FBI: boyfriend admitted killing Petito

The boyfriend of US traveller Gabby Petito admitted killing her in a notebook discovered near his body in a Florida swamp, the FBI says.

It's the first time authorities have squarely pinned the blame for Petito's death on Brian Laundrie, though he was the prime suspect all along.

Investigators also announced that Laundrie, 23, had sent text messages to intentionally deceive people that Petito, 22, was still alive after he beat and strangled her in late August, according to a statement released on Friday 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.

"The investigation did not identify any other individuals other than Brian Laundrie directly involved in the tragic death of Gabby Petito."

Petito's body was found on September 19 at a campground near Grand Teton National Park in northwestern 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 Petito's phone, according to the statement.

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.