French police have named two brothers as suspects in the attack on the Paris offices of satirical magazine Charlie Hebdo, as a manhunt continues.
They issued photos of Cherif and Said Kouachi, said to be “armed and dangerous”, and arrest warrants. A third suspect reportedly surrendered.
France is holding a day of mourning for the 12 people killed in the attack.
A minute’s silence will be observed at midday across the country and the bells of Notre Dame in the capital will toll.
Security forces carried out a major search operation in the eastern city of Reims overnight but no arrests were made. Police cordoned off a block of flats and forensic teams could be seen inside.
The country has been placed on the highest terror alert and extra troops have been deployed to guard media offices, places of worship, transport and other sensitive areas.
Vigils have been held in Paris and in cities across the world in tribute to those killed in Wednesday’s attack. Many carried placards reading “Je suis Charlie” (I am Charlie) in solidarity with the victims.
Eight journalists – including the magazine’s editor – died along with two policemen, a maintenance worker and a visitor when masked men armed with assault rifles stormed the Charlie Hebdo offices.
The magazine has angered some Muslims in the past by printing cartoons of the Prophet Muhammad. The offices were firebombed in 2011.
The gunmen were heard shouting “we have avenged the Prophet Muhammad” and “God is Great” in Arabic (“Allahu Akbar”).
French media, citing police documents, initially named a third suspect as Hamyd Mourad, 18. However, it was later reported that Mr Mourad had handed himself in to police after seeing his name circulating on social media.
Officials then issued photographs of the Kouachi brothers and said arrest warrants had been issued for them.
Cherif Kouachi has been described in the French media as a militant sentenced in 2008 to three years in prison for belonging to a group sending jihadist fighters to Iraq.
French President Francois Hollande said the country’s tradition of free speech had been attacked and called on all French people to stand together.
In a sombre televised address late on Wednesday he said: “Today the French Republic as a whole was the target.”
Thursday’s national day of mourning is only the fifth held in France in the past 50 years.
The attack took place as the magazine was holding its weekly editorial meeting. French media have named three cartoonists killed as Cabu, Tignous and Wolinski, as well as Charlie Hebdo contributor and French economist Bernard Maris.
Charlie Hebdo editor Stephane Charbonnier, 47, had received death threats in the past and was living under police protection.
Eleven people were wounded, some seriously.
Cartoonist Corinne Rey said the gunmen entered the building after forcing her to enter the code to open the door.
“They said they belonged to al-Qaeda,” she said, adding they had spoken in fluent French.
Witnesses said they heard as many as 50 shots fired both inside the Charlie Hebdo office and on the streets outside.
The gunmen were filmed on amateur video shooting one injured police officer at point blank range in the head on the pavement outside.
Police said the men fled to northern Paris, before abandoning their car and hijacking a second one.
The attack was swiftly condemned by world leaders, with US President Barack Obama offering to help France track down those responsible.
Al-Azhar University in Cairo, Sunni Islam’s leading centre of learning, called the attack “criminal” and said “Islam denounces any violence”.
The Arab League also condemned the attack. Pope Francis called it “abominable”.
Thousands of people gathered at the Place de la Republique in central Paris for a vigil, many holding up placards saying “Je suis Charlie” (I am Charlie), in solidarity with the victims.
Piles of pens – symbolising freedom of expression – and candles were laid across the square.
Similar scenes were repeated at vigils across France and in cities around the world.