The bombing was not a spontaneous act. A small group of anarchists, led by a man named Nogues Molas, had originally planned to attack the procession the day before, on June 6, at Barcelona Cathedral itself. At the last minute, Molas and his group abandoned the plot. But the following day, as the procession moved into its second stage through the old city's winding streets, another group -- whether connected to the first or acting independently remains unclear -- carried out the attack. The bomb was an Orsini device, a type of explosive designed to fragment on impact, and it was dropped from above into the dense crowd of worshippers passing below.

The Spanish authorities responded with sweeping arrests. Hundreds of suspected anarchists were detained in Barcelona, and the confessions that followed were extracted through torture. The case became one of the most controversial episodes of state repression in late 19th-century Spain -- the Proces de Montjuic, named for the fortress where prisoners were held and interrogated. Whether the people convicted were actually responsible for the bombing, or whether the state used the attack as a pretext to crush Barcelona's anarchist movement, has been debated by historians ever since. The true perpetrator or perpetrators were never conclusively identified.

Late 19th-century Barcelona was a city of sharp contradictions. The industrial boom had created enormous wealth for factory owners while leaving workers crowded into the old city's tenements, laboring long hours for meager wages. Anarchism found fertile ground in this inequality, and Barcelona became the most anarchist city in Europe. The Corpus Christi bombing was neither the first nor the last act of political violence. In 1893, an anarchist had thrown bombs into the audience at the Liceu opera house, killing twenty people. The cycle of attack and repression fed on itself -- each bombing provoked a crackdown, each crackdown deepened the rage.

The Corpus Christi bombing was a harbinger of the turbulence that would consume Barcelona for the next four decades. Thirteen years later, in 1909, workers staged a general strike that erupted into the Semana Tragica -- the Tragic Week -- during which churches and convents were burned across the city and the military response killed over a hundred people. By the time the Spanish Civil War reached Barcelona in 1936, the city had become the capital of anarchist-controlled Catalonia, where unions ran factories and collectivized businesses. The narrow street where the bomb fell in 1896 sits quietly today in Barcelona's Born district, a neighborhood of boutiques and wine bars. Nothing marks the spot where twelve people died during a religious procession, their deaths an early tremor in a long political earthquake.