Pakistani security personnel still guard Karachi's Jinnah airport using versions of Jim McCormick's phoney bomb detector. They are not alone in having a seemingly unshakeable belief in the ADE 651

It is is one of the world's most obvious terrorist targets. So how did a group of 10 militants armed with guns, bomb vests, rocket-launchers and grenades get into Karachi's Jinnah airport? Part of the answer, incredibly, may lie in the fact that Pakistani security personnel still guard the outer perimeter using versions of the phoney bomb detector sold by the convicted British conman Jim McCormick.

McCormick's device, which he called the ADE 651, was itself a variation of a common design. Essentially, a telescopic radio aerial is attached by a hinge to a plastic handgrip. When used by a "properly trained" operator, who must first sensitise it to the "molecular frequency" of explosives, it was supposed to point out bombs by swinging towards them.

In fact, all this was nonsense. The aerial swings because of unconscious movements by the operator, known as the ideomotor effect – the same thing that gives rise to the common belief in dowsing. Nevertheless, McCormick and other fraudsters, such as Gary Bolton, exported thousands to clients around the world, including in Iraq and Pakistan. Less ambitious criminals used to sell them as golf ball detectors in the 1990s.

Jim McCormick leaving the Old Bailey after being convicted of fraud. Photograph: Gavin Rodgers/Rex

Like dowsers, though, many security personnel continued to keep the faith. In 2010, even after McCormick had been charged with fraud, Pakistan's Airport Security Force admitted to the Dawn newspaper that they were continuing to use a device of their own design that operates on the same principle.

Iraq, which had been McCormick's largest market, still uses them too, despite repeated warnings. In 2009, the New York Times confronted bomb squad commander Major General Jehad al-Jabiri with evidence of the ADE 651's fraudulence, yet he insisted that it was effective, saying: "Whether it's magic or scientific, what I care about is it detects bombs." In 2011, al-Jabiri was charged and later jailed for – of all things – taking bribes from McCormick. And still the ADE 651s were being used, as recently October 2013.

At the same time it was apparently common to find Beirut security guards still scanning cars for explosives with an ADE 651, or something similar. And there are reports of other devices being used in the south of Thailand. Indeed, according to Detective Sergeant Steve Mapp, who led the investigation into McCormick, some people's belief in the ADE 651 is almost unshakeable. As he told Business Week, "In Kenya they said, 'No, we know about Mr McCormick's conviction, but we're really glad we've got them – and they do work.'"