The boyfriend of Gabby Petito claimed responsibility for her death before taking his own life, according to the FBI.

Brian Laundrie had been on a cross-country trip in the US with Ms Petito when she disappeared.

Her body was found in Wyoming in September, sparking a manhunt for Laundrie, who officials believe returned to Florida without her.

However, his remains were found a month later and investigators confirmed that he had killed himself.

The FBI found a notebook, revolver and backpack.

“A review of the notebook revealed written statements by Laundrie claiming responsibility for Ms Petito’s death,” the FBI’s Denver Field Office said in a statement released Friday.

The notebook was found in October, in the same area as Laundrie’s remains in Florida’s Myakkahatchee Creek Environmental Park.

More on Gabby Petito

In their statement, the FBI added that both families have reached an agreement on how the couple’s belongings will be distributed.

What happened to Gabby Petito?

Laundrie, 23, was a person of interest in the killing of Ms Petito, who was reported missing on 11 September by her parents while the couple were on a road trip.

Ms Petito’s body was found on 19 September at a campground near Grand Teton National Park in northwestern Wyoming.

A coroner determined she’d died about three weeks earlier of “blunt-force injuries to the head and neck, with manual strangulation”.

The FBI found Laundrie’s body a month later, along with the belongings and Ms Petito’s family suspected Laundrie was impersonating her in text messages after her death.

The FBI did not release specific details of what was written in the notebook but said that they had also found texts between the couple, sent after Ms Petito’s death.

“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.

Laundrie and Ms Petito grew up on Long Island, New York, but were living in Florida in recent years.

The couple documented their trip on Instagram. They crossed Colorado and on 12 August were stopped by police in Utah, after the two got into a scuffle.

Police video showed Ms Petito describing a fight that had escalated.