The filing, submitted Thursday in Florida's Sarasota County court on behalf of Petito's parents and penned by attorney Patrick J. Reilly, alleges that Laundrie, 23, told his parents Petito was dead on Aug. 28, 2021.
On the same date, the Petito lawyer asserts, the Laundries spoke with attorney Steve Bertolino.
"While Gabrielle Petito's family was suffering, the Laundrie family went on vacation to Fort DeSoto Park on September 6-7, 2021," Reilly said. "They went on vacation knowing that Brian Laundrie had murdered Gabrielle Petito, it is believed that they knew where her body was located and further knew that Gabrielle Petito's parents were attempting to locate her."
Fort De Soto Park is located in Pinellas County.
Records obtained by Fox News later that month showed Roberta Laundrie, Brian's mother, had checked into the Florida park.
She canceled reservations for two people at the St. Petersburg campground Aug. 31 and made new reservations for three people at the same park Sept. 3 for the weekend of Sept. 6-8.
Duane "Dog" Chapman, also known as Dog the Bounty Hunter, said he had learned that Laundrie's parents spent the night there twice in early September.
"I will no longer give that dog credibility or dignify his false claims with the time of my reply," Steven Bertolino, Laundrie’s attorney, told Fox News at the time. However, he told local media that the family camped out on Sept. 6 and 7 and that "they all left the park."
The documents revealed the checkout was actually recorded on Sept. 8, three days before Gabby Petito officially became a missing person.
Thomas Rutherford, another camper, and his wife were celebrating their anniversary at the campground, three spots down from the Laundries, he previously told Fox News.
He said he could "vaguely remember" seeing their truck and camper because he and his wife rarely see campers attached to vehicles anymore. Other than that, he had no interaction with the family.
Reilly wrote that Roberta Laundrie also reportedly blocked Nicole Schmidt – Gabby's mother – on her cell phone and Facebook on or around Sept.10.
Petito’s body was found Sept. 19 at a campground near Grand Teton National Park in northwestern Wyoming.
While the Petito family was searching for information, Reilly said, the Laundries were keeping Brian's whereabouts secret, and it is believed they were "making arrangements for him to leave the country."
He accused Christopher and Roberta Laundrie of acting "with malice of great indifference to the rights of Joseph Petito and Nichole Schmidt."
"Christopher Laundrie and Roberta Laundrie exhibited extreme and outrageous conduct, which constitutes behavior, under the circumstances, which goes beyond all possible bounds of decency and is regarded as shocking, atrocious and utterly intolerable in a civilized community," he said.
Petito's parents and stepparents filed a lawsuit in Florida in March that seeks more than $100,000 in damages for alleged negligence, pain and suffering.
At the end of the month, Bertolino filed a motion to drop the lawsuit with prejudice, arguing that there are no facts to support their claims of "intentional infliction of emotional distress."
He asked that the judge prevent the Petito family from filing an amended lawsuit should his dismissal request be granted.
Bertolino said the Laundries were exercising their constitutional right to remain silent.
When asked if anything in the updated document shows the Laundries violating any laws, Bertolino told Fox News there was "nothing in my view."
"They had to reword to avoid being defective as to form. Now the judge can decide the motion to dismiss as to the law," he said.
KDFW noted that the lawsuit does not provide evidence of when Brian Laundrie's parents knew Gabby Petito was dead and where her remains might have been located.
Fox News' Stephanie Pagones, Audrey Conklin, Laura Ingle and The Associated Press contributed to this report.