It was described as dirtier than Watergate, and involved US government dealings with Iraq, Libya, Korea and even the late British publishing tycoon Robert Maxwell. The story is deep, dark and complex; a web of strange dealings and dubious characters, it implicates wealthy arms dealers, Israeli intelligence services, the Soviet KGB, MI5 and the CIA. But unlike Watergate, this scandal, from a particularly dark chapter in American history, has appeared in no Hollywood film and is yet to reach a satisfying conclusion.

It began in the late 1970s, when the Washington-based software developer Inslaw pioneered people-tracking technology, designed to be used by prosecutors to monitor case records. Known as the Prosecutor's Management Information System (PROMIS), the software was developed under grants from the US department of justice. The US government, as it helped fund the creation of PROMIS, had been licensed to use the software on condition that it did not modify, distribute or create derivative versions of it. The government, however, did not stick to this agreement.

Under the Ronald Reagan administration's covert intelligence initiative known as "'Follow the Money", the US National Security Agency (NSA) misappropriated PROMIS for sale to banks in 1982. The version of PROMIS sold by the NSA had been "espionage-enabled" through a back door in the programme, allowing the agency to covertly conduct real-time electronic surveillance of the flow of money to suspected terrorists and other perceived threats to US national interests.

A letter from the US department of justice in 1985, later obtained by Inslaw, documented more plans for the covert sale and distribution of the espionage-enabled version of PROMIS, this time to governments in the Middle East (which would surreptitiously allow the US to spy on foreign intelligence agencies). The letter outlined how sales of the software were to be facilitated by the late Saudi billionaire Khalid bin Mahfouz and the arms dealers Adnan Khashoggi and Manucher Ghorbanifar. PROMIS should be delivered without "paperwork, customs, or delay", it stated, and all of the transactions paid for through a Swiss bank account.

In the years that followed, friends of then attorney general Edwin Meese, including a Reagan associate, Dr Earl Brian of the government consultancy firm Hadron, Inc, were reportedly allowed to sell and distribute pirated versions of PROMIS domestically and overseas. As a House judiciary committee report found in 1992, these individuals were apparently permitted to do so "for their personal financial gain and in support of the intelligence and foreign policy objectives of the United States".

Brian, who was later jailed for four years on an unrelated fraud charge in 1998, has since denied any association with the Inslaw case. According to the former arms broker and CIA "contract operative" Richard Babayan, however, he was instrumental in selling PROMIS to the governments of Iraq, Libya and Korea. When Brian was unable to market PROMIS further, it is claimed that, with the help of Rafi Eitan, a high-ranking Israeli intelligence officer, the British publisher Robert Maxwell was recruited to assist.

In a sworn affidavit, the investigative author Gordon Thomas recounts how Eitan told him Maxwell alone sold over $500m worth of espionage-enabled versions of PROMIS – including licences to the UK, Australia, South Korea, Canada and the Soviet KGB. The British counter-intelligence agency MI5, according to Eitan (who himself was an adviser to the UK secret service MI6), used PROMIS to track members of the Irish Republican Army (IRA), as well as Irish republican political leaders including Gerry Adams.

Inslaw alleges the US government, by selling PROMIS to other governments around the world, engaged in what equates to "multibillion-dollar theft". This claim was supported by two separate courts in 1988, which ruled that it "took, converted, stole" PROMIS from Inslaw "through trickery, fraud and deceit". Three years later, however, a court of appeal overturned both rulings on a "jurisdictional technicality" after pressure from the federal justice department.

Now more than two decades since he pioneered PROMIS, the Inslaw president Bill Hamilton today believes the story illustrates an enduring, fundamental problem at the heart of the US justice system. "[It] chronicles the continued inability of the US government to enforce federal criminal laws in cases involving national security issues, or even to render ordinary civil justice," he says. "National security appears to suspend the checks and balances built into the system of government in the United States, to the detriment of the citizens."

Some, including the US government, have tried to dismiss the Inslaw saga as conspiracy. But a message relayed to Bill Hamilton and his wife from the former chief investigator of the Senate judiciary committee, Ronald LeGrand, seems to confirm that the strange PROMIS affair – which remains unresolved – is much more than just a case of chronic paranoia.

"What Mr and Mrs Hamilton think happened, did happen," LeGrand wrote, conveying information he had received from a trusted government source. "The Inslaw case is a lot dirtier for the Department of Justice than Watergate was, in both breadth and depth. The Department of Justice has been compromised in the Inslaw case at every level. "

Ryan Gallagher is a freelance journalist based in London, currently working for the Frontline Club. His website is here.