A crucial ingredient for a "dirty bomb" at risk of falling into the hands of ISIS leaders has been recovered by Iraqi government officials.

Iraqi and U.S. officials had been monitoring two caches of cobalt-60 — a synthetic radioactive isotope of cobalt — stored in a Mosul college campus since ISIS captured the city in 2014, according to The Washington Post.

But when Iraqi military forces were able to access the campus storage room earlier this year, they found the stockpile undisturbed.

Cobalt-60 is used by medical professionals to kill cancer cells, but its high levels of radiation can also be employed in warfare with devastating effects.

The threat was so great that independent nuclear experts tried to estimate the damage the supply could cause but details were kept secret in case ISIS was not aware of the resource at their disposal, the Post reported.

U.S. officials and nuclear experts spectulated ISIS operatives left the cobalt untouched because they did not know "how to dismantle the machines' thick cladding without exposing themselves to a burst of deadly radiation."

Similar equipment exists in other conflict zones, the newspaper said.