Over the last few years South Korea has been releasing a lot of what it knows about the North Korean espionage effort against the south. The latest revelations describe how North Korea has trained its spies. For decades North Korea has recruited some of its most capable people and put them through extensive training to operate as spies in the south. These operatives were expected to enter South Korea via submarine or a third country. False documents gave the spy a new identity that could pass all but the most intense scrutiny. But what really made these agents so effective was their training in the culture of South Korea. Until the last decade most North Koreans had always lived in near total isolation from the south. The North Korean accent was distinctive and trainee spies had to lose that and then learn thousands of facts about South Korean culture so they would not stand out. To make this training work North Korea had long kidnapped South Koreans (using commandos landing on the coast from a sub and escaping the same way or grabbing them in third countries) who were then forced to spend the rest of their lives providing cultural training for spies. These kidnapped trainers ensured that the accent of the new spies was flawless and that their knowledge of the culture (including the latest TV shows, movies, pop songs, and celebrity gossip) was extensive enough to make them appear as natives.
North Korea selected 200 trainee spies a year, usually after they had graduated from high school. About half managed to graduate from the four year course and were sent south or to other countries where they would spy while pretending to be South Koreans. The deal was that after 5-10 years as spies they would return to North Korea and be given comfortable careers for their rest of their lives. Many North Korean officials got their start via this route and these are among the most trusted members of the bureaucracy because they “went south” and returned, despite all the temptations to stay. But some did stay, either because they got caught or because they did become disenchanted with the north and turned themselves in and became double agents. Thus, many of the North Korean agents sent south had orders to hunt down and kill North Korean spies who had either stopped reporting back or were suspected of operating as double agents. Not all of these assassination efforts worked but enough did succeed to force South Korea to put a lot more effort into protecting its defector spies.
Last year South Korea revealed some of the special weapons captured from these North Korean death squads. Two of the weapons were disguised to look like a pen. One contained a poison tipped needle. The poison used is a muscle relaxant that causes paralysis and eventually suffocation (as the lungs stop working). Another version of this poison pen fires a small bullet containing the same type of poison. A third weapon was a small flashlight that contained three bullets that could be fired one at a time. This weapon was tested by South Korean technicians and found to work perfectly. Some of these weapons were found in the possession of North Korean agents who had managed to get past the screening to find such agents among refugees. These spies would then make contact with another agent in South Korea and obtain the weapons.
As more and more people have escaped from North Korea and made their way to South Korea it was first suspected, and then confirmed, that North Korea was sneaking spies into South Korea by having agents pretend to be refugees. None of these agents was ever caught until 2008. Most of those caught since then appear to have been sent to locate and kill refugees who were officials in the North Korean government and were a constant source of embarrassment to North Korea because of all that inside knowledge, or activists trying to aid in the overthrow of the North Korean government.
The first of these assassins to be discovered was a 35 year old North Korean woman, Jong Hwa Won. She was arrested after being observed by South Korean intelligence for three years. The South Koreans were hoping Won would lead them to other North Korean spies but she appeared to be operating alone. Won had been sent to northern China a decade earlier to help the Chinese identify North Korean refugees (who were then sent back to North Korea, where they were punished and sometimes killed). Won had a secondary mission, to arrange the kidnapping of South Korean businessmen and transporting them to North Korea for the spy training program. Eventually Won was ordered to get into South Korea as a refugee from the north. She did this in 2001, by the simple expedient of marrying a South Korean man doing business in China. As soon as Won got to South Korea she divorced her husband and offered her services to the South Korean army as a lecturer on conditions in North Korea. Won is apparently quite convincing in whatever she does, and she was soon going around to South Korean military bases lecturing on the evils of communism.
Won's main mission in South Korea was to locate high ranking North Korean defectors living in the south and kill them. She was never able to make much progress in that area. She was able to collect a lot of low level intel on the South Korean military. She did this by getting friendly with South Korean officers and using sexual relationships to obtain classified information, especially anything on high level North Korean defectors. This is apparently how she was found out but at least one officer, a captain nine years younger than Won, continued passing along classified info even after he figured out she was a North Korean spy. Won would travel to China to pass information to North Korean intelligence officials, who would carry it back to North Korea. As far as the South Korean's can tell, she never got anyone into bed who had access to really useful stuff. But the skill and professionalism of Won was scary to South Korean counter-intelligence officials, who knew that if there were more like Won in the south, they were probably hard to detect and producing useful intel for the north.
There are over 20,000 North Korean refugees living in South Korea, and the number arriving each week has gone from 30 to over a 100 in the last seven years. There are over 3,000 North Koreans who have obtained asylum in other countries. Many more are getting out of North Korea but it's difficult to get from China to South Korea. This is usually done by travelling across China to a Southeast Asian nation, like Thailand, and asking for political asylum there. That usually results in the South Korean government stepping in and transporting the North Korean refugees to South Korea. There are believed to be over 600,000 North Korean refugees in northern China, nearly all of them there illegally.
Attracted by the opportunity to settle in prosperous South Korea, many of the millions of Chinese, of Korean ancestry, try to sneak in as North Korea refugees. There are schools in northern China that will train these Chinese, who already speak Korean (with a North China accent) to sound and act like North Koreans and pass the intense interrogation South Korean intelligence officials give each refugee who makes it to South Korea. Most of these Chinese appear to succeed. That is worrisome, as North Korea also runs training programs to help their agents get past the South Korean security screening. Thus it is likely that many North Korean agents have gotten past the screening and are operating. But are they? It's just as likely that many of these agents realized, once in South Korea, that all the North Korean propaganda about South Korea and the West was a lie. At that point all they have to do is go dark and hope for the best, or become double agents for South Korea. Perhaps in light of all this, South Korean intelligence increased the screening process from 90 days to 180.