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.

Iraqi military commanders were apprised of the potential threat as they battled Islamic State fighters block by block through the sprawling complex where the cobalt was last seen. Finally, earlier this year, government officials entered the bullet-pocked campus building and peered into the storage room where the cobalt machines were kept.

They were still there, exactly as they were when the Islamic State seized the campus in 2014. The cobalt apparently had never been touched.

“They are not that smart,” a relieved health ministry official said of the city’s former occupiers.

Why the Islamic State failed to take advantage of its windfall is not clear. U.S. officials and nuclear experts speculate that the terrorists may have been stymied by a practical concern: how to dismantle the machines’ thick cladding without exposing themselves to a burst of deadly radiation.

More certain is the fact that the danger has not entirely passed. With dozens of Islamic State stragglers still loose in the city, U.S. officials requested that details about the cobalt’s current whereabouts not be revealed.

They also acknowledged that their worries extend far beyond Mosul. Similar equipment exists in hundreds of cities around the world, some of them in conflict zones.

“Nearly every country in the world either has them, or is a transit country” through which high-level radiological equipment passes, said Andrew Bieniawski, a vice president for the Washington-based Nuclear Threat Initiative who once led U.S. government efforts to safeguard such materials.

“This,” he said, “is a global problem.”

The worries began within hours of the Islamic State’s stunning blitz into Iraq’s second-largest city. As TV networks showed footage of triumphant terrorists parading through Mosul’s main thoroughfares, intelligence agencies took quiet inventory of the vast array of military and material wealth the Islamist militants had suddenly acquired. The list included three Iraqi military bases, each supplied with U.S.-made weapons and vehicles. It also included bank vaults containing hundreds of millions of dollars in hard currency, as well as factories for making munitions and university laboratories for mixing chemicals used in explosives or as precursors for poison gas.

U.S. officials also were aware that the Islamic State had gained control of small quantities of natural or low-enriched uranium — the remnants of Iraq’s nuclear projects from the time of Saddam Hussein’s presidency — as well as some relatively harmless radioactive iridium used in industrial equipment.

But a far bigger radiological concern was the cobalt. Intelligence agencies knew of the existence in Mosul of at least one powerful radiotherapy machine used for cancer treatment, one that could potentially provide the Islamic State with a potent terrorist weapon.

Outside experts were becoming aware of the threat as well…..

Leaders of the Islamic State and al-Qaeda are known to have sought materials for a dirty bomb, a threat that has added urgency to efforts by U.S. agencies and private groups to improve security for machines with heavy concentrations of cobalt-60, or other radioactive elements such as cesium-137, which comes in a powdery form that is even easier to disperse.

The machines are a necessary fixture in many cancer clinics around the world, but in Western countries efforts are underway to replace the most dangerous models with new technology that cannot be easily exploited by terrorists, said Bieniawski, the former Energy Department official. His organization, the Nuclear Threat Initiative, has raised money to try to speed up the transition, but for now, he said, older machines such as the ones in Mosul are commonly found in developing countries where the risk of theft or terrorism is greatest.

“The ones we see overseas are in the highest category — the highest levels of curies — and they are also portable,” he said. “They are exactly the ones we are most worried about.”

Morris reported from Beirut. Mustafa Salim in Irbil, Iraq, contributed to this report. https://www.washingtonpost.com/world/national-security/how-isis-nearly-stumbled-on-the-ingredients-for-a-dirty-bomb/2017/07/22/6a966746-6e31-11e7-b9e2-2056e768a7e5_story.html?utm_term=.564e04f5c470