January 8, 2015 6:18 AM EST

The French, who have seen two devastating world wars and a revolution on their soil, are known for keeping cool heads in the face of tragedy and violence. Yet little could have prepared them for the gruesome events of Jan. 7. It seemed a gray, rainy Wednesday like any other, until two gunmen wearing black ski masks stormed into the offices of the satirical weekly newspaper Charlie Hebdo just before midday, in Paris’ congested 11th district, and let loose a fusillade of bullets. Outside on the street they shot dead a police officer and fled in a black car driven by a third man–a horrifying sequence of events filmed by a witness and seen around the world.

The men killed 12 people in the attack, among them three of France’s best-known cartoonists, the paper’s top editor and two police officers. Rushing to the scene, President François Hollande stood ashen-faced on the chilly sidewalk, some 3 miles (about 5 km) from his grand Elysée Palace, and appealed for the French to “show we are a united country.” “This was an act of exceptional barbarity,” he said, and declared the attack “a terrorist operation.”

The massacre–France’s worst terrorist attack in memory–was something else too: it was an act foretold.

For months, French officials have expressed concerns that the country was becoming increasingly vulnerable to a terrorist attack. Last month their nervousness appeared well justified, when two separate drivers rammed their cars into Christmas crowds in the provincial cities of Nantes and Dijon, injuring dozens of people, while a third attacker wielded a knife, shouting, “Allahu akbar,” or “God is great.”

Those car rammings now seem amateurish and opportunistic–more the work of lone-wolf terrorists acting without support. In contrast, the men who mounted the Jan. 7 massacre worked like a well-drilled cell primed to inflict maximum damage.

Their target was clear: Hard-line Islamists have threatened Charlie Hebdo for years for publishing countless caustic commentaries and cartoons directed at extremist Muslims (as well as ultra-religious Jews and conservative Catholics). In 2011 Muslims firebombed Charlie Hebdo’s previous office building in Paris amid widespread protests after the paper published an issue purportedly edited by the Prophet Muhammad, with a cartoon lampooning him. French people quickly assumed that the assault had been an act of retaliation against the paper’s antireligious stance.

The attack’s ruthless efficiency shocked Parisians as much as the result. Arriving at the very moment Charlie Hebdo’s journalists were in their weekly editorial meeting–the only time the entire staff gathers in the building–the two assailants sprang from their vehicle carrying automatic rifles and wasted no time killing as many people as they could. “They were well equipped, they had military weapons, they had probably bulletproof vests,” French terrorism consultant Jean-Charles Brisard told the BBC after watching video footage from a closed-circuit security camera outside the building. “These individuals were well trained.”

The attackers are precisely the sort of terrorists the French government has most feared. With more than 5 million Muslims, France has the largest Islamic population in all of Europe. Many French Muslims emigrated from French-speaking North Africa or were raised as children of immigrants from those countries. Their communities have been hard hit by years of recession as France struggles with record-high unemployment now nearing 11%.

Some young French Muslims, disillusioned by the economic hardship and what they see as a French population increasingly hostile to outsiders, have looked abroad for direction and meaning, to the jihadist groups fighting in Syria and Iraq. French police believe about 1,200 French citizens have joined Islamist groups in Syria and Iraq since 2011–by far the most fighters to have joined the jihadists from any Western country. French officials now fear that those filtering home might return with professional military skills and a desire–or even instructions–to harm France.

In response to the attack, Hollande quickly ordered France’s security threat raised to its highest level in years, while Interior Minister Bernard Cazeneuve vowed to hunt down the attackers “so they can be punished with the severity that their barbarous acts are worthy of.”

Those words sound reassuring–for now. And as darkness fell on Jan. 7 Parisians came together in mass gatherings to show their sympathy for the dead and to insist on their right to express themselves free of the threat of violence. But citizens and security officials alike know that unity will not necessarily deter future attacks.

This appears in the January 19, 2015 issue of TIME.

Contact us at letters@time.com.

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