During Trip, 'There Appeared to Be More and More Tension' Between Gabby Petito and Fiancé: Warrant

Police believe the relationship between Gabby Petito and her fiancé Brian Laundrie had become more strained during their tragic road trip, according to a newly unsealed search warrant connected to the case.

The warrant, filed on Sept. 17, allowed detectives with the North Port, Fla. police to examine an external hard drive in their investigation of Gabby's disappearance. Gabby, 22, last made verbal contact with the family on Aug. 25 during her cross-country road trip with Laundry, 23.

In the "probable cause for issuance" section of the warrant application, North Port police detective Daniel Alix wrote that texts between Gabby and her mother showed that "there appeared to be more and more tension between her and Laundrie" during the road trip.

Alix also lists the "domestic disturbance" between the couple, which led to an encounter between the couple and police that was seen in widely-circulated body cam footage, as a reason he was seeking the warrant.

The person who called 911 to report the disturbance said he witnessed Laundrie striking and "slapping" Gabby.

The warrant also states that Gabby's mother, Nicole Schmidt, received an "odd text" from her daughter's phone on August 27 — three days after she last spoke to her family verbally.

According to the warrant, the message read: "Can you help Stan, I just keep getting his voicemails and missed calls."

Schmidt told investigators that Stan is Gabby's grandfather, but that she doesn't usually refer to him by his first name, the warrant states.

"The mother was concerned that something was wrong with her daughter," Alix wrote.

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This is the last communication that Gabby's phone had with her family. After August 27, her phone wasn't operational, and she did not post anything on social media, Alix said in the warrant.

"Per her family, this was not normal behavior for [Gabby], and they became more worried about her," the search warrant reads.

Gabby was first reported missing on Sept. 11. On Sunday, the FBI confirmed they discovered a body in Wyoming matching Gabby's description.

Laundrie was identified as a person of interest in the case and refused to sit down with investigators to discuss Gabby's whereabouts. He has also since gone missing, with authorities in Florida currently conducting searches for him.

He has not been named a suspect at this time. Laundrie's lawyer has not responded to PEOPLE's requests for comment.

On Sept. 15, PEOPLE spoke with Gabby's father, Joseph Petito, who said his daughter was "amazing."

"She's so artistic. She's an amazing artist. She loves to draw," he said. "She has a great eye with decorating … She's that flower child type of personality. The old soul, you know?"

"If Woodstock was still going on, she'd still be there, man," Joseph added. "She's a hippie from the '60s, all right? She's a Beatles fan, round glasses … a free spirit."