Paris: Masked gunmen shouting “Allahu akbar!” stormed the Paris offices of a satirical newspaper Wednesday, killing 12 people including the editor and a cartoonist before escaping. It was France’s deadliest terror attack in at least two decades.

With a manhunt on, French President Francois Hollande called the attack on the Charlie Hebdo weekly, whose caricatures of the Prophet Muhammed have frequently drawn condemnation from Muslims, “a terrorist attack without a doubt.” He said several other attacks have been thwarted in France “in recent weeks.”

There was no immediate claim of responsibility for the attack.

France raised its security alert to the highest level and reinforced protective measures at houses of worship, stores, media offices and transportation. Top government officials were holding an emergency meeting and Hollande planned a nationally televised address in the evening. Schools across the French capital closed their doors.

World leaders including President Barack Obama and German Chancellor Angela Merkel condemned the attack, but supporters of the militant Islamic State group celebrated the slayings as well-deserved revenge against France.

The Islamic State group has repeatedly threatened to attack France. Just minutes before the attack, Charlie Hebdo had tweeted a satirical cartoon of that extremist group’s leader giving New Year’s wishes. Another cartoon, released in this week’s issue and entitled “Still No Attacks in France,” had a caricature of an extremist fighter saying “Just wait—we have until the end of January to present our New Year’s wishes.”

The 12 dead included two men who went by the pen names: Charb—the editor and a cartoonist as well—and the cartoonist Cabu, spokeswoman Agnes Thibault-Lecuivre of the Paris prosecutor’s office confirmed.

Two police officers were also among the dead, including one assigned as Charb’s bodyguard after prior death threats against him, a police official told The Associated Press, speaking on condition of anonymity because of the ongoing investigation.

Just before noon, multiple masked men armed with automatic weapons attacked the newspaper’s office in central Paris, nearby worker Benoit Bringer told the iTele network. The attackers went to the second floor and started firing indiscriminately in the newsroom, said Christophe DeLoire of Reporters Without Borders.

“This is the darkest day of the history of the French press,” DeLoire said.

Video images on the website of public broadcaster France Televisions showed two gunmen in black at a crossroads who appeared to fire down one of the streets. A cry of “Allahu akbar!”—Arabic for “God is great”—could be heard among the gunshots.

Luc Poignant of the SBP police union said the attackers left in a waiting car and later switched to another vehicle that had been stolen.

Obama’s top spokesman said U.S. officials have been in close contact with the French since the attack. “We know they are not going to be cowed by this terrible act,” spokesman Josh Earnest said.

On social media, supporters of militant Islamic groups praised the move. One Twitter user who identified themselves as a Tunisian loyalist of al-Qaida and the Islamic State group called the attack well-deserved revenge against France.

Elsewhere on the Internet, the hashtag (hash)JeSuisCharlie was trending as people expressed support for weekly and for journalistic freedom.

Charlie Hebdo has been repeatedly threatened for its caricatures of the Prophet Muhammad and other controversial sketches. Its offices were firebombed in 2011 after a spoof issue featuring a caricature of the prophet on its cover.

Nearly a year later, the publication again published crude Muhammad caricatures, drawing denunciations from around the Muslim world.

Wednesday’s attack comes the same day of the release of a book by a celebrated French novelist depicting France’s election of its first Muslim president. Hollande had been due to meet with the country’s top religious officials later in the day.