PARIS -- Three masked gunmen shouting "Allahu akbar!" stormed the Paris offices of a satirical newspaper Wednesday, killing 12 people, including its editor, before escaping in a car. It was France's deadliest postwar terrorist attack.

Security forces were hunting for the gunmen who spoke flawless, unaccented French in the military-style noon-time attack on the weekly newspaper Charlie Hebdo, located near Paris' Bastille monument. The publication's caricatures of the Prophet Muhammed have frequently drawn condemnation from Muslims.

President Francois Hollande called the slayings "a terrorist attack without a doubt," and said several other attacks have been thwarted in France "in recent weeks."

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

There was no immediate claim of responsibility for the attack, which also left four people critically wounded, and was condemned by world leaders including President Barack Obama and Russian President Vladimir Putin, but praised by supporters of the militant Islamic State group.

Clad all in black with hoods and carrying machine-guns, the attackers forced one of the cartoonists arriving at the office building with her young daughter to open the door with a security code.

The staff was in an editorial meeting and the gunmen headed straight for the paper's editor, Stephane Charbonnier -- widely known by his pen name Charb -- killing him and his police bodyguard first, said Christophe Crepin, a police union spokesman. Minutes later, two men strolled out to a black car waiting below, calmly firing on a police officer, with one gunman shooting him in the head as he writhed on the ground.

Ten journalists were killed and two police, Crepin said, one of them assigned as Charb's bodyguard and another who had arrived on the scene on a mountain bike. Among the dead was Bernard Maris, an economist who was among the newspaper's contributors and appeared regularly on French radio.

"Hey! We avenged the Prophet Muhammad! We killed Charlie Hebdo," one of the men shouted in French, according to a video shot from a nearby building and broadcast on French TV. Other video 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.

The video showed the killers moving deliberately and calmly. One even bent over to toss a fallen shoe back into the small black car before it sped off. The car was later found abandoned in northern Paris, police said.

Luc Poignant of the SBP police union said the attackers switched to another vehicle that had been stolen.

Corinne Rey, the cartoonist who said she was forced to let the gunmen in, said the men spoke fluent French and claimed to be from al-Qaida. In an interview with the newspaper l'Humanite, she said the entire shooting lasted perhaps five minutes.

The security analyst group Stratfor said the gunmen appeared to be well-trained, "from the way they handled their weapons, moved and shot. These attackers conducted a successful attack, using what they knew, instead of attempting to conduct an attack beyond their capability, failing as a result."

Both al-Qaida and the Islamic State group have repeatedly threatened to attack France. Just minutes before the attack, Charlie Hebdo had tweeted a satirical cartoon of the Islamic State's leader giving New Year's wishes.

Charlie Hebdo has been repeatedly threatened for its caricatures of the Prophet Muhammad and other sketches. Its offices were firebombed in 2011 after an issue featured a caricature of the prophet on its cover. Nearly a year later, the publication again published Muhammad caricatures, drawing denunciations from the Muslim world because Islam prohibits the publication of drawings of its founder.

Another cartoon, released in this week's issue and entitled "Still No Attacks in France," had a caricature of a jihadi fighter saying "Just wait -- we have until the end of January to present our New Year's wishes." Charb was the artist.

"This is the darkest day of the history of the French press," said Christophe DeLoire of Reporters Without Borders.

In the winter 2014 edition of the al-Qaida magazine Inspire, a so-called chief describing where to use a new bomb said: "Of course the first priority and the main focus should be on America, then the United Kingdom, then France and so on."

In 2013, the magazine specifically threatened Charb and included an article titled "France the Imbecile Invader."

An al-Qaida tweeter who communicated Wednesday with AP said the group is not claiming responsibility, but called the attack "inspiring."

Obama offered U.S. help in pursuing the gunmen. In a statement, he offered prayers and support for France, which he called "America's oldest ally."

Mohammed Moussaoui, president of the Union of French mosques, condemned the "hateful act," and urged Muslims and Christians "to intensify their actions to give more strength to this dialogue, to make a united front against extremism."

On social media, supporters of militant Islamic groups praised the move. One self-described Tunisian loyalist of al-Qaida and the Islamic State group tweeted that the attack was well-deserved revenge against France.

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

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Associated Press writers Angela Charlton, Sylvie Corbet and John Leicester contributed.