Panorama's Jane Corbin describes how she tracked a black market nuclear network which supplied Libya, Iran and North Korea with atomic knowhow and technology. She also writes about her exclusive interview with one of the engineers who worked inside it.
In a four month investigation of the nuclear black market run by rogue Pakistani scientist AQ Khan I clocked up a lot of miles, from Johannesburg to Vienna, Holland to Washington and on to the wilds of Montana.
I found people in all these places who knew Khan, had worked for him or had tried and failed to stop him selling nuclear technology.
Dutch technical photographer Frits Veerman befriended Khan in 1975 when the young scientist came to work for URENCO, the European consortium developing centrifuge technology in Almelo, Holland.
High tech centrifuges are used to enrich uranium for electricity but at the highest levels of enrichment for the Bomb.
Veerman suspected Khan was spying when he saw classified centrifuge documents lying around in his house.
"I tried to warn my bosses three times about Khan" said Veerman, "but they did nothing".
A few months later Khan returned to Pakistan with stolen centrifuge designs and a valuable list of western suppliers of specialist centrifuge technology.
His government wanted the Bomb and gave Khan free rein to set up his own laboratory to develop centrifuges - and create a clandestine buying network for the technology he needed from abroad.
Thanks to Khan, within a few years Pakistan was able to develop its own enrichment programme and build nuclear weapons.
The security failure in Holland was the first in a series of disastrous mistakes.
In a Montana trailer park I found a CIA whistle blower who tracked the Khan story during the eighties for me.
Richard Barlow was a rising star at the agency who discovered the extent of Pakistan's procurement network and that Khan was behind it.
But when he went public before Congress he sparked a political storm.
Hawks in the US Administration saw Pakistan as a vital ally in the Cold War, the supporter of the Afghan mujahideen against the Soviet invaders.
Those hawks were afraid Barlow's evidence would push Congress into imposing sanctions on Pakistan for its nuclear ambitions and the US would lose its valuable ally.
So Barlow had to go but Khan carried on buying.
"He was allowed to continue to operate," Barlow told me, "and that allowed the buying network to become a selling network providing critical nuclear technology to North Korea, Iran and Libya."
So what caused Khan, the patriotic scientist, to secretly turn his skills from acquiring technology for Pakistan to procuring the same kit to sell to other countries who wanted atomic knowhow?
I asked this question of former Pakistani Brigadier-General, Feroz Khan, a man closely involved with his country's nuclear programme who knew Khan well.
"His work was so clandestine he feared he would never get the recognition he deserved and he wanted to show off, that was part of his personality" said the Brigadier-General.
"What he did was motivated by a mixture of ego, greed and anger with the West" he continued.
AQ Khan found eager business partners, contacts he had made in the old days at URENCO who had already provided him with technology for Pakistan and would now earn millions by joining Khan in his freelance venture based out of the free-wheeling port city of Dubai.
Khan's first customers, in 1987, were the Iranians, locked in a war of attrition with Iraq which had cost a million lives.
The ayatollahs wanted the ultimate weapon.
I travelled to Vienna and the skyscrapers that house the UN's International Atomic Energy Agency to meet the deputy director, the man who heads the teams investigating both Iran and the Khan network.
Olli Heinonen explained to me what the network had offered Iran - materials for 2,000 centrifuges, one sample machine and specifications for "a complete plant".
The Iranians tried to develop their own centrifuges but were forced to go back to the network when they could not engineer the complex machines themselves.
"Around '93 or '94, Iran has told us, the network came with another proposal" said Heinonen "and this is one of the puzzles for us to find out the dynamics of that second deal."
It has become urgent to get to the bottom of this mystery as Iran defies the international community with its nuclear programme.
The IAEA knows that Khan ultimately sold Iran designs for an advanced centrifuge which can enrich uranium faster but Iran is still stalling on how far it has got with this technology.
It claims it wants to enrich uranium for electricity but western governments suspect it is after nuclear weapons.
By the nineties, Khan was becoming untouchable as I discovered in Washington talking to former UN weapons inspector David Albright.
He had been in Iraq after the 1991 Gulf War when the Coalition invaded Iraq and the UN discovered how close Saddam had been to developing nuclear weapons.
Documents were found detailing an offer made by Khan to sell a complete nuclear warhead design to the Iraqis.
They had turned it down fearing a western sting operation.
"The western intelligence community was outraged that this Pakistani, supposedly an ally of ours was prepared to sell a weapon to Saddam to use against coalition forces" said Albright.
But although Khan's activities were now undeniable the US failed to take action.
"At the very heart of the US government there was no desire to step on Pakistan hard enough to get them to stop Khan" Albright continued "and they let it fester. They let Khan get out of control."
So out of control had Khan become that by 1997 he was selling a complete nuclear weapons package to the pariah regime of Libya and Colonel Gadaffi, the man behind the Lockerbie bombings.
Centrifuge parts manufactured in Malaysia, machine tools and training provided from Europe, electrical parts from Turkey. It was a one-stop shop that investigators would later call the nuclear Walmart.
And so I followed the trail which by now spanned three continents to South Africa, a country with its own secret nuclear history.
The apartheid regime had developed nuclear weapons but when it fell this nation gave them up. But the expertise was still there waiting to be exploited by Khan's network.
Daniel Geiges, a nuclear engineer, had agreed to talk exclusively to Panorama about his work as project manager on the uranium enrichment plant built here for Libya.
"I never had the slightest doubt what it was for" Geiges admitted. He and others at a small engineering works outside Johannesburg called Tradefin, began building the three-storey plant in 2002.
Geiges's boss, a German with links to the network had been paid a commission of $4 million.
The plans had been supplied and annotated by Khan and Geiges knew the plant was capable of enriching uranium to bomb grade.
"We're talking about just under 24 kilos of highly enriched stuff a year" Geiges told me. Enough for a bomb, I asked. "Yes" he replied "maybe two if you get a good design."
Geiges justified his actions to me even though he knew the plant was destined for Libya.
"What qualifies the Americans to have 10,000 nuclear devices and not others?" he demanded.
Daniel Geiges could face 15 years in jail if found guilty of charges of nuclear proliferation.
The network was eventually busted in the autumn of 2003 but no one knows the extent of the damage it has done and what the consequences will be for all of us.
The nuclear genie is out of the bottle thanks to Khan and it may be too late to prevent the spread of this deadly technology.
The Nuclear Wal-Mart will be broadcast on Sunday 12 November at 2215 on bbc.co.uk/panorama and on BBC One.