The BBC's Security Correspondent Frank Gardner has said the massacre at the French satirical magazine Charlie Hebdo "did not come out of the blue."
A major manhunt has been launched in Paris for three gunmen who shot dead 12 people at the office of the French satirical magazine Charlie Hebdo.
Eight journalists, including the magazine's editor, and two policemen were among the dead.
Protests over the killings are being held in cities across France. It is the country's deadliest attack in decades.
President Francois Hollande called it a "cowardly murder" and declared a day of national mourning on Thursday.
He called on all French people to stand together. "Our best weapon is our unity," he said in a televised address.
Security has been stepped up across France in the wake of the attack, with Paris placed on the highest alert.
The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was firebombed in November 2011 a day after it carried a caricature of the Prophet Muhammad.
A firefight erupted outside the offices between the attackers and police
Wednesday's attack is France's deadliest since 1961
Thousands have gathered at a central square in Paris for a silent vigil
France's chief prosecutor, Francois Molins, said 11 people were wounded in the attack, four of them seriously.
He told reporters all efforts were being made to find those responsible for the attack, without giving any details about the investigation.
"The investigations have been numerous and in-depth, because of course, the police have been mobilised, and these inquiries are going on."
Police said the masked gunmen fled to northern Paris, before abandoning their car and hijacking a second one.
Eyewitnesses said they heard as many as 50 shots fired by the attackers both inside the Charlie Hebdo office and on the streets outside.
The gunmen were captured on amateur video shooting one injured police officer at point blank range in the head on the pavement outside.
Five of the victims known to have died in the attack, including deputy chief editor Bernard Maris, Georges Wolinsky, Jean Cabut, Stephane Charbonnier and Bernard Verlhac
They were heard shouting "we have avenged the Prophet Muhammad" and "God is Great" in Arabic ("Allahu Akbar").
The attack took place as the magazine was holding its daily editorial meeting. French media have named three cartoonists killed in the attack as Cabu, Tignous and Wolinski, as well as Charlie Hebdo contributor and French economist Bernard Maris.
Charlie Hebdo editor Stephane Charbonnier, 47, had received death threats in the past and was living under police protection.
Video uploaded to YouTube shows the attackers fleeing after shooting a police officer
People had been "murdered in a cowardly manner", President Hollande told reporters at the scene. "We are threatened because we are a country of liberty," he added, appealing for national unity.
French government officials have held an emergency meeting, and President Hollande is due to give a televised address at 19:00 GMT.
The killings have been condemned by leaders worldwide, with US President Barack Obama offering to help France track down those responsible. Reuters quotes FBI director James Comey as saying that the US agency was working with France to help investigate the attack.
Thousands of people have gathered at the Place de la Republique in central Paris for a silent vigil, many holding up placards saying "Je suis Charlie" ("I am Charlie"), referring to a hashtag that is trending on Twitter in solidarity with the victims.
At least 20,000 people also joined rallies in the cities of Lyon and Toulouse, French news agency AFP quotes police as saying.
Charlie Hebdo's website, which went offline during the attack, is displaying the single image of "Je suis Charlie" ("I am Charlie) on a black banner.
The latest tweet on Charlie Hebdo's account was a cartoon of the Islamic State militant group leader, Abu Bakr al-Baghdadi.
Analysis: Hugh Schofield, BBC News, Paris
Charlie Hebdo is part of a venerable tradition in French journalism going back to the scandal sheets that denounced Marie-Antoinette in the run-up to the French Revolution.
The tradition combines left-wing radicalism with a provocative scurrility that often borders on the obscene. Its decision to mock the Prophet Muhammad in 2011 was entirely consistent with its historic raison d'etre.
The paper has never sold in enormous numbers - and for 10 years from 1981, it ceased publication for lack of resources.
But with its garish front-page cartoons and incendiary headlines, it is an unmissable staple of newspaper kiosks and railway station booksellers.
The gunmen targeted the Charlie Hebdo office as a meeting was taking place
France has raised its security alert for Paris to the highest level
The attackers switched cars after fleeing the scene
Gilles Boulanger, who works in the same building as the office, told French TV channel Itele: "There were several shots heard in the building from automatic weapons firing in all directions. So then we looked out of the window and saw the shooting was on Boulevard Richard-Lenoir, with the police. It was really upsetting. You'd think it was a war zone."
Wandrille Lanos, a TV reporter who works across the road, was one of the first people to enter the Charlie Hebdo office after the attack.
"As we progressed into the office, we saw that the number of casualties was very high. There was a lot of people dead on the floor, and there was blood everywhere," he told the BBC.
The country was already on the alert for Islamist militant attacks after several incidents just before Christmas, although the French government has denied the attacks were linked.
It is the deadliest militant attack in France since 1961, when a bomb on a train killed 28 people.
Danish newspaper Jyllands-Posten, which published cartoons of the Prophet Muhammad in 2005 sparking riots in Muslim countries, says it has stepped up security in the wake of the Charlie Hebdo attack.