"I've got questions, but the only one that matters is, 'Where'd you leave Gabby? Where did you last see her?'" Joseph Petito tells PEOPLE

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Joseph Petito has questions about his missing daughter, Gabrielle, and tells PEOPLE the one person who may hold the answers he seeks — her fiancé, Brian Laundrie — is not talking.

Joseph spoke to PEOPLE on Wednesday, shortly after North Port Police declared Laundrie, 23, a person of interest in the 22-year-old Long Island, N.Y., woman's disappearance.

"Oh, he's been a person of interest in my opinion, since this thing started," says Joseph of Laundrie, who returned to his parents' Florida home on Sept. 1 following a cross-country trip with Gabby that began in July.

"I've got questions, but the only one that matters is, 'Where'd you leave Gabby? Where did you last see her?'" Petito says.

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Laundrie, police said Wednesday, has refused to cooperate with their investigation.

Gabby Petito was last heard from on Aug. 25 — six days before Laundrie, 23, was back in Florida.

Gabby's mother reported her missing on Sept. 11.

The couple had been traveling across America in a white Ford Transit van since early July, and Gabby had been documenting her travels on YouTube.

gabby petito
Gabrielle Petito
| Credit: Facebook

They started their trip traveling from Florida to New York. Last month, they arrived in Salt Lake City, Utah, staying only a short time as wildfire smoke blanketed the region.

The couple apparently stopped in Grand Teton National Park on Aug. 25 before heading for Yellowstone.

On Saturday, detectives attempted to interview Laundrie and his parents at their home, but were told to contact the family's attorney, who has instructed his clients not to talk.

"The only reason you invoke your Fifth [Amendment right], in my opinion, is because you don't want to incriminate yourself," Joseph says. "I'm not trying to accuse nobody, but right now, it looks like you are guilty. Innocent people don't clam up."

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Prior to Gabby vanishing, Joseph says he had no reason to suspect that Laundrie would harm her.

"I hate to say it, but they were happy," he says. "There are no red flags that stick out."

Gabby is a white female, approximately 5 feet 5 inches tall and 110 pounds. She has blonde hair and blue eyes, and several tattoos, including one on her finger and one on her forearm that reads "let it be."

Anyone with information on Petito's whereabouts is urged to call 800-CALL-FBI.