By Steve Chao in Seoul, South Korea
When Im Chun-yong made his daring escape from North Korea, with a handful of his special forces men, there were many reasons why the North Korean government was intent on stopping them.
They were, after all, part of Kim Jong-il's elite commandos - privy to a wealth of military secrets and insights into the workings of the reclusive regime
But among the accounts they carried with them is one of the most shocking yet to emerge – namely the use of humans, specifically mentally or physically handicapped children, to test North Korea's biological and chemical weapons.
"If you are born mentally or physically deficient, says Im, the government says your best contribution to society… is as a guinea pig for biological and chemical weapons testing."
Even after settling into the relative safety of South Korea, for 10 years Im held on to this secret, saying it was too horrific to recount.
But with Kim's health reportedly failing, and the country appearing increasingly unpredictable, Im felt it was time he spoke out.
Daughter given up
The former military captain says it was in the early 1990s, that he watched his then commander wrestle with giving up his 12-year-old daughter who was mentally ill.
The commander, he says, initially resisted, but after mounting pressure from his military superiors, he gave in.
Im watched as the girl was taken away. She was never seen again.
One of Im's own men later gave him an eyewitness account of human-testing.
Asked to guard a secret facility on an island off North Korea's west coast, Im says the soldier saw a number of people forced into a glass chamber.
"Poisonous gas was injected in," Im says. "He watched doctors time how long it took for them to die."
Other North Korean defectors have long alleged that the secretive nation has been using political prisoners as experimental test subjects.
Some have detailed how inmates were shipped from various concentration camps to so-called chemical "factories".
But Im's is the first account of mentally-ill or physically challenged children being used.
Security analysts believe Kim oversees one of the most aggressive and robust biochemical weapons programmes in the world.
A member of the special forces' Brigade No.19, Im says he was trained on how to use biochemical weapons against the "enemy" – including how to fire them from short-range "bazooka-style" weapons.
He says such training was normal practice for all elite units.
Today it is estimated the country has accumulated a stockpile of more than 5,000 tonnes of biochemical weaponry; from mustard gas, to nerve agents such as sarin, to anthrax and cholera.
The extent of the stockpile is a concern to Kim Sang-hun, a retired UN official who has spent years investigating the North's chemical and biological weapons programme.
He believes over the past 20 years, the programme has advanced at a startling pace, specifically because the country’s rulers approve and support the use of human test subjects.
"If you are born mentally or physically deficient, the government says your best contribution to society… is as a guinea pig for biological and chemical weapons testing"
Im Chun-yong, former North Korean commando
"Human experimentation is a widespread practice," Kim says.
"I hoped I was wrong, but it is the reality and it is taking place in North Korea and it is taking place at a number of locations."
There are some who question claims that the North conducts human trials. But Kim says he has interviewed hundred of defectors who, more times than not, volunteer personal vivid accounts.
"The programme is now a commonly known fact in the North Korean public," he says.
As a former member of the elite special forces, Im agrees.
While the government may be secretive about a lot of things, he says "when it comes to human experimentation, most know it happens".
Investigating what he says are serious UN violations regarding the rights of children and prisoners, Kim Sang-hun has amassed a vast amount of evidence.
Compiled in folders at his home in Seoul are reams of testimonies and documents.
Some bear what appear to be official government stamps approving the transfer of prisoners from camps to chemical "factories".
He says he believes these are, in reality, experimental weapons sites.
He has pinpointed at least three to five labs that he believes are situated in different parts of the country, including one just a few kilometres north of the capital, Pyongyang.
Security analysts suspect there are as many as 20 such plants across the country.