French President François Hollande said the shooting at the Paris headquarters of French satirical magazine Charlie Hebdo Wednesday was “undoubtedly a terror attack” as the city raised its anti-terror alert to the maximum level. Hollande called the attack “an exceptional act of barbarism” and also said that several terrorist attacks were thwarted in France in recent weeks, according to the BBC.

At least 12 people were killed and five critically injured in the attack on the magazine’s office. Paris police spokesman Rocco Contento confirmed to French media that nine members of Charlie Hebdo’s staff were killed in the attack, as well as two police officers, the Guardian reported. The assault on the office happened late in the morning local time after masked gunmen entered the building and began shooting with automatic weapons. Some reports suggest that up to 50 shots were fired by gunmen. Police sources said that the attackers are still at large.

The satirical magazine gained international attention in 2012 after it published cartoons of the Muslim Prophet Muhammad. The uproar resulting from the controversial publication forced France to temporarily close its embassies and schools in more than 20 countries over fears of violent reprisals, according to NBC. Its offices were also the target of a firebomb attack in 2011 after the magazine published a caricature of Muhammad on its cover.

"Nobody in France should think that they can behave against the principle of the Republic and harm the spirit of the Republic, embodied by a newspaper," Hollande, who visited the scene of the attack Wednesday, said. "Today I am thinking about the victims -- 11 are dead, four are fighting for their lives. We should do whatever we can to find those responsible and to call for national unity."

Prosecutors later confirmed that 12 people had been killed. France has reinforced security at houses of worship, stores, media offices and transportation in the wake of the attack, the AP reported.