Members of the French GIPN intervention police forces secure a neighbourhood in Corcy, northeast of Paris January 8, 2015. Armed and masked anti-terrorism police swooped on woodland villages northeast of Paris on Thursday in a manhunt for two brothers suspected of being the Islamist gunmen who killed 12 people at a French satirical weekly.
A day after the Paris attack, officers carried out house-to-house searches in the village of Corcy, a few km (miles) from a service station where police sources said the brothers were sighted in ski masks. Helicopters flew overhead, reuters report.
The fugitive suspects are French-born sons of Algerian-born parents, both in their early 30s, and already under police surveillance. One was jailed for 18 months for trying to travel to Iraq a decade ago to fight as part of an Islamist cell. Police said they were "armed and dangerous".
United States and European sources close to the investigation said on Thursday that one of the brothers, Said Kouachi, was in Yemen in 2011 for a number of months training with Al Qaeda in the Arabian Peninsula (AQAP), one of the group's most active affiliates.
In Paris, a policewoman was killed in a shootout with a gunman wearing a bulletproof vest on Thursday morning, setting a tense nation further on edge. Police sources were unable to say whether that incident was linked to the previous day's assault at the Charlie Hebdo weekly newspaper, but the authorities opened another terrorism investigation.
Bewildered and tearful French people held a national day of mourning. The bells of Notre Dame pealed for those killed in the attack on Charlie Hebdo, a left-leaning slayer of sacred cows whose cartoonists have been national figures since the Parisian counter-cultural heyday of the 1960s and 1970s.
The newspaper had been firebombed in the past for printing cartoons that poked fun at militant Islam and some that mocked the Prophet Muhammad himself. Two of those killed were police posted to protect the paper.
While world leaders described the attack as an assault on democracy, al Qaeda's North Africa branch praised the gunmen as "knight(s) of truth".
Searches were taking place in Corcy and the nearby village of Longpont, set in thick forest and boggy marshland about 70 km north of Paris, but it was not clear whether the fugitives who had been spotted in the area were holed up or had moved on.
Corcy residents looked bewildered as heavily armed policeman in ski masks and helmets combed the village meticulously from houses to garages and barns.
Police released photographs of the two suspects, Cherif and Said Kouachi, 32 and 34. The brothers were born in eastern Paris and grew up in an orphanage in the western city of Rennes after their parents died.
The younger brother's jail sentence for trying to fight in Iraq a decade ago, and more recent tangles with the authorities over suspected involvement in militant plots, raised questions over whether police could have done more to watch them.
Cherif Kouachi was arrested on Jan. 25, 2005 preparing to fly to Syria en route to Iraq. He served 18 months of a three-year sentence.
In 2010 he was suspected of being part of a group that tried to break from prison Smain Ali Belkacem, a militant jailed for the 1995 bombings of Paris train and metro stations that killed eight people and wounded 120. The case against Cherif Kouachi was dismissed for lack of evidence.
A third person wanted by police, an 18-year-old man, turned himself into police in Charleville-Mézières near the Belgian border late on Wednesday. A legal source said he was the brother-in-law of one of the brothers. French media quoted friends as saying he was in school at the time of the attack.
In the wake of the killings, authorities tightened security at transport hubs, religious sites, media offices and stores. Police also increased their presence at entry points to Paris.
At Porte d'Orleans, one of the capital's main gateways, more than a dozen white police vans lined up the main avenue. Officers stood guard with bulletproof jackets and rifles.
The defense ministry said it sent 200 extra soldiers from parachute regiments across the country to help guard Paris.
Tens of thousands of people attended vigils across France on Wednesday, many wearing badges declaring "Je suis Charlie" in support of the newspaper and the principle of freedom of speech.
Charlie Hebdo's lawyer Richard Malka said the newspaper would be published next Wednesday with one million copies compared to its usual print run of 60,000.