A Florida search warrant mentions text messages between Gabby Petito and her mother.
The warrant said the messages seemed to show growing "tension" between Petito and Brian Laundrie.
On Monday, the FBI also executed a search warrant at Laundrie's home, declaring it a crime scene.
A search warrant filed last week by Florida law enforcement to search Brian Laundrie's computer cited phone communications between Gabby Petito and her mother, Nichole Schmidt, as evidence of growing tension in Laundrie's and Petito's relationship.
The warrant was filed by the North Port Police Department after Laundrie was named a person of interest last week in the late-August disappearance of his 22-year-old fiancée, Petito, after they embarked on a cross-country road trip.
"During the trip, the subject" - Petito - "sent multiple text messages and had many talks with her mother, Nichole Schmidt, via cellphone communication," the warrant said, adding: "During these conversations there appeared to be more and more tension between her and Laundrie."
The warrant added that Schmidt said she received "odd texts" from Petito, including the last one sent August 27, when Petito referred to her grandfather by his first name, Stan, which was unusual to her mother and cause for concern that someone else may have been using Petito's phone.
The tenor of the messages and, later, the lack of communication from Petito were what led her mother to contact authorities, according to the warrant.
The warrant was filed to gain access to Laundrie's computer and electronic devices.
Separately on Monday, the FBI executed a court-obtained search warrant at Laundrie's family's home in North Port, Florida, declaring the location a crime scene.
The home was declared a crime scene a day after a body was found in Grand Teton National Park in Wyoming. An autopsy was expected to be completed Tuesday to identify the body.
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