On Sept. 8, just three days before the anniversary of the 1973 military coup that deposed Chile’s socialist President Salvador Allende, an explosion rocked a metro station at an upscale shopping center in the capital, Santiago. The blast injured 14 people, two of them seriously, and sent authorities scrambling to investigate Chile’s worst bomb attack in more than two decades.

The country’s deputy interior minister, Mahmud Aleuy, declared that the blast was the work of “demented criminals,” but the facts pointed to a much more troubling explanation. It wasn’t common criminals, demented or otherwise, who had carried out the attack. The operation bore the hallmarks of dozens of other recent explosions in Santiago. ...