Authorities identified the three men as Said Kouachi and Cherif Kouachi, both French and in their early 30s, and 18-year-old Hamyd Mourad, whose nationality wasn't immediately clear.
One of the officials, who spoke on condition of anonymity because they weren't authorized to publicly discuss the investigation, told The Associated Press that the men were linked to a Yemeni terrorist network. And Cherif Kouachi was convicted in 2008 of terrorism charges for helping funnel fighters to Iraq's insurgency and sentenced to 18 months in prison.
Authorities have not reported any arrests in the case.
The gunmen, armed with AK-47s, attacked the offices of Charlie Hebdu, a publication that has enraged Muslims for publishing cartoons depicting the prophet Muhammad. On their way in, they killed a maintenance worker, then stormed into an editorial meeting, where they killed eight journalists, including Stephane Charbonnier, the magazine's editorial director, and Bank of France economist Bernard Maris, a columnist. A security officer and a guest were also gunned down. As they fled, they killed a second police officer.
Eleven other people were injured, four of them critically, officials said.
Because the masked, black-clad gunmen attacked with militaristic precision and left the scene with shouts of "Allahu Akbar," the killers were suspected to be well-trained Islamic extremists. But their motive and backgrounds are still being investigated.
While authorities hunted the suspects, shock and mourning spread across Paris and the rest of France, a country with an estimated 5 million Muslims. France has a long, troubled relationship with its Arab immigrants and a more recent history of unrest among young native-born Muslims. There has been growing concern about young men and women returning to France after joining jihadist activity in the Middle East.
Thousands of Parisians took to the streets in spontaneous and defiant demonstrations of unity. They lighted candles and held signs declaring "Je Suis Charlie" ("I am Charlie") in reference to the magazine. At the Place de la Republique, they crammed themselves up onto the monument in the middle of the square and chanted "Charlie! Liberty!" Candles, posters and signs covered the three statues representing Liberty, Equality and Fraternity — the bedrock of French values.
"They want to scare French citizens and prohibit any criticism of religion, so here we are to remind them that religion can be freely criticized," said Sasha Reingewirtz, 28, president of the Jewish Students Union.
President Francois Hollande declared Thursday a day of national mourning and called for a minute of silence at noon. Flags will fly at half-staff for three days. The government raised its terrorism threat level to its highest grade and announced that security forces would be deployed at media outlets, major shopping venues, sites of religious worship and transportation networks in the Paris region.
