NORTH PORT, Fla. - Fugitive Brian Laundrie appeared "very much in control" of his late fiancée, Gabby Petito, during their cross-country road trip before she disappeared, former FBI Behavioral Science Unit researcher Dr. Ann Wolbert Burgess says.

Laundrie and Petito were traveling in a van from New York to Oregon over the summer before Petito's parents reported her missing on Sept. 11. The FBI discovered her remains eight days later at a dispersed camping area within Wyoming's Bridger-Teton National Forest on Sept. 19.

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"He really was very much in control the whole time," Burgess, author of the forthcoming book "A Killer by Design: Murders, Mindhunters, and My Quest to Decipher the Criminal," told Fox News.

Referring specifically to Aug. 12 bodycam footage from police in Moab, Utah, following a domestic incident, Burgess noted that Laundrie appeared to have control over Petito's van because he was driving it; control over her finances because he reportedly used her credit card; and, ultimately, control over her because he "kept her isolated" by traveling to remote areas and camping, Burgess said. She also noted that Petito's remains were found in an isolated area.

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Burgess also mentioned that Laundrie continued to refer to Petito as his fiancée to police in the video despite the fact that her mother, Nichole Schmidt, told media outlets earlier in the saga that the pair had decided to hold off on getting married to travel.

"To Laundrie this is very, very important, and I think what sets into motion the final days for her," Burgess said of their engagement.

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Laundrie, 23, returned home to North Port, Florida, on Sept. 1 in the van without Petito. Laundrie has been named a person of interest in the case, and his whereabouts have been unknown for weeks. His parents reported him missing to North Port Police Sept. 17.

On Sept. 23, the FBI issued an arrest warrant for Laundrie, accusing him of bank card fraud. Authorities alleged at the time that he used an unidentified person’s Capital One card and the personal identification number to charge or withdraw over $1,000 between Aug. 30 and Sept. 1, a period when Petito was missing.