image source, Find Gabby/Facebookimage captionGabrielle "Gabby" Petito has not spoken to her family since late August

US authorities are searching for a Florida woman who went missing during a road trip with her fiancé.

22-year-old Gabrielle "Gabby" Petito has not spoken to her family since late August, when she was travelling with her boyfriend in Wyoming.

Police say her boyfriend, Brian Laundrie, 23, who has returned home, is now a person of interest in the case.

The couple had been documenting their cross-country campervan trip on social media.

Mr Laundrie returned to their Florida home on 1 September, police say. Ms Petito's parents reported her missing 10 days later.

But police say Mr Laundrie is refusing to speak to them about her disappearance.

"We are pleading with anyone, including Brian, to share information with us on her whereabouts in the past few weeks," said North Port police chief Todd Garrison.

"The lack of information from Brian is hindering this investigation. The answers will eventually come out."

But Mr Laundrie's lawyer, Steve Bertolino, defended his silence.

"In my experience, intimate partners are often the first person law enforcement focuses their attention on in cases like this and the warning that 'any statement made will be used against you' is true, regardless of whether my client had anything to do with Ms Petito's disappearance.

"As such, on the advice of counsel Mr Laundrie is not speaking on this matter," Mr Bertolino said.

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The couple set off on a nomadic nationwide road trip in a white Ford Transit van in July, posting updates on social media.

They filmed themselves smiling, kissing, and running on beaches in a YouTube video, "Beginning Our Van Life Journey", which has been viewed over 600,000 times.

Two weeks before Ms Petito went missing, police in Utah were called to an incident involving the couple.

Officers found Ms Petito crying and complaining about her mental health, according to a police report. They did not file any charges.

Police have now seized the van and are processing it for clues.

Ms Petito's family have called on Mr Laundrie to give answers, including when he last saw her and "why he left Gabby all alone and drove her van to Florida".

"These are critical questions that require immediate answers," they said in a statement.