PARIS - As CBS News' Clarissa Ward reports, France is on its highest terror alert, with more than 80,000 security forces deployed across the country. The hunt is on for the Kouachi brothers - Cherif and Said, the Paris-born sons of Algerian parents - who are wanted for the precision attack Wednesday that killed 12 people at Charlie Hebdo, a satirical weekly that lampooned radical Muslims and the Prophet Muhammad himself.
Officers blocked roads and searched houses in the Picardy region, north of Paris, after reports that the two heavily armed men had robbed a gas station, Ward reports. Police are going through towns and villages across this entire area. They're on foot and they're going door to door looking for any potential hiding places.
The attack on satirical newspaper Charlie Hebdo has thrown France into shock, reports CBS News' Elizabeth Palmer. A national day of mourning has been under way with the bells pealing from Notre Dame, the lights on the Eiffel Tower dimmed and a moment of silence held for the 12 murdered people.
The French are defiant against the terror attack, Palmer reports. They hold a symbol to one another, the pen. The pen is mightier than the sword.
Charlie Hebdo's surviving staff has moved to offices at Liberation, a major newspaper, Palmer reports. Unintimidated, they're determined to go ahead and publish next week.
Late Wednesday it was learned that two brothers at the heart of the manhunt had been on the U.S. no-fly list, reports CBS News' Bob Orr.
U.S. sources say French investigators have evidence that Said traveled to Yemen in 2011 and linked up with the terrorist affiliate al Qaeda in the Arabian Peninsula. CBS News has been told Said "spent several months" in Yemen training with the group known as AQAP, Orr reports.
Investigators believe Said returned home with the intention of using his training to carry out an attack on a target in France. But, reports Orr, law enforcement officials are struggling to explain the three year gap between Said's homecoming and the mass shooting at Charlie Hebdo.
As the investigation continues, more is being learned about the duo.
The younger brother, Cherif, was a ladies' man who belted out rap lyrics before the words of a radical preacher persuaded him to book a flight to Syria to wage holy war.
Less is known about his elder sibling Said - except that his ID card was found in the getaway car used by the gunmen in the newspaper-office massacre in Paris.
Cherif Kouachi, 32, the younger brother, was no stranger to French counterterrorism authorities. He is a former pizza deliveryman who appeared in a 2005 French TV documentary on Islamic extremism and was sentenced to 18 months in prison in 2008 for trying to join up with fighters battling in Iraq.
It was the teachings of a firebrand Muslim preacher that put him on the path to jihad in his rough-and-tumble neighborhood of northeastern Paris, Kouachi was quoted as saying in the documentary.
The cleric "told me that (holy) texts prove the benefits of suicide attacks," Kouachi was quoted as saying. "It's written in the texts that it's good to die as a martyr."
Associated Press reporters who covered the 2008 trial, which exposed a recruiting pipeline for Muslim holy war in the multi-ethnic and working-class 19th arrondissement of Paris, recalled a skinny young defendant who appeared very nervous in court.
Cherif Kouachi's lawyer said at the time that his client had fallen in with the wrong crowd.
During the trial, Kouachi was said to have undergone only minimal training for combat - going jogging in a Paris park to shape up and learning how a Kalashnikov automatic rifle works by studying a sketch.
He was described at the time as a reluctant holy warrior, relieved to have been stopped by French counterespionage officials from taking a Syria-bound flight that was ultimately supposed to lead him to the battlefields of Iraq.
French Interior Minister Bernard Cazeneuve, however, said Thursday that Kouachi had been described by fellow would-be jihadis at the time as "violently anti-Semitic."
Imprisonment changed him, his former attorney Vincent Ollivier told Le Parisien newspaper in a story published Thursday.
Kouachi became closed off and unresponsive and started growing a beard, the lawyer said, adding that he wondered whether the stint behind bars transformed his client into a ticking time bomb.
There was a time, though, when he had very different interests.
Footage in the documentary, part of a prestigious French public television series titled "Evidence for the Prosecution," shows him in 2004, when, according to the narrator, the lanky young man in a black T-shirt with extremely close-cropped hair and a chunky wristwatch was keener on spending time with pretty girls than on going to the mosque. He appears relaxed and smiling as he pals around with friends.
At one point, with his baseball cap worn backward, Kouachi belts out some rap music and breaks into a joyful dance.
After he was released from prison, he worked in a supermarket's fish section in the Paris suburbs for six months beginning in 2009. Supervisors said he gave no cause for concern.
In 2010, police detained him again in a probe of an alleged plot to free an Islamic militant sentenced to life in prison for bombing a Paris train line in 1995. Kouachi was ultimately released with no charges ever brought.
Much less has become public about the older brother, Said, but Cazeneuve said the jobless resident of the city of Reims was also known to authorities, despite never having been prosecuted, because he was "on the periphery" of the illegal actions his younger sibling was involved in.
If French authorities are now hunting for the right suspects, it may be because of Said, Cazeneuve hinted.
In the stolen Citroen abandoned Wednesday by the gunmen, police found a French identity card in the older Kouachi's name, the minister said.
Moreover, after the attackers dumped the first car, they grabbed another, and Cazeneuve said the elder Kouachi had been identified as "the aggressor" by witnesses shown his photo.
A third suspect identified by French authorities in the attack turned himself in Wednesday night. Mourad Hamyd, 18, surrendered at a police station after learning his name had been linked to the case in the news, said Agnes Thibault-Lecuivre, spokeswoman for the Paris prosecutor.
She did not specify his relationship to the Kouachis.