President Francois Hollande, visiting the scene of France's deadliest such attack in more than half a century, called the assault on the weekly newspaper Charlie Hebdo "an act of exceptional barbarism."
France raised its terror alert system to the maximum Attack Alert and bolstered security with more than 800 extra soldiers to guard media offices, places of worship, transport and other sensitive areas.
French brothers Said and Cherif Kouachi, in their early 30s, should be considered armed and dangerous, according to a police bulletin released early today.
Mourad Hamyd, 18, surrendered at a police station in Charleville-Mezieres, a small town in France's eastern Champagne region, said Paris prosecutor's spokeswoman Agnes Thibault-Lecuivre. She did not offer details on Hamyd's relationship with the men.
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Heavily armed police moved into the nearby city of Reims, searching for the suspects without success, Thibault-Lecuivre said.
Video from BFM-TV showed police dressed in white apparently taking samples inside an apartment. It was not immediately clear who lived there.
The officials spoke on condition of anonymity because they were not authorised to publicly discuss the sensitive and ongoing investigation.
Cherif Kouachi was sentenced to 18 months in prison after being convicted of terrorism charges in 2008 for helping funnel fighters to Iraq's insurgency. He said he was outraged at the torture of Iraqi inmates at the US prison at Abu Ghraib near Baghdad.