The search for Brian Laundrie, Gabby Petito’s fiancé, resumes today in the Carlton Reserve.

(CNN) — Brian Laundrie, the fiancé of Gabby Petito, has been missing for almost a week.

His disappearance comes after he and Petito embarked on a cross-country trip in June, only for him to return alone in September. The search for Petito has made national headlines with daily developments, and Laundrie’s disappearance has only added to the confusion.

North Port Police said Tuesday morning they are searching for him on the Venice side of the Carlton Reserve, a 25,000-acre nature reserve near Venice, Florida.

Laundrie has not been charged and is not suspected of a crime at this point, authorities said. He had refused to speak with police, leaving authorities stumped and powerless in their investigation.

Here’s why the search for Laundrie has been so difficult for authorities:

Laundrie had a multiple-day head start

Laundrie’s family told police Friday night they have not seen the 23-year-old since Tuesday. His family told police he left home with his backpack and told them he was going to the Carlton Reserve.

Police in North Port, Florida, tweeted Saturday that authorities were searching the reserve, an effort that included the use of drones and bloodhounds who had used articles of Laundrie’s clothing taken from his home to get his scent, said police spokesperson Josh Taylor.

In a place like a nature reserve, foliage and the lack of sunlight affects visibility, according to Chris Boyer, executive director of the non-profit National Association for Search and Rescue (NASAR). The night can also hamper efforts, especially if the person being sought has no source of light or fire.

But when it comes to people eluding authorities, the search becomes much more difficult. An evasive person is more than likely wearing clothing that helps them blend in with their surroundings, Boyer said. In order to avoid being seen by helicopters or drones, an individual may also crawl in creek beds and avoid leaving tracks — such as footprints, trash or evidence of a fire.

Boyer said technology like night vision goggles, drones and thermal sensors could help in pinning down a person’s location.

Before he disappeared, Laundrie had been home in North Port for about two weeks.

He and Petito had been touring Western states together in her van for months until he returned to North Port on September 1 without her.

The family told police the last time they saw him was September 14.

Cheryl Dorsey, a retired Los Angeles police sergeant, told CNN’s Jake Tapper on Monday she was curious why Laundrie’s parents did not alert authorities about his leaving Tuesday.

“I get that he’s a grown man,” Dorsey said, adding that he’s still just in his early 20s. “What influence, if any, do (his parents) have over him? He decides to go backpacking and they couldn’t stop him?”

Wilderness searches are difficult

North Port police said Monday they shifted the focus of their search for Laundrie and are no longer looking for him in the nature reserve.

“At this time, we currently believe we have exhausted all avenues in searching of the grounds there,” Taylor said.

However, police said Tuesday morning they were again searching for him at the reserve.

Boyer, the NASAR director, said trying to find a person in the wilderness can be very difficult.

“It’s really hard to find people even when they want to be found,” he told CNN on Monday.

What makes finding Laundrie difficult, though, is the distance he could have already traveled before authorities started looking for him.

“The search area starts to grow every hour he could be in a car or be on foot,” Boyer said. “It gets pretty daunting, to be honest.”

Laundrie has not been charged with a crime

Laundrie has not cooperated with police in their search for Petito, and because he has not been charged with a crime or been named a suspect, police are unable to do any more than file a search warrant.

The FBI executed a search warrant Monday on Laundrie’s parents’ home, where he lived with Petito.

The FBI removed Christopher and Roberta Laundrie from the home, executed the search warrant, and then brought them back inside for questioning, Taylor said. The search of the home concluded Monday evening, the FBI tweeted.

Police visited the home last week but the family refused to talk and instead gave authorities their attorney’s information, Taylor said.

On Saturday, Taylor reiterated police were limited in what they could do because “we don’t have a crime.”

“Laundrie is not a suspect in a crime. We think he is likely one of the last people to see Gabby Petito alive, and for that reason he’s a very important witness,” said Andrew McCabe, CNN senior law enforcement analyst and former deputy director of the FBI.

Before he vanished, Laundrie was silent about Petito’s disappearance. North Port Police Chief Todd Garrison told CNN’s Don Lemon last week that Laundrie had invoked his Fifth Amendment right, which generally means a person cannot be forced to make statements they feel might be negative or used against them.

Steve Moore, retired FBI supervisory special agent, told CNN on Monday that in order to obtain a search warrant, authorities would need to have probable cause there had been a crime and the person at the home was involved in the crime.

“What I believe people in law enforcement are doing right now are making sure they have all the t’s crossed and i’s dotted because I think they believe — and I believe — they know who did this and they want to make sure their case is perfect at this point,” Moore said.

McCabe told CNN’s Ana Cabrera on Monday police had gotten to the point where “the search warrant absolutely has to be executed.”

“Primarily, I think what the investigators will be looking for are anything that he may have written, any recordings of his thoughts, if he wrote any notes, if he kept a journal,” or any electronic activity and history he may have, McCabe said.

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