Transcript for Former prisoners in North Korea recall the conditions they were held in: Part 3
Reporter: Seoul, South Korea. A teeming, high-tech metropolis birthplace of that 21st century classic -- ??? gangnam style ??? Reporter: "Gangnam style." A stark contrast to life just 35 miles north, across the border, call it Pyongyang style. This satellite video tells it all. South Korea, ablaze with lights at night. The north, grim and dark. Right across the river is north Korea, and it is absolutely dark. There's no other country that's like North Korea that's left in the world. In fact, I would say the whole country is a slave state. We have 25 million people who are imprisoned. Reporter: This man knows all about the enslavement of the north, and the kind of ordeal Otto Warmbier may have faced. He's American Kenneth Bae, now living in Seoul, after spending over two years imprisoned in the north. Bae was leading a tour of north Korea in 2014 when he ran into trouble with the authorities -- accused of supposed hostile acts against the regime. Just like Otto, Bae was sentenced to 15 years hard labor. I was devastated to find out what happened to Otto. I just cannot imagine what he was going through at the time, knowing that he's actually going to prison in north Cree yard for 15 years. Reporter: Kenneth Bae and Otto Warmbier had been caught up in the capricious, brutal world of north Korean justice. Infractions as minor as not showing the proper affection for the dear leader Kim Jong-un can earn you a one-way ticket to the gulag of secret prison camps known as kwanliso. These camps are places of unimaginable suffering. They're the very machinery of a a widespread and systematic campaign of enslavement and of executions. Reporter: Scott Edwards of amnesty international says the facilities are so secretive, the only way to see what they look like is from space, through satellite imagery. What's this we're looking at here? So, this is one of these political prison camps that amnesty has recently imaged. This area is about the size of all of New York City. There's another one up here in the north, kwanliso 16. It's about three times the size of D.C. Reporter: Edwards says within these gargantuan facilities -- whole industries are devoted to slave labor. The different areas are different sorts of commercial or agricultural or mining activity. What goes on in the camps that isn't visible from space is torture, is the intentional starving of populations. It's infanticide. It's rape. It's mutilation. Reporter: How do you know this? We know, primarily, from the testimony of those who have left the camps. Reporter: Although no photos exist of camp conditions, former prisoners have sketched scenes of the grim reality of life inside. Witnesses actually described literal torture chambers, stress positions. People told of forging for whatever they could find, eating frogs, finding mice to field to infants. One of these drawings actually shows, you know, the rendition of bodies being carted off to presumably a crematorium. Reporter: There he goes. Of course, to that dear leader, Americans like Otto are too valuable as political tools to be subject to the wanton killing in these labor camps. But while they escape the biggest horrors, their fate is still tied to the whims of the regime and its irrational notions of what punishment fits the crime. It depends. There have been some Americans that have talked about being abused. There are others that have said they are treated properly. It varies. Reporter: Take the case of American Jeff Fowle, who was convicted in North Korea in 2014 after deliberately leaving a bible behind at a night club. Fowle was detained at that same mysterious luxury hotel, the yanggakdo, where Otto was staying. After just a six-month stay at the hotel and at a guest house, Fowle was allowed to go home. I interviewed him after his release. How did they treat you physically? Physically, I was not abused at all. I got three meals a day. Reporter: Was it pretty good food? What they served their guests. Reporter: But there was no luxury hotel for Kenneth Bae. He was sent to a labor camp specifically reserved for foreigners. I was carrying maybe coal from one place to the other, or carrying rock or digging the hole and any type of manual labor, I have to endure ten hours a day for six days a week. Reporter: Why the harsh treatment? It's because Bae was on a mission to spread the word of god in North Korea through his tour group. Why is North Korea so afraid of christianity? In North Korea, do you know there's only one religion? It's worshipping the leader Kim il-sung. We are afraid of someone like you coming in, trying to convert our people or influence our people with your religions, because if this spreads, our country will fall. Reporter: Kenneth Bae languished for 735 days in captivity, the longest held American prisoner ever. Bae confessed that he had committed serious crimes against the regime, after extensive negotiations with the U.S., the north Koreans finally released him in November of 2014. Today, Bae is involved with a human rights group in south Korea, determined to help get the remaining three American prisoners held in North Korea released without the tragedy consequences of Otto's case. What happened to Otto should not happen again. I do believe that something can come out from it. And impact the people in north Korea and also all the detain needs that are waiting to come home.
This transcript has been automatically generated and may not be 100% accurate.