On a Sunday morning in the fall of 2009, a 26-seat bus carrying one ton of explosives made its way toward the Ministry of Justice in Baghdad. Security in Iraq had been improving for more than a year. A U.S. troop surge seemed to have worked, and in a demonstration of the country’s stability, Prime Minister Nouri al-Maliki had ordered security measures reduced. Blast walls were taken down. Traffic was allowed closer to government buildings.

The bus was not searched at any of the checkpoints near the administrative center. The driver pulled right up to the ministry building, and then the payload detonated with such tremendous force that, to those nearby, it felt as if a meteor had struck. Cars spun through the air and landed on top of one another; debris rocketed through windows a dozen stories above the street. As an opaque plume of smoke and dust rose hundreds of feet into the air, a van carrying another bomb exploded just up the street, in front of the Provincial Administration building.

In a matter of seconds, 155 people had been killed and more than 500 injured. Iraq’s most hopeful post-Saddam interlude was over, shattered by a new wave of devastating bombings. Two days later, a little-known Al Qaeda–linked group, who would later take on the name ISIS, claimed credit. But the government was less concerned with who had carried it out than with how the attack was even possible: How could nearly 4,000 pounds of explosives have passed, undetected, into one of the city’s most tightly controlled areas?

The first impulse was to look for infiltrators and collaborators. Some 60 suspects who had worked at checkpoints and local police posts near the bombings were rounded up. But the real problem, it turned out, was with the handheld bomb detectors being used at just about every Iraqi-run checkpoint in the country. They were toys. They had never worked. It had all been an expensive charade—costing the Iraqi government tens of millions of dollars and hundreds of lives—engineered by a portly middle-aged salesman in Britain.

War has always provided a deep reservoir for profiteers. A wash of international money floods into countries torn by conflict, and those countries often have little capacity for oversight. This opportunity was exploited by an unlikely group of con artists, including a couple from outside London, an elderly eccentric from the United States, a former security-equipment salesman from Sevenoaks, in Kent, and a policeman turned telecommunications worker from Hambridge, in Somerset. They eventually fell out and went their separate ways, but one of them, managing his global operation out of a sleepy West Country town, would earn at least $80 million from his scam.

The “bomb detectors” sold to Iraq—and, it would later emerge, the versions bought by security forces in dozens of other countries—were based on a gag gift that had been around for decades. When the group modified the devices to sell them all over the world, they invented technical-sounding names like the A.D.E. 651, the Quadro Tracker, the Positive Molecular Locator, the Alpha 6, and the GT200. But all of them were simply rebranded versions of a hollow, five-ounce plastic toy sold as “Gopher: The Amazing Golf Ball Finder!,” whose packaging claims, limply, that “you may never lose a golf ball again!” Even as a toy, the Gopher is unimpressive. It would barely pass muster as a prop in a fourth grader’s camcorded Star Wars tribute. It consists of a cheap plastic handle with a free-swinging antenna, and the way it works is simple: When you tilt the device to the right, the antenna swings right. When you tilt it to the left, the antenna swings left. If you’ve been primed by the right sales pitch, you believe that the antenna has moved as a result of something called nuclear quadrupole resonance, or electrostatic attraction, or low-frequency radio waves bouncing off the ionosphere—each force supposedly drawing the antenna toward the substance you’re scanning for, such as, in the beginning, “the elements used in all golf balls.” Because the salesman is glib and confident, or because people around you aren’t questioning it, or because your superior has ordered you to use it, you ignore the far more obvious force that has actually moved the antenna: gravity.