There is plenty of reason to be confident that if ISIS could reliably and easily make a dirty bomb, they would do so.
In the last three years, I have not spent much time wondering whether ISIS has access to radioactive material. I know they have had access, because I had a hand in getting it to them.
In 2005, while working for an air cargo company in Mosul, I delivered a large wooden box, marked for consignment to the University of Mosul. To fly it in, we needed a special plane, an Antonov-12, whose cargo hold was cavernous compared to our usual 727s and DC-8s. The box contained, according to its air waybill, radiological imaging equipment for the university’s teaching hospital. The next day, workers from the hospital met me at my office, and I gently forklifted the crate into their truck. The load seemed off-balance, and I winced when I heard a corner of the box splinter as we strapped it down. But they drove away, and unless that million-dollar piece of medical equipment fell off the back of the truck and ended up strewn across the road, it probably made it safely to the hospital, where it was captured by ISIS nine years later.