A major manhunt has been launched for three gunmen who shot dead 12 people at the Paris office of the French satirical magazine Charlie Hebdo.
Eight journalists and two policemen were among the dead. Police named three suspects, one of whom is reported to have surrendered.
A police operation took place in Reims, east of Paris, without arrests.
Protests and vigils over the attack, the deadliest the country has seen in decades, have been held across France.
President Francois Hollande called it a “cowardly murder” and declared a day of national mourning on Thursday.
He said the country’s tradition of free speech had been attacked and called on all French people to stand together. “Our best weapon is our unity,” Mr Hollande said in a televised address late on Wednesday.
Security has been stepped up across France in the wake of the attack, with Paris placed on the highest alert.
The three suspects were named in a police document circulated to regional forces as Hamyd Mourad, 18, and brothers Said Kouachi, 34, and Cherif Kouachi, 32.
However, a source close to the case told AFP news agency that Hamyd Mourad handed himself in to police after seeing his name circulating on social media. He was arrested and taken into custody, AFP reported.
Paris police later said arrest warrants had been issued for Cherif and Said Kouachi who were believed to be “armed and dangerous”.
Media reports described Cherif Kouachi as a militant sentenced in 2008 to three years in prison for belonging to a group sending jihadist fighters to Iraq.
Police did not give any details but forensic teams were later seen searching an apartment in Reims.
The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was firebombed in November 2011 a day after it carried a caricature of the Prophet Muhammad.
The attack took place as the magazine was holding its weekly editorial meeting. French media have named three cartoonists killed in the attack 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.
Cartoonist Corinne Rey, said the hooded 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.
Eyewitnesses said they heard as many as 50 shots fired by the attackers both inside the Charlie Hebdo office and on the streets outside.
The gunmen were captured on amateur video shooting one injured police officer at point blank range in the head on the pavement outside.
They were heard shouting “we have avenged the Prophet Muhammad” and “God is Great” in Arabic (“Allahu Akbar”).
Police said the masked gunmen fled to northern Paris, before abandoning their car and hijacking a second one.
The Paris prosecutor, Francois Molins, said 11 people had been wounded in the attack, four of them seriously.
He told reporters all efforts were being made to find those responsible, without giving any details about the investigation.
“The investigations have been numerous and in-depth, because of course, the police have been mobilised, and these inquiries are going on.”
