ISIS terrorists nearly stumbled on the main ingredient for a 'dirty bomb' when they overran Mosul in 2014.

Two caches of cobalt, a metallic substance with lethally high levels of radiation, was found inside two radiotherapy machines at the University of Mosul.

However Iraqi forces discovered the cobalt-60 machines had not been touched when they liberated the city this month.

A health ministry official said of the terror group: 'They are not that smart'.

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ISIS terrorists failed to locate the main ingredient for a 'dirty bomb' when they overran Mosul in 2014

Western intelligence agencies were aware of the cobalt and watched anxiously for three years for any signs ISIS fighters might try to use it.

It is used to kill cancer cells when it is contained within the heavy shielding of a radiotherapy machine.

However, in the terrorists' hands, cobalt could have been used to create a 'dirty bomb'.

Fears were intensified in late 2014 when ISIS claimed it had obtained radioactive material and then last year when they took over laboratories at the same Mosul college campus with the apparent aim of building new weapons.

A November 2015 draft report found that the radioactive cores of the material, when new, 'contained about nine grams of pure cobalt-60 with a potency of more than 10,000 curies — a standard measure of radioactivity.'

A person standing three feet from the unshielded core would receive a fatal dose of radiation in less than three minutes, the Washington Post reported.

US officials have requested their current location not be disclosed.

Iraqi Army soldiers celebrate the liberation of Mosul from ISIS this month

It is not clear why ISIS failed to take advantage of the cobalt stored at the Mosul college campus.

Nuclear experts suggested they may have been concerned about how to remove the machines' thick cladding without exposing themselves to deadly radiation.

A 'dirty bomb' made from cobalt could have resulted in 'panic and an expensive disruptive clean-up', David Albright, a nuclear weapons expert and former UN weapons inspector told the Washington Post.

He added: 'There would likely not have been that many deaths, but the panic could have been profound, leading to the emptying of parts of the city as residents fled, fearful of the effects of radiation.'

ISIS insurgents seized control of Mosul in the summer of 2014.

The east side of the city was recaptured by Iraqi fighters in January this year, though the West side of the city took far longer.

An aerial bombardment on the West, which includes the Old City, started in February and lasted until early July.

ISIS fighters had turned the city into a fortress, holding tens of thousands of civilians as human shields.

Iraqi forces often turned to artillery and US-led coalition airstrikes. The Old City was the last battlefield.