“Habibi! Aluminium!”

The call echoes through the courtyard of a trash-strewn home in Tal Afar, a remote outpost in northern Iraq. It is late September and still hot, the kind of heat that seems to come from all sides, even radiate up from the ground, and the city is empty except for feral dogs and young men with guns.

“Habibi!” Damien Spleeters shouts again, using the casual Arabic term of endearment to call out for Haider al-Hakim, his Iraqi translator and partner on the ground.

Spleeters is a field investigator for Conflict Armament Research (CAR), an international organization funded by the European Union that documents weapons trafficking in war zones. He is 31 years old, with a 1980s Freddie Mercury mustache and tattoos covering thin arms that tan quickly in the desert sun. In another context, he’d be mistaken for a hipster barista, not an investigator who has spent the past three years tracking down rocket-­propelled grenades in Syria, AK-47-style rifles in Mali, and hundreds of other weapons that have found their way into war zones, sometimes in violation of international arms agreements. The work Spleeters does is typically undertaken by secretive government offices, such as the US Defense Intelligence Agency’s Military Material Identification Division, known as Chuckwagon. But while Chuckwagon is barely discoverable by Google, Spleeters’ detailed reports for CAR are both publicly available online and contain more useful information than any classified intelligence I ever received when I was commanding a bomb disposal unit for the US military in Iraq in 2006.

In that fight, guerrillas ambushed American soldiers with IEDs. The devices I saw during my tours were largely hidden in the ground or deployed as massive car bombs, detonated in marketplaces and schools so that the gutters filled with blood. But the majority of those devices were crude, held together with duct tape and goopy solder. The few rockets and mortars the fighters possessed were old and shoddy, lacked the correct fuzes, and often failed to detonate.

Many of ISIS’ leaders were veterans of that insurgency, but as they began ramping up their war against the Iraqi government in 2014, they knew they needed more than IEDs and AK-47s to seize territory and create their independent Islamic State. A conventional war required conventional arms—mortars, rockets, grenades—which, as an international pariah, ISIS could not buy in sufficient quantities. Some they looted from the Iraqi or Syrian governments, but when those ran out they did something that no terrorist group has ever done before and that they continue to do today: design their own munitions and mass-produce them using advanced manufacturing techniques. Iraq’s oil fields provided the industrial base—tool-and-die sets, high-end saws, injection-­molding machines—and skilled workers who knew how to quickly fashion intricate parts to spec. Raw materials came from cannibalizing steel pipe and melting down scrap. ISIS engineers forged new fuzes, new rockets and launchers, and new bomblets to be dropped by drones, all assembled using instruction plans drawn up by ISIS officials.

At Iraqi military intelligence headquarters in Baghdad, weapons inspector Damien Spleeters (left) and his coworker, Haider al-Hakim, look through crates of ISIS ammunition. Andrea DiCenzo

Since the early days of the conflict, CAR has conducted 83 site visits in Iraq to collect weapons data, and Spleeters has participated in nearly every investigation. The result is a detailed database that lists 1,832 weapons and 40,984 pieces of ammunition recovered in Iraq and Syria. CAR describes it as “the most comprehensive ­sample of Islamic State–captured weapons and ammunition to date.”

Which is how, this autumn, Spleeters came to be hovering over a 5-gallon bucket of aluminum paste in a dingy home in Tal Afar, waiting for his fixer to arrive. Al-Hakim is bald, well-dressed, and gives off a faint air of the urban sophisticate, such that at times he looks a bit out of place in a sewage-filled ISIS workshop. The two men have an easy rapport, though one in which al-Hakim is the host and Spleeters always the respectful guest. Their job is to notice small things; where others might see trash, they see evidence, which Spleeters then photographs and scours for obscure factory codes that can give a clue to its origins.