"There was always something below the surface where things weren't 100% wonderful," says Laundrie's friend, Ben Matula

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On the surface, Gabby Petito and Brian Laundrie seemed as perfectly-matched as any two people could be. But friends who spoke with PEOPLE say there was often tension between the couple, who'd dated since high school.

"One minute, they'd be all over each other, the next minute, he'd be like, 'We're fighting,'" says Laundrie's friend, Ben Matula, describing their relationship when they were teenagers in Long Island, New York. "They always had some drama."

Matula, who spoke to PEOPLE exclusively for this week's issue, adds: "There was always something below the surface where things weren't 100% wonderful."

Over the weekend, Gabby's body was found in Wyoming's Grand Teton National Park, and officials have since determined her death was a homicide. Meanwhile, the search continues for Laundrie, her fiancé, who hasn't been seen in over a week.

For nearly two months, Gabby, 22, and Laundrie, 23, had been traveling across America in a white, repurposed Ford Transit van. Police say Laundrie returned to his parents' Florida home in the van on Sept. 1, and that he was alone.

gabby petito cover

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Gabby was reported missing on Sept. 11 — more than two weeks after her mother last spoke to her by phone. Laundrie and his parents refused to meet with detectives handling the case.

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Last Wednesday, Laundrie was named a person of interest in the case. Last Friday, he was reported missing. Laundrie has not been named a suspect in the case.

Gabby Petito, Brian Laundrie
Gabby Petito, Brian Laundrie
| Credit: Find Gabby/Facebook

Alyssa Chen, 23, said her vlogger friend Gabby's on-again, off-again relationship with Laundrie had many ups and downs.

"They had times they were toxic and times where everything seemed a lot more healthy," Chen tells PEOPLE. "They had very low lows and very high highs. But they really seemed to love each other. When things were good, you'd be like, 'Why can't I have a relationship like that?' When they were bad, you'd be like 'Oh my God, just break up and spare yourself from the drama and everyone else from having to hear about it.' "

At this point, investigators have yet to say how Gabby was killed, but believe she died sometime between Aug. 27 and Aug. 30.

Police believe the relationship between Gabby and Laundrie became 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 Petito
Gabby Petito, Brian Laundrie
| Credit: Gabby Petito Instagram

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.

Matula says the tragedy of their road trip has made him reflect upon what he thought he knew about the couple.

"The guy who he's being made out to be isn't the guy I knew," Matula insists. "But then I have to look at what's going on and realize that he did some stuff that seems like it was shady — seems like maybe he wasn't on the up and up. And because he did that stuff, a girl might be dead and he might be in trouble.

Gabby Petito and Brian Laundrie
Brian Laundrie, Gabby Petito
| Credit: North Port Police Department/Facebook

"It seemed like they were seeing the world with the person they loved," Matula says. "They were living the dream that all of us want to do. But they were doing it. All their friends were either still in college or searching for entry level jobs, and they're out there killing it. I was jealous, and I know I'm not the only one. I was happy for them."

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

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"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."

If you have information on this case, call 1-­800-­CALL-­FBI (225­-5324).