The partner of slain US woman Gabby Petito, Brian Laundrie, died by suicide, an attorney for the Laundrie family has told CNN.

"Chris and Roberta Laundrie have been informed that … the manner of death was suicide," said attorney Steven Bertolino.

Laundrie's remains were found in a Florida nature reserve last month after a weeks-long manhunt.

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He disappeared just days after his fiancée Gabby Petito was reported missing.

The couple had been traveling across the country in a converted van and documenting their travels on social media.

The case gained national attention as authorities searched for both.

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Laundrie returned to his parents house in Florida on September 1 without Ms Petito.

As police were trying to question the family about Ms Petito's whereabouts, Laundrie went missing on September 13 or 14 after leaving home with a backpack, according to his parents.

Ms Petito's remains were found in Wyoming on September 19.

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Her death was ruled a homicide by manual strangulation.

Laundrie's remains were found in an area of the park that had been under water during previous searches of the 25,000-acre nature reserve in North Port, Florida.

If you need help in a crisis, call Lifeline on 13 11 14.

For further information about depression, contact beyondblue on 1300 224 636 or talk to your GP, local health professional or someone you trust.