Her body was found on September 19 at a Wyoming campground, with an autopsy determining she had been strangled. Photo / Supplied
Gabby Petito's parents are suing Brian Laundrie's parents, accusing them of knowing he had killed their daughter and planning to help him flee the country.
Gabby, 22, went missing late last year during a cross-country trip in the US with her fiance Brian. The mysterious disappearance of the aspiring influencer sent shockwaves across the world.
Her body was found on September 19 at a Wyoming campground, with an autopsy determining she had been strangled.
Brian, 23, later fatally shot himself, leaving behind a notebook where he took responsibility for her death.
Gabby's parents Nichole Schmidt and Joseph Petito filed the civil lawsuit on Thursday, local time, in Sarasota County, Florida, listing Christopher and Roberta Laundrie as defendants.
The lawsuit shockingly claims that Brian's parents had "full knowledge that Gabrielle Petito had been murdered by their son" when they released a statement on September 14 saying the Laundrie family hoped the search for Gabby was successful and she could be reunited with her family.
Gabby's parents also claim in the days before, when they were "suffering" trying to locate their daughter, the Laundrie family went on a vacation on September 6 and 7.
They further allege Brian's mother Roberta blocked Gabby's mother Nichole on her phone and Facebook about September 10 to avoid any contact.
The legal filing goes on to claim a letter from Nichole and Joseph pleading with Brian's parents to tell them where their daughter was ignored, and while the pair were "desperately searching for information", Christopher and Roberta were "keeping the whereabouts of Brian Laundrie secret, and it is believed were making arrangements for him to leave the country".
The heartbreaking letter sent on September 16, in part, is quoted to say, "We beg you to tell us. As a parent, how could you let us go through this pain and not help us? As a parent how can you put Gabby's younger brothers and sisters through this?
"Gabby lived with you for over a year. She was going to be your daughter in law. How can you keep her location hidden? … Please if you have any decency left, please tell us where Gabby is located. Tell us if we are even looking in the right place."
The lawsuit seeks damages of at least $US30,000 ($41,000) for "pain and suffering, mental anguish, inconvenience, loss of capacity for enjoyment of life experienced in the past and to be experienced in the future".
Gabby was reported missing by her parents on September 11 after Brian returned from their road trip to his home in North Port, Florida, without her.
He had refused to address the last time he saw her.
A search was organised of a remote area within the Grand Teton National Park and the Bridger-Teton National Forest in Wyoming.
rian quickly became the sole person of interest in the case before he disappeared.
His remains were discovered at Carlton Reserve near his parents' home on October 20.