In a statement on their website, French national police ask for information on the whereabouts of suspects Cherif Kouachi and Said Kouachi, warning that both could be armed and dangerous.
Police released photos of the two men, who Paris Deputy Mayor Patrick Klugman told CNN are brothers in their 30s.
Police found an ID document of Said Kouachi at the scene of the shooting, CNN affiliate BFMTV reported. "It was their only mistake," said Dominique Rizet, BFMTV's police and justice consultant, reporting that the discovery helped the investigation.
Citing sources, the Agence France Presse news agency reported that an 18-year-old suspect in the attack had surrendered to police. CNN has not independently confirmed whether the suspect has surrendered.
Police fanned out across France in an intense manhunt for the suspects, who were masked and dressed in black when they burst into the satirical magazine's office Wednesday, killing 12 people.
A tactical unit was deployed in an operation about a 144 kilometers (about 90 miles) from Paris in Reims, France, following the attack, CNN affiliate BFMTV reported. Authorities haven't revealed details about the target of the operation, but speculation surged in French media that investigators could be closing in on the suspects.
French authorities vowed to step up security and apprehend those responsible.
"Everything will be done to arrest (the attackers)," French President Francois Hollande said in a speech Wednesday night. "... We also have to protect all public places. Security forces will be deployed everywhere there can be the beginning" of a threat.
It's too soon to say whether the suspects were operating alone, CNN terrorism analyst Paul Cruickshank said.
Some experts warned that how well the gunmen wielded their weapons
, hid their identities and apparently planned their escape showed a marked difference from previous "lone wolf" attacks -- and could be a game changer.
Video shows gunman target seemingly wounded man
These developments come after at least two heavily armed men entered the Charlie Hebdo
office in Paris' 11th district, close to Place de la Bastille, and opened fire, Paris Prosecutor Francois Molins said.
The gunmen said they were avenging the Prophet Mohammed and shouted "Allahu akbar," which translates to "God is great," Molins said.
A witness who works in the office opposite the magazine's told BFMTV that he saw two hooded men, dressed in black, enter the building heavily armed.
"We then heard them open fire inside, with many shots," he said. "We were all evacuated to the roof. After several minutes, the men fled, after having continued firing in the middle of the street."
The men reportedly spoke fluent French with no accent.
One unsettling video, posted to YouTube, shows two men shooting on a Paris street, then walking up to and firing point-blank at a seemingly wounded man as he lay on the ground.
Video shows a gunman approaching his getaway car and raising his finger in the air in what appears to be a signal, possibly to another vehicle or other people who might have played a role in the attack, a Western intelligence source briefed on the French investigation told CNN.
In addition to the 12 dead
, 11 people were wounded, including four in "serious condition," the prosecutor said. Two police officers were among those killed, the French President said.
editor and cartoonist Stephane Charbonnier, known as "Charb," is among the dead in the attack, a police spokesman in the district where the office is located told CNN. At least three other well-known cartoonists -- known by the pen names Cabu, Wolinski and Tignous -- were also killed.
'Parisians will not be afraid'
At an event in Paris' Place de la Republique, demonstrators held up pens in honor of the slain cartoonists and chanted, "We are Charlie!" Pictures posted online showed similar demonstrations in other cities, including Rome, Berlin and Barcelona.
"Parisians will not be afraid," Klugman said. "We will fight terrorism with our common values, freedom, freedom of speech, freedom of the press. ... We are at war, but we still want to behave as a leading democracy."
Armed soldiers could be seen standing guard outside monuments, in transit stations and elsewhere in well-trafficked spots around France by Wednesday evening.
Prime Minister Manuel Valls raised France's security to its highest level -- "attack alert" -- after the Charlie Hebdo
bloodshed. That means there will be reinforced security at media company offices, major stores, religious centers and on public transport, his office said in a statement.
U.S. counterterrorism agencies are looking at a number of groups, including ISIS
and al Qaeda, that might be responsible for the attack. Charlie Hebdo has been singled out as a target for Islamist extremists in al Qaeda's publication, Inspire.
Police impounded a black Citroen in northeastern Paris similar to the one purportedly used by the attackers as a getaway car. Video from CNN affiliate BFMTV shows the vehicle being towed from Porte de Pantin, in Paris' 19th district.
Investigators will do a complete DNA work-up on the Citroen, including soil signatures that might suggest where the gunmen came from, a Western intelligence source briefed on the probe told CNN.
The same source said that French authorities are searching all travel records from the past 17 days to see whether any of the attackers entered the European nation over the holidays. This includes checks at Charles de Gaulle and Orly airports, as well as whatever limited information is available from train stations.
Thursday will be a national day of mourning for those killed in the attack, Hollande said. He asked for a moment of reflection Thursday and said flags will be at half-staff for three days.
Satirical magazine has drawn anger
is no stranger to controversy for lampooning a variety of subjects, including Christianity. But what it's done on Islam has gotten the most attention and garnered the most vitriol.
Its last tweet before Wednesday's attack featured a cartoon of ISIS leader Abu Bakr al-Baghdadi.
The magazine has a controversial history of depicting Mohammed, often in an unfavorable light, which has angered many Muslims around the world
Earlier cartoons depicting the Prophet Mohammed spurred protests and the burning of the magazine's office three years ago.
In November 2011, Charlie Hebdo's office was firebombed
the day it was due to publish a cover making fun of Islamic law.
A year later, in an interview with Le Monde newspaper, Charbonnier gave little indication that he planned to change Charlie Hebdo's ways.
"It may sound pompous," he said, "but I'd rather die standing than live on my knees."
The attack on the magazine spurred a wave of support for the publication and its practices around France and the world.
On social media, people tweeted past covers from the magazine as well as the words "Je suis Charlie," or "I am Charlie