SEOUL The godfathers of North Korea's nuclear weapons program were an elderly trio: a nuclear physicist, a military general, and a broker with contacts in Pakistan.
The broker is reported to have died in 2014, but together, the three helped lay the groundwork that led to the isolated country's fourth nuclear test on Wednesday.
Roughly 6,000 people are involved in North Korea's nuclear and missile program, according to a 2009 report by South Korea's Science & Technology Policy Institute, and they are an elite corps. Many are given brand new houses and those at the very top are awarded medals listing them as heroes of the state.
"Just like the Manhattan Project, in order to build a nuclear bomb you need eggheads, logistics personnel, and military personnel," said Michael Madden, an expert on the North Korean leadership.
"That's guys like So Sang Guk, Jon Pyong Ho, and O Kuk Ryol".
Even by the standards of secretive North Korea, the three have remained firmly behind the scenes. Some sketchy details however have pointed to their role in the isolated state's nuclear program.
The brain, experts say, is 77-year-old So Sang Guk, a skilled scientist and PhD who rose to become the head of Kim Il Sung University's Department of Nuclear Physics.
According to North Korean state media, So is the author of forty books, with titles including "Quantum Mechanics" and "Elementary Particle Theory".
A decorated hero, So was given a spread of food for his 60th birthday in 1998 by then-leader Kim Jong Il, state media reported at the time.
"He was Kim Jong Il's tutor on nuclear physics and nuclear science," said Madden.
So was appointed to the Organisation and Guidance Department (OGD), a secretive body used by Kim Jong Il to wrest power from his father before becoming leader in 1994.
"The reason he had the OGD title was to supervise personnel and give people jobs in the weapons program," said Madden. "A job there also effectively gave him security clearance so that he could have access to secret documents as he needed them".
Using connections forged as a young student in the Soviet Union, So made a solo trip to Russia to arrange a deal to import nuclear parts, South Korea's Yonhap news reported in 2006.
His daughter won a place at a top music school thanks to her father's patronage, according to Jang Jin-sung, a North Korean defector who left in 2004 but studied alongside So's daughter as a student.
"She once complained she could not study abroad because her father was engaged in some big, secretive project," Jang told Reuters.
So was listed as a target of financial sanctions by the European Union in 2009.
GENERAL O: THE MILITARY MAN
Late leader Kim Jong Il built patronage with gifts and around 2001, he gave two of his most-trusted aides in the nuclear department a large American van each as a present, according to Jang, the defector.
So Sang Guk received one van. Jang said the other was awarded to O Kuk Ryol, a slender man with thinning hair and tinted spectacles said to be the military coordinator behind the nuclear program.
A health nut who walks several miles a day, the 85-year-old O is an "anomaly" among the North Korean elite for avoiding excessive partying, according to Madden.
He was promoted to general in 1984 and was born in China, according to South Korea's Ministry of Unification.
His rapid rise in the Korean People's Army was aided by being related to a guerrilla soldier who fought alongside North Korea's founder President Kim Il Sung during the Japanese colonial period.
O was named in U.S. sanctions against North Korea's weapons program in 2013 specifically to "further impede North Korea's weapons of mass destruction and ballistic missile program".
He was last seen in state media alongside Kim Jong Un in November last year.
JON PYONG HO: THE LOGISTICIAN
In July 2014, state media said Jon Pyong Ho, the ruling Workers' Party bureaucrat credited with the development of Pyongyang's ballistic missile program, had died.
He had helped broker a deal with Pakistan in the 1990s that provided Pyongyang with critical technology for its uranium enrichment program, experts have said. Jon was the main contact for North Korea with Abdul Qadeer Khan, the father of Pakistan's nuclear program who confessed in 2004 to selling nuclear secrets to Iran, North Korea and Libya.
Hwang Jang-yop, a former mentor to the late Kim Jong Il and North Korea's highest-ranking defector before he died in 2010, once told a Japanese newspaper that Jon had approached him to ask if they could "make a few more nuclear bombs".
"Can we buy some more plutonium from Russia or somewhere?" Hwang quoted Jon as asking him.
"By the autumn of 1996, he said 'We've solved a big problem. We don't need plutonium this time. Due to an agreement with Pakistan, we will use uranium'".
A graduate of Moscow State University and a close adviser to Kim Jong Il, Jon worked for more than four decades as a senior figure in the production and development of North Korean arms before retiring from public life in 2011.
Jon was the logistician who oversaw the factories, scientists and technicians working on the isolated country's missile and nuclear program, according to Madden.
"Although he has passed away," an obituary for Jon in state media said, "the exploits he performed on behalf of the party, the revolution and the country will shine on".
(Editing by Raju Gopalakrishnan)