The Panama Papers leak has recently drawn attention to the shady world of offshore accounts, but such accounts are nothing new. Case in point: such accounts were at the center of the 1986 deal that has been described as one of the most corrupt transactions in global trading history: the Al Yamamah (“the dove”) deal.

The deal, which netted British arms company BAE over $61 billion, was on its face a straightforward sale by arms company BAE of British-made jet fighters, naval vessels and infrastructure works to the Saudi government. Behind the scenes, however, there lurked a web of shadowy interactions between Saudi and British royals and politicians, BAE executives, shady middlemen and the Bank of England. Those involved with the deal have been accused in the years since of various forms of corruption: payments to agents well connected in the kingdom; barter agreements, where military hardware was exchanged for oil with extra barrels of crude added to the transfers to be sold by a Saudi dealer; and, the simple mechanism of overcharging for various aspects of the contract.

BAE eventually admitted to false accounting and other arms export related charges, but the use of offshore accounts in this case underlines a key point made clear by the Panama Papers scandal as well. Even if nothing technically illegal is taking place, it can become nearly impossible to trace the billions of dollars that move around the globe in these cases—and the consequences of that fact can be grave.

A pivotal figure in the Al Yamamah deal was the Saudi Prince Bandar bin-Sultan, a senior member of the Saudi royal family and the son of the kingdom’s Defense Minister at the time of the deal. He has described the 25-minute negotiation with the British Prime Minister, Margaret Thatcher, as the easiest arms deal he ever clinched. Thatcher’s son received about $17 million as an agent on the deal. Meanwhile, Prince Bandar received $1.4 billion in payment—and, for his birthday in 1998, a customized Airbus worth $105 million, painted in the colours of the prince’s beloved Dallas Cowboys. Police estimated later that $8.5 billion was paid on the deal in total, through a British Virgin Islands-based company.

From 2003 The Guardian newspaper started to report on the situation, after a series of tip-offs from whistle-blowers linked to the contracts. At first they thought the story was that there was a massive slush fund the company utilized to benefit Saudi officers and members of the royal family. For example, one of the most senior Saudi royals, head of the Saudi Air Force, was not only paid cash but his mistress, an aspiring actress, has said that she was accommodated, at BAE’s expense, in one of London’s most expensive neighborhoods, her drama classes paid, and cash and expensive gifts regularly supplied.

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But the real story, it turned out, was the labyrinthine maze of offshore jurisdictions through which hundreds of agents were being paid. It was alleged that through this complex and secretive system — an early and more complex forerunner of what was revealed in the ‘Panama Papers’ — Prince Bandar had been directly collecting more than £100 million a year, paid quarterly into Riggs Bank of Washington D.C., which was not only the capital’s oldest and largest financial institution but also its most august. Banker to 22 Presidents and most of the world’s Washington-based embassies, Riggs was so much a part of the American establishment that its majestic headquarters building was featured on the ten-dollar bill for decades.

Another £1 billion was said to be routed through companies in the British Virgin Islands, which was then moved into Swiss bank accounts thought to be linked to agents and to Prince Sultan, Prince Bandar’s father and the Defense Minister who signed the deal. Some of these commissions were offset by massive overcharging, up to 32% in the case of a certain model of jets.

The scandal lived on for decades, as it later came to light that in 2000, about two weeks after a man named Omar al-Bayoumi opened bank accounts for two of the 9/11 hijackers, his wife began receiving monthly payments totaling tens of thousands of dollars through a Riggs bank account held by Princess Haifa bint Faisal, the wife of Prince Bandar bin-Sultan, who by then had become the long-time Saudi ambassador to the U.S. The source of that money? Prince Bandar’s commission on the Al Yamamah deal, which the FBI and later the 9/11 Commission ultimately stated was not intentionally being routed to fund terrorists.

In late 2006, under pressure from British Prime Minister Tony Blair, the U.K.’s Serious Fraud Office was forced to end its two and a half year investigation into the Al Yamamah arms deal. The United States Departments of Justice and State, however, fined BAE over $400 million and forced the company to acknowledge not only numerous violations of the Arms Export Control Act and the International Traffic in Arms Regulations, but also that it had paid unauthorized commissions on a number of arms deals.

BAE remains one of the world’s largest defense contractors. Riggs Bank is no more, having been taken over by PNC Financial Services Group of Pittsburgh.

The Al Yamamah deal is featured in Shadow World, a documentary feature premiering at the Tribeca Film Festival on April 16. This article is drawn from the book The Shadow World: Inside the Global Arms Trade by Andrew Feinstein {St. Martins Press, 2012} on which the film is based.

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