Authorities returned Thursday to the sprawling Florida wildlife refuge where they have been searching for Brian Laundrie, who is wanted for questioning in the disappearance of his fiancée, Gabby Petito, whose death was ruled a homicide.

But Laundrie is likely to have fled the state and may have had help, experts in missing persons cases said.

“It seems unlikely that he’s still in that reserve. They did not find any trace of him so far. It’s been weeks,” said former FBI agent Bryanna Fox, an associate professor of criminology at the University of South Florida in Tampa.

Laundrie, 23, of North Port, Florida, may have had financial or transportation assistance eluding authorities, Fox said, adding that if someone picked him up hitchhiking or if he boarded a bus, witnesses probably would have gone to police or the media.

“The idea that nothing has been said reinforces to me that he was likely getting help,” Fox said. “Obviously, that person would have to be very loyal and very close to him … to not compromise his identity.”

Laundrie’s family said he went hiking Sept. 13 in the Carlton Reserve, which has been searched multiple times. Law enforcement officers were back at the 25,000-acre wildlife refuge Thursday, with his father, Chris Laundrie, helping, family attorney Steven Bertolino said.

Again, the search came up empty, Bertolino said in a text message to Telemundo affiliate WRMD of Tampa.

“Chris Laundrie accompanied members of law enforcement into the Reserve to show them the trails and places Chris and Brian have hiked and which Brian was known to frequent,” Bertolino said. “There were no discoveries but the effort was helpful to all. It seems the water in the Reserve is receding and certain areas are more accessible to search. … Hopefully Brian will be located soon.”

The unforgiving terrain and the weather could easily erase any evidence of Laundrie’s presence, Fox said.

“These factors that are here in Florida, a subtropical environment, are known to degrade evidence,” she said. “Imagine a massive landscape that’s extremely dangerous. There are snakes, gators and animals. There is rain, torrential downpours that come down out of the blue. It’s hot.”

Laundrie was on a cross-country road trip with Petito, 22, when she disappeared in late August. Her remains were found Sept. 19 at a campground in Wyoming near Grand Teton National Park. A coroner ruled the manner of death a homicide; an official cause of death is pending autopsy results.

A grand jury indictment said the FBI wanted to speak with Laundrie about his “activities following the death of Gabrielle Petito.” A federal arrest warrant was issued Sept. 22 in Wyoming alleging that he “knowingly and with intent to defraud, used one or more unauthorized access devices, namely a Capital One Bank debit card” owned by Petito and personal identification numbers for two accounts.

Thomas Lauth, a private investigator in Indianapolis who specializes in missing persons cases, said Thursday that he agreed with the theory that Laundrie left his home state and is likely to have had help.

“I have never thought he would stay in Florida very long,” Lauth said. “I believe he had some outside resources available to him early on, but those outside resources have likely cut him off because he’s a fugitive.”

Laundrie has the skill set to vanish in the wilderness, Lauth said.

“He’s been a savvy traveler for many years. When I say ‘savvy,’ he’s able to live off the land, and he knows how to travel alone and lightly,” Lauth said.

Reports of possible Laundrie sightings have flooded law enforcement agencies in Canada, Alabama and Montana, NBC New York reported, but none have been publicly confirmed by the FBI.

A spokesperson for the FBI could not be reached for comment Thursday afternoon.

A misstep by Laundrie or a critical clue uncovered by the FBI is likely to lead to his being found, Fox said.

“He has to get lucky all the time for him not to get caught,” she said. “They only have to get lucky once. In the end, there’s a good chance law enforcement will either find him or learn what happened to him sooner than later.”