One of two brothers suspected of carrying out the deadly shooting at a French satirical weekly visited Yemen in 2011 to train with al Qaeda-affiliated militants, US and European sources close to the investigation said on Thursday.
The sources said Said Kouachi, 34, was in Yemen for a number of months training with al Qaeda in the Arabian Peninsula (AQAP), one of the group's most active affiliates.
The two suspects in the Charlie Hebdo attack, Said Kouachi and his brother Cherif, were killed Friday when police stormed the building where they were holed up, sources close to the investigation said.
The two brothers are French-born sons of Algerian-born parents. Both men had been under police surveillance. Cherif was jailed for 18 months for trying to travel to Iraq a decade ago to fight as part of an Islamist cell.
A Yemeni official familiar with the matter said the government was aware of the possibility of a connection between Said Kouachi and AQAP, and was looking into possible links.
The sources said that after Said Kouachi returned to France from Yemen, both brothers appeared to have refrained from any activities that might have drawn the attention of French law enforcement or spy agencies.
They also said that in the months leading up to Wednesday's attack, the men were not treated as priority targets by French counter-terrorism agencies.
US government sources said both Said and Cherif Kouachi were listed in two US security databases - a highly-classified database, containing information on 1.2 million possible counter-terrorism suspects, called TIDE, and the much smaller "no fly" list maintained by the Terrorist Screening Center, an interagency unit.
ABC News reported that the brothers had been listed in the databases for "years."
Dave Joly, a spokesman for the Terrorist Screening Center, said he could neither confirm nor deny whether the Kouachis were listed in counter-terrorism databases.
"Disclosure of an individual's inclusion or non-inclusion in the TSDB (screening databases) would significantly impair the government's ability to investigate and counteract terrorism," Joly said.
At the time Said Kouachi went to Yemen, one of AQAP's top inspirational and organisational leaders was Anwar al Awlaki, a US-born preacher prominent in spreading the group's militant message to European and English-speaking audiences. It is not known if Said Kouachi had any contact with Awlaki, who was killed in September 2011 in a drone strike widely attributed to the CIA.
Some investigators believe Awlaki's death could have contributed to the brothers' decision to lie low, but other investigators say that it was too early to reach such a conclusion. Investigators are trying to establish the significance, if any, of the brothers' links with AQAP or any other radical Islamist group.
One of those killed in the Paris attack was Charlie Hebdo's top editor, Stephane Charbonnier, who drew cartoons under the rubric "Charb."
Last spring, "Inspire," an English-language online magazine published by AQAP, featured a "Wanted dead or alive" graphic which included Charbonnier's name and photograph. There was no immediate evidence that the graphic actually inspired the Paris attack.
Armed and masked anti-terrorism police swooped on woodland villages northeast of Paris on Thursday in a manhunt for the two attackers.
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.
On Thursday, US President Barack Obama made an unannounced visit to the French Embassy in Washington to pay his respects.
He wrote in a condolence book, "As allies across the centuries, we stand united with our French brothers to ensure that justice is done and our way of life is defended. We go forward together knowing that terror is no match for freedom and ideals we stand for - ideals that light the world."
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".
Many European newspapers either re-published Charlie Hebdo cartoons or lampooned the killers with images of their own.
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.
"We have not found them, there is no siege," an interior ministry official in Paris said.
Corcy residents looked bewildered as heavily armed policeman in ski masks and helmets combed the village meticulously from houses to garages and barns.
"We're hearing that the men could be in the forest, but there's no information so we're watching television to see," said Corcy villager Jacques.
In neighbouring Longpont, a resident said police had told villagers to stay indoors because the gunmen may have abandoned their car there. Anti-terrorism officers pulled back as darkness fell. The silence was broken by the sound of a forest owl.
Thursday's shooting of the policewoman on the streets of Paris's southern Montrouge district -- whether related or not --caused more fear.
Montrouge Mayor Jean-Loup Metton said the policewoman and a colleague came under fire while responding to a reported traffic accident.
Witnesses said the assailant fled in a Renault Clio. Police sources said he wore a bullet-proof vest and had a an assault rifle and a handgun.
A police officer at the scene told Reuters he did not appear to resemble the Charlie Hebdo shooter suspects.
Prime Minister Manuel Valls, asked on RTL radio whether he feared a further attack, said: "That's obviously our main concern and that is why thousands of police and investigators have been mobilised to catch these individuals."
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 defence 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.
Newspapers in many countries republished Charlie Hebdo cartoons. Britain's Daily Telegraph depicted two masked gunmen outside the doors of Charlie Hebdo saying to each other: "Be careful, they might have pens."
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.
Muslim leaders condemned the shooting, but some have expressed fears of a rise in anti-Islamic feeling in a country with a large Muslim population. The window of a kebab shop next to a mosque in the town of Villefrance-sur-Saone was blown out by an overnight explosion. Local media said no one was hurt.
(With inputs from Reuters, AFP, AP)