the statement.
“The timing and content of these messages are indicative of Mr Laundrie attempting to deceive law enforcement by giving the impression that Ms Petito was still alive,” the statement said.
Grand Teton was the last place the couple visited during a cross-country van trip last summer that ended with Laundrie mysteriously returning home alone in the vehicle.
The couple documented their trip on Instagram. They crossed Colorado and on August 12 were stopped by police in Moab, Utah, after the two got into a scuffle.
Police video showed Ms Petito describing a fight that escalated. Police concluded Ms Petito was the aggressor but officers decided to separate them for the night rather than file charges.