Could Brian Laundrie still be alive if he's in Florida's Carlton Reserve? Here's what survival experts think

Video shows Gabby Petito alive for last time Authorities have released footage of Gabby Petito and Brian Laundries shopping at a Whole Foods in Wyoming hours before her murder.

For more than three weeks, local and federal authorities have been combing through a 25,000-acre nature reserve searching for Brian Laundrie, the fiancé of 22-year-old Gabby Petito.

On Sept. 1, Laundrie returned in Petito's van to the couple's North Port, Fla. home from a cross-country road trip the couple embarked on earlier in the summer, police have said. Petito's remains were later found in Wyoming's Teton County.

Laundrie has not been accused of any wrongdoing in Petito's killing. Laundrie's parents told police he left home on Sept. 13 -- a day earlier than they initially told investigators -- to hike in the nearby Carlton Reserve.

Dozens of officers from agencies including the FBI, the North Port Police Department, Florida Wildlife Commission, several sheriff's offices and K9 search and rescue teams have been searching the reserve since Laundrie's parents first talked to police on Sept. 17.

"The Carlton Reserve is a vast and unforgiving location at times. It is currently (waist) deep in water in many areas," North Port police said in a Facebook post last month. "This is dangerous work for the search crews as they are wading through gator and snake infested swamps and flooded hiking and biking trails."

CNN asked survival experts if it's possible Laundrie is alive if he has been in the reserve since mid-September. Here's what they said.

IT DEPENDS ON HIS SUPPLIES AND EXPERIENCE

In an ABC interview that aired last week, Laundrie's sister, Cassie Laundrie, said he had previously taken trips for up to five days on the Appalachian Trail. She said her brother is a "mediocre survivalist."

Robert Urban, founder and chief instructor of the Urban Survival Academy, said he believes it's highly unlikely that someone who is not professionally trained in survival tactics would be able to survive in the reserve for this long.

"Florida's climate is very, very difficult," Urban told CNN. "I'm an expert with plenty of experience, and it would be everything that I could do to survive for (more) than three weeks. Someone with no experience, you cannot be lucky and survive in that sort of climate."

The list of challenges Laundrie could be facing isn't a short one. There's wildlife that could hurt him, including water moccasins, hogs, bears and alligators, survival experts told CNN. There have also been panther sightings in the reserve in the past. If Laundrie was in the reserve for this long, he would likely also not be operating under the best mindset, several experts added, due to the anxiety of trying to evade authorities and trying to survive, which could hinder his survival skills.

"When you're tired, you're not as defensive, you're not as vigilant as you should be," Urban said.

Jason Marsteiner, founder and president of The Survival University, said when he sends trainees of his program in the wilderness -- for just about a week after they've gone through a 50-day course -- they struggle.

"Twenty-five days in that area is extremely tough. I wouldn't want to do it and I've got jungle training, I've got mountain training," he said. "He's not sleeping well, and when you don't sleep you slowly drive yourself insane, so he would be making bad decisions, bad choices and I think that would cause him to be found or get injured or perish."

HE WOULD NEED SHELTER, WATER, FOOD

It's unclear what Laundrie took with him when he left. His parents told police he left home with a backpack. Experts say a big part of whether he could still be alive if he was in the reserve depends on how prepared he was when he allegedly went in and how able he is to come up with the three basics: shelter, water and food.

"If he had ways to disinfect the water through filtration and (cover) to protect himself from the weather and things like that, he could survive for a certain period of time," survival expert Dave Canterbury told CNN's Erin Burnett on Thursday. "If he had ways to gather food and process that food, he could survive even longer."

But Laundrie would likely have to process and cook the food he gathers, Canterbury added, and if his water filtration system failed, he would have to find another way to disinfect the water. Both of those things would likely require him starting a fire. But North Port police spokesperson Josh Taylor told CNN's Randi Kaye on Friday authorities have not found any physical evidence of Laundrie in the reserve. Police also disputed that a recently used campsite was found -- information that was given to CNN from a source close to the Laundrie family.

"Starting fire is going to be a big no-no for (Laundrie) if he doesn't want to get found or seen, so I think it's going to be very difficult for him to stay out there for a very long period of time," Canterbury said. But he added that Laundrie could have had enough in his backpack to survive up to this point.

Alan McEwen, a cattle rancher and outdoorsman who lives near the reserve, said the area feels like "muck," with parts of the reserve reaching knee- and waist-deep water levels -- making camping in that wilderness extremely difficult.

"The palmettos out there alone are enough to trip you up, when you're walking through them, when you you stumble through them," McEwen told CNN's Chris Cuomo Friday night. "The mosquitos will carry you off, anybody out there more than a day without mosquito spray, you'd go insane with the bugs getting you and everything else."

"There's just no possible way for anybody to survive out there like that," he added. "Nobody."

The way Byron Kerns, founder of the Itchatad Outdoors Survival School, sees it, Laundrie's most immediate need would be water, and if he had a water filtration pump with him, that could last him for gallons. But after three weeks, Laundrie would need to get food, and if he's not trained and doesn't have the right tools, knowing what to eat and how to get it can be tricky.

"Bugs can also be a source of protein in the wilderness, but does he know that?" said Kerns.

EXPERTS THINK HE'S EITHER NOT ALIVE OR NOT IN THERE

Laundrie family attorney Steven Bertolino told CNN in response to a question he is "hopeful" that Laundrie is still alive. Taylor, the North Port police spokesperson, told CNN it is "certainly possible" that Laundrie is alive.

"Nothing to suggest one way or another," Taylor said in an email.

The survival experts CNN spoke to seem to agree that Laundrie is either no longer alive or not in the reserve.

"(Authorities) probably have some of their best people in the nation out there looking for him and they haven't found no sign of him," Marsteiner said. "The 25,000 acres, it sounds big, but when you've got that many people scouring the area it's really not."

It's highly unlikely he could be camping out and continuing to evade the search teams that have been looking for him, Urban added.

"Some non-trained guy just used to walking on trails is not going to evade a team of experts like that, just by hiding," Urban said, adding that he also believes Laundrie is either not in the reserve or not alive in there.

If Laundrie is in the reserve but no longer alive, finding him becomes harder as more days go by, Urban added, with the movement of water and sand, as well as the animals that go by. But there have been no indications that Laundrie has died either.

"When you have something dead out there, you're going to have buzzards flying everywhere," McEwen told Cuomo on Friday. "You can't see any buzzards flying anywhere, and no buzzards have been flying in any direction of this place."