An explosive argument between Gabby Petito and Brian Laundrie at a restaurant is one of the two "most important" clues surrounding the 22-year-old's death, an FBI researcher has said.
Gabby remains were found at a dispersed camping area within Wyoming's Bridger-Teton National Forest on September 19.
The YouTuber was witnessed having an altercation with him just "days" before she disappeared.
Gabby and Laundrie, 23, were on a cross-country road trip when the social media star disappeared on August 26.
A coroner has now ruled she died of strangulation and that she had probably been dead for three or four weeks before her body was discovered.
Laundrie went missing shortly after returning home without Gabby, and has now been gone for nearly a month.
He has been named a "person of interest" by police.
According to a witness named Nina Celie Angelo, the two had been in a heated argument while at the Merry Piglet restaurant in Jackson, Wyoming.
The witness told ABC News that Laundrie was "aggressive" towards staff while Gabby cried and apologised for her boyfriend's behaviour.
"It looked as though they were like almost getting kicked out," Angelo said.
"It wasn't necessarily between them, it was more so Gabby abruptly leaving the restaurant crying, and Brian was just evidently really upset, pissed off, I would say.
"I would say Gabby was upset, he was angry, and he was just being very temperamental towards the restaurant staff."
Angela, who was sitting within a foot of the couple, said Laundrie left the restaurant four times, leaving and returning only to start the whole argument again.
Dr Ann Wolbert Burgess, who pioneered the FBI's Behavioral Science Unit, told Fox News that Gabby's alleged apologising for Laundrie's behaviour may be indicative of the tragic fate that awaited her.
She said: "How she has to apologise for his behaviour is something that's kind of characteristic, and this probably enrages him even more because he wants to appear in a certain way.”
The restaurant incident, Dr Burgess said, was "on the day or very close to the day that she is murdered".
Dr Burgess went on to say the second "most important" clue about the couple's dangerous relationship is the domestic dispute police were called to on August 12 in Moab, Utah.
It comes after friends have spoken of how terrified Gabby must have felt in the moment leading up to her death.
"It makes me sick to my stomach," friend Alyssa Chen told news outlet People.
"I wonder what was going through her mind, knowing she was probably about to die. She must have been so scared."
Ms Chen added: "She didn't deserve to die that way."
Gabby and Laundrie started their trip in early July from New York's Long Island, where her parents live.
Laundrie then returned without her on September 1 to his family's North Port home, in Florida, and driving Gabby's van.
A total of 10 days later her mother Nicole Schmidt reported her missing.
