Police in North Port, Florida say Brian Laundrie was under their surveillance before he disappeared, but they were limited in what they could do.

As the search for Gabby Petito's fiancé approaches its fourth week, no police activity was visible Friday at the Carlton Nature Reserve, where Laundrie's parents believe he went before disappearing.

Laundrie returned to his parents’ home in Petito's white van without Petito on Sept. 1. Before he left his parents’ home on Sept. 13, he told them he was headed to the 25,000-acre reserve.

North Port Police say while Laundrie was under their watch, they were limited in what they could do because he had not been charged for a crime.

"If you talk to a lot of people who have experience in law enforcement— the guy goes for a walk in the Carlton Reserve, he's not wanted for a crime, I mean – what are we supposed to do? Go tree to tree? Tree to tree, following him back through the woods? It just wasn't there with the information we had in this case,” says Public Information Officer Josh Taylor.

Police also confirmed they do not have the cell phones Laundrie or Petito used during their cross country trip.

Laundrie bought a new cell phone from an AT&T store in North Port on Sept. 4 and left it behind on Sept. 13.

Laundrie has not been charged in connection with Petito's death, but he is suspected of using a debit card Petito's family says belonged to her to access over $1,000 after her death. A federal warrant has been issued for his arrest on that charge.

In an interview with Fox News that aired Thursday, Petito's family pleaded with Laundrie to turn himself in.

"People want to know how I am feeling…I am feeling upset. Turn yourself in. That's all I wanted,” said Petito’s mother, Nichole Schmidt. “It's just getting more and more frustrating as days go on. I don't know what is taking so long."

On Long Island, the Gabby Petito Foundation is gearing up for its first fundraiser. It will be held at the North Music Venue in Patchogue on Sunday, Oct. 17.