Parents in Mysterious ‘Missing Dial’ Case on Painful Quest to Find Vanished Son in Costa Rican Jungle

— -- Legendary explorer Roman Dial has been on a painful, very personal quest in a Central American rain forest for the past two years.

Dial and his wife Peggy had been searching Costa Rica’s wild and dangerous Osa peninsula for their missing son, Cody Roman Dial. The 27-year-old biology grad student was last seen in July 2014 hiking by himself and heading off toward a remote part of the jungle.

National Geographic cameras documented the Dials’ harrowing journey in the new series, “Missing Dial.”

Roman Dial is a world renowned biologist, a National Geographic explorer and a pioneering, fearless ice climber, who raised his family in Alaska. He and Cody shared adventures together.

“When he was 6 we did a 60-mile walk, just the two of us, across an island in the Aleutians,” Roman Dial told "Nightline." “He was not an adrenaline junky at all, and I would admit that I am.... He and I liked sort of science and adventure and doing it together.”

Watch the full story on "Nightline" tonight at 12:37 a.m. ET

Since his son has disappeared, Roman has struggled to shake a sense of guilt.

“I did at least, think I shouldn’t have spent all that time doing things that I like doing and dragging him into it,” he said. “Did I want him to follow in my footsteps? I mean, I guess. I liked having him as a companion. I liked having him do stuff with me.”

When Cody said he was going solo to Central America, Roman supported his son’s decision. He said Cody was usually pretty consistent with checking in with his family over email. But at one point, Roman realized several days had gone by and they hadn’t heard from Cody.

“I checked my emails and I realized that this email about topographic maps that I hadn’t opened was actually, ‘Here’s where I’m going to go, I’m going to do this five-day trip across Corcovado Park. I’m going by myself and I’m going off-trail,’” Roman Dial said. “And I was like, ‘Woah, when did he send this email?’ Two weeks old ... so he’s like 10 days overdue.”

Dial called the Costa Rican authorities and they began searching for Cody. Then he got on a plane.

“It never crossed my mind not to go,” he said. “He was hurt. That was my initial thought. That he was hurt. Not that he was lost. He was really careful about not getting lost.”

Within the first five days after landing in Costa Rica, Dial said they found a gold miner who was the last person who saw Cody alive.

“He took us to the spot, and then I kept trying to get back to that spot but it’s within the national park.... They wouldn’t let me go in,” Dial said. “Typically in searches and investigations the family members are kind of pushed off to the side because we’re too emotional.”

After two weeks and still no sign of Cody, the Costa Rican authorities called off the search, but the Dials did not.

“I held out all kinds of hope,” Roman Dial said. “I mean, we had psychics who said, ‘Oh, I can see him. He’s under boards and alive but he’s in a jail,’ sort of thing.

“After a while I was like, ‘He would have contacted us, kidnappers would have sent us a demand for money,’” he continued. “I came to a realization after three months or so that he was most likely dead.”

The Dials brought in their own private investigators. Then Cody’s backpack was discovered at a local hostel on the edge of the national park and locals said they had also spotted him.

“The Red Cross had said to me, ‘Oh your son was seen with this local bad guy, this drug dealer, and they walked across this trail,’ and I was like, ‘That doesn’t sound like my son,’” Dial said.

A guide, who goes by the name Pata de Lora, or Parrot’s Foot, was questioned. He claimed gold miners had killed Cody, but his story didn’t add up.

Still, the OIJ, which is Costa Rica’s FBI equivalent, changed Cody Dial’s case status from missing person to homicide. The Dials went to Washington to ask the American FBI to get involved.

“It’s been hard, he is on our mind constantly,” Peggy Dial said. “It’s hard to move forward and to go on with life because we get called back to look for him. We have to keep going back to find answers. I feel like I can’t move forward I have to find the answer. I won’t have closure until I have an answer. I would like to have his body home. It’s been emotional and a rough ride.”

The Dials then went home to Alaska and National Geographic began editing their missing person show without a body when a call came in this past May from Costa Rican authorities saying human remains and camping equipment had been found less than a mile from where Cody was last seen.

“The teeth, our dental records, we sent them and they said the teeth match,” Roman Dial said. “They sent us a photograph of the skull in the place ... I about broke down right there.”

Now, the new theory is that Cody had set up camp and a tree had fallen on him in what could have been a freak accident. Fallen trees are common in that area.

“The top of his pack had his passport and his money in it,” Roman Dial said. “If somebody killed him, why would they kill him in a super remote place and leave his passport and his money?”

DNA test results are expected soon to confirm the remains found are Cody Roman Dial. The Dials hope their theory that their son died in an accident is true.

“At this point, I’m happy with the narrative I have for myself, and if it’s true, or not true ... the feeling is right for me,” Roman Dial said. “I’ve see pictures of his bones. Yeah, freaking heartbreaking, man. It’s the worst possible thing to lose your children. I can’t think of anything worse.... But at least it wasn’t a murder scene.”

The Dials said they have gone back to the spot where the body was found, and they are planning to have a memorial service.

“I have a chronic pain that will never go away. I probably cry every day,” Roman said. “This is a horrible thing to say, but I think adventuring is a pretty selfish activity.... I have kind of scaled back what I do because I don’t want my family to suffer another loss.”

ABC News' Nick Watt contributed to this story