Police arrested several people in connection with the shooting at the officers of Charlie Hebdo, but the hunt is still on for the two men who carried out Europe's deadliest terror attack in more than three years.
Heavily-armed police scoured streets in tactical gear in the city of Reims, 144 kilometers (about 90 miles) from Paris.
Their target: brothers Cherif Kouachi, 32, and Said Kouachi, 34, the suspects in Wednesday's brazen lunchtime attack that killed 12 people and wounded 11.
"They are still free, they are heavily armed, so we can be afraid of further violence," French Prime Minister Manuel Valls said in an interview with French broadcaster RTL.
'Their only mistake'
The two masked men apparently left behind a getaway car, which police impounded. CNN affiliate BFMTV reported that police found an identification card of one of the Kouachi brothers during their investigations.
"It was their only mistake," said Dominique Rizet, BFMTV's police and justice consultant, reporting that the discovery helped the investigation.
The Kouachi brothers returned from war-torn Syria in the summer, USA Today reported without saying where it got the information.
Officials were running their names through databases to look for connections with ISIS and al Qaeda.
A third suspect, 18-year-old Hamyd Mourad, has turned himself in to police, a source close to the case told the AFP news agency. Mourad did so late Wednesday after seeing his name mentioned on social media, the source said.
The victims may have been killed over cartoons the magazine published lampooning the Muslim prophet, Mohammed.
The magazine is known for its irreverence towards religion, society and politics. Its offices were fire-bombed in 2011, on the same day the magazine was due to release an issue with a cover that appeared to poke fun at Islamic law.
On Wednesday, gunman ran into the building housing Charlie Hebdo's offices, not far from the famed Notre Dame cathedral and the Place de la Bastille.
On their way into the building, they asked exactly where the offices were. The men reportedly spoke fluent French with no accent.
They barged in on the magazine's staff, while they were gathered for a lunchtime editorial meeting. The gunmen separated the men from the women and called out the names of cartoonists they intended to kill, said Dr. Gerald Kierzek, a physician who treated wounded patients and spoke with survivors.
The shooting was not a random spray of bullets, but more of a precision execution, he said.
It was the deadliest attack in Europe since July 2011 when Anders Behring Brevik killed 77 people in attacks on government buildings in Oslo, Norway, and at a youth camp on the island of Utoya.
Avenging the prophet
In Paris on Wednesday, journalist Martin Boudot was working nearby when the attack happened.
Boudot and his colleagues made a mad dash for the roof. They didn't know what to do.
"We knew that there were victims a few meters away from us, but there might be, you know, some explosives somewhere or maybe a third guy," Boudot said.
Cell phone cameras caught two gunmen as they ran back out of the building, still firing. One of them ran up to a wounded man lying on a sidewalk, who appeared to wear a dark blue uniform. The gunman shot him at point blank.
The two said they were avenging the Prophet Mohammed and shouted "Allahu akbar," which translates to "God is great," Paris Prosecutor Francois Molins said.
'Parisians will not be afraid'
Cartoonist Stephane Charbonnier -- "Charb" for short -- was killed. He was the magazine's editor. Three other well-known cartoonists, known by the pen names Cabu, Wolinski and Tignous, were also shot dead.
On Wednesday night, Parisians poured into streets in hordes.
At an event in Paris' Place de la Republique, demonstrators held up pens and chanted, "We are Charlie!"
Similar demonstrations took place in other cities, including Rome, Berlin and Barcelona.
"Parisians will not be afraid," said Paris Deputy Mayor Patrick Klugman. "We will fight terrorism with our common values, freedom, freedom of speech, freedom of the press."
On Thursday, the victims' names were splashed across newspapers as heroes for the freedom of expression. "Liberty assassinated." "We are all Charlie Hebdo," the headlines blared.
French President Francois Hollande declared Thursday a national day of mourning.