ISIS never realized it had access to a core ingredient of a potentially devastating "dirty bomb" when the jihadists overran Mosul in 2014, The Washington Post reported.
"They are not that smart," one unnamed health ministry official said of the terrorists.
But the possibility worried U.S. officials for three years.
According to the Post, 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 used in a radiotherapy machine, cobalt-60 is used to kill cancer cells. In terrorists' hands, it could be the core ingredient of a weapon to spread radiation and panic.
Western intelligence agencies knew about the stash and watched for signs ISIS would try to use it – especially after the militants boasted of obtaining radioactive material in late 2014, and again early last year as they took over laboratories at the same Mosul college campus where the cobalt was stored.
But earlier this year when government officials entered the campus building and looked into the storage room where the cobalt machines were kept, they were still there exactly as they'd been since ISIS seized the campus in 2014, the Post reported.
The machines are now in secure storage and "weren’t used by Daesh," Laith Hababa, a physician and head of the Nineveh provincial health ministry, told the Post, using the Arabic acronym for ISIS.
“Its leaders were preoccupied elsewhere, and [perhaps] did not learn about the sources in Mosul, or have a chance to think through the opportunities," David Albright of the Institute for Science and International Security, a nonprofit organization in Washington that monitors global nuclear threats, told the Post.
But the danger isn't entirely gone; dozens of ISIS fighters remain on the loose in Mosul, the Post reported.
According to the Post, Albright's institute obtained documents showing that two different medical centers in Mosul had obtained cobalt-60 machines in the 1980s.
The institute shared its findings with U.S. intelligence and military officials in late 2015 but declined to publish its report, fearing that ISIS would benefit from the information.
Because cobalt-60 decays over time, the potency of the Mosul machines’ 30-year-old cobalt cores would have been far less than when the equipment was new, but still easily enough to deliver a lethal dose at close range, the report said.
"The worst case would have been the Islamic State widely dispersing the radioactive cobalt in a city, causing panic and an expensive, disruptive cleanup," institute president David Albright told the Post. "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."