Hayat Boumeddiene, the 26-year-old partner of one of the attackers, left for Turkey and then Syria, sources say
A woman hunted by French police as a suspect in the attacks on a satirical paper and Jewish supermarket in Paris left France several day before the killings and is believed to be in Syria, Turkish and French sources said.
After killing the gunmen behind the worst assault in France for decades, French police launched in an intensive search for Hayat Boumeddiene, the 26-year-old partner of one of the attackers, describing her as "armed and dangerous".
But a source familiar with the situation said that Boumeddiene left France last week and traveled to Syria via Turkey. A senior Turkish official corroborated that account, saying she passed through Istanbul on January 2.
Security forces remained on high alert before a march on Sunday which will bring together European leaders in a show of solidarity for the 17 victims killed in three days of violence that began with an attack on the Charlie Hebdo weekly on Wednesday and ended with Friday's dual sieges at a print works outside Paris and a kosher supermarket in the city.
French security forces shot dead the two brothers behind the Hebdo killings after they took refuge in the print works. They also killed an associate - Boumeddiene's partner - who planted explosives at the Paris deli in a siege that claimed the lives of four hostages.
On Saturday, police maintained a heavy presence around the French capital, with patrols at sensitive sites including media offices, and local vigils were held across France. The Interior Ministry said about 700,000 people attended including 120,000 in Toulouse, 75,000 in Nantes, and 50,000 in Marseille.
"It's no longer like before," said Maria Pinto, on a street in central Paris. "You work a whole life through and because of these madmen, you leave your house to go shopping, go to work, and you don't know if you'll come home."
The attack on Charlie Hebdo, a journal that satirised Islam as well as other religions and politicians, raised sensitive questions about freedom of speech, religion and security in a country struggling to integrate five million Muslims.
A source familiar with the situation said that Boumeddiene left France last week and traveled to Syria via Turkey.
"On January 2, a woman corresponding to her profile and presenting a piece of identity took a flight from Madrid to Istanbul," a source familiar with the situation told Reuters.
The source said she was accompanied by a man and had a return ticket for January 9, but never took the flight.
A senior Turkish security official said Paris and Ankara were now cooperating in trying to trace her, but said she arrived in Istanbul without any warning from France.
"After they informed us about her ... we identified her mobile phone signal on Jan 8," the source said. "We think she is in Syria at the moment but we do not have any evidence about that ... She is most probably not in Turkey," the source said, adding the last signal from her phone was detected on Thursday.
An official police photograph of Boumeddiene shows a young woman with long dark hair hitched back over her ears. French media, however, released photos purporting to be of a fully-veiled Boumeddiene, posing with a cross-bow, in what they said was a 2010 training session in the mountainous Cantal region.
French media described her as one of seven children whose mother died when she was young and whose delivery-man father struggled to keep working while looking after the family. As an adult, she lost her job as a cashier when she converted to Islam and started wearing the niqab.
Le Monde said Boumeddiene wed Amedy Coulibaly in a religious ceremony not recognised by French civil authorities in 2009. The two were questioned by police in 2010 and Coulibaly jailed for his involvement in a botched plot to spring from jail the author of a deadly 1995 attack on the Paris transport system.
Participation of European leaders including Germany's Angela Merkel, Britain's David Cameron and Italy's Matteo Renzi in a silent march through Paris with President Francois Hollande will pose further demands for security forces on Sunday.
Arab League representatives and some Muslim African leaders as well as Turkish Prime Minister Ahmet Davutoglu will attend.
Political and security chiefs were reviewing how two French-born brothers of Algerian extraction, Cherif and Said Kouachi, could have carried out the Charlie Hebdo attacks despite having been on surveillance and "no-fly" lists for many years.
Paris chief prosecutor Francois Molins said late Friday the three men killed on Friday in the two security operations had a large arsenal of weapons and had set up booby traps. They had a loaded M82 rocket launcher, two Kalashnikov machine guns and two automatic pistols on them.
With one of the gunmen saying shortly before his death that he was funded by al Qaeda, Hollande warned that the danger to France - home to the European Union's biggest populations of both Muslims and Jews - was not over yet.
"These madmen, fanatics, have nothing to do with the Muslim religion," Hollande said in a televised address.
"France has not seen the end of the threats it faces," said Hollande, facing record unpopularity over his handling of the economy but whose government has received praise from at least one senior opposition leader for its handling of the crisis.
An audio recording posted on YouTube attributed to a leader of the Yemeni branch of al Qaeda (AQAP) said the attack was prompted by insults to prophets but stopped short of claiming responsibility for the assault on the offices of Charlie Hebdo.
Palestinian Islamist militant group Hamas condemned the strike as an unjustifiable terrorist attack.
Before his death at the printing works, Cherif Kouachi told a television station he had received financing from an al Qaeda preacher, Anwar al Awlaki, in Yemen.
Al Awlaki, an influential international recruiter for al Qaeda, was killed in September 2011 in a drone strike. A senior Yemeni intelligence source told Reuters that Kouachi's brother Said had also met al Awlaki during a stay in Yemen in 2011.
Paris prosecutor Molins said there had been sustained contact between Boumeddiene and the wife of Cherif Kouachi, with records of no fewer than 500 phone calls between the two last year. The wife of Kouachi is being questioned by French police.
Coulibaly had also called BFM-TV, to claim allegiance to Islamic State, saying he wanted to defend Palestinians and target Jews. He said he had jointly planned the attacks with the Kouachi brothers, and police confirmed they were all members of the same Islamist cell in northern Paris.