Locked away in a storage room on a Mosul college campus were two caches of cobalt-60, a metallic substance with lethally high levels of radiation. When contained within the heavy shielding of a radiotherapy machine, cobalt-60 is used to kill cancer cells. In terrorists’ hands, it is the core ingredient of a “dirty bomb,” a weapon that could be used to spread radiation and panic.

Western intelligence agencies were aware of the cobalt and watched anxiously for three years for signs that the militants might try to use it. Those concerns intensified in late 2014 when Islamic State officials boasted of obtaining radioactive material, and again early last year when the terrorists took over laboratories at the same Mosul college campus with the apparent aim of building new kinds of weapons.

In Washington, independent nuclear experts drafted papers and ran calculations about the potency of the cobalt and the extent of the damage it could do. The details were kept under wraps on the chance that Mosul’s occupiers might not be fully aware of what they had.