"Hamyd Mourad handed himself in to police... on Wednesday at 11:00 pm (2200 GMT) after seeing his name circulating on social media," the source told AFP. "He has been arrested and taken into custody," another source confirmed.
Meanwhile, French police on Thursday published photos of the two brothers wanted as suspects and launched an appeal to the public for information. (See the pictures here)
Paris police said arrest warrants had been issued for Cherif Kouachi, 32 and his 34-year-old brother Said who were "likely armed and dangerous". (Paris Attackers Shouted 'We Have Avenged the Prophet': Police)
Several thousand police were deployed to find the gunmen, one of whom was convicted in 2008 for involvement in a network sending fighters to Iraq, the source said. (France Puts Paris on Highest Alert Status After Shooting)
In a sombre televised address, President Francois Hollande declared a day of national mourning on Thursday -- only the fifth in the past 50 years -- after the worst attack on French soil in decades.
The Paris attack on Charlie Hebdo, a magazine that has long been in confrontation with Islamists, triggered impromptu demonstrations of solidarity in cities across the world, including Moscow, Washington, London and Tokyo.
More than 100,000 gathered across France, many protesters carrying banners reading: "I am Charlie" while the hashtag #JeSuisCharlie was trending worldwide including in Arabic.
Hollande called the massacre "an act of exceptional barbarity" and "undoubtedly a terrorist attack."
"Nothing can divide us, nothing should separate us. Freedom will always be stronger than barbarity," said the president, who ordered flags flown at half-mast for the next three days.
Charlie Hebdo gained notoriety in February 2006 when it reprinted cartoons of the Prophet Mohammed that had originally appeared in Danish daily Jyllands-Posten, causing fury across the Muslim world.
The killers on Wednesday shouted "we have avenged the prophet, we have killed Charlie Hebdo", according to prosecutors.
- 'Like a movie' -
The drama began in broad daylight in a quiet Paris street when the gunmen stormed into the weekly's offices as the journalists were in an editorial meeting, first shooting a person in the reception area.
They picked off eight journalists, including some of France's best-known cartoonists, a policeman and a visitor. One person survived by hiding under a table.
Chilling amateur video footage filmed after the carnage then showed them outside, running toward a wounded policeman as he lay on the pavement. (Video of Gunmen Shooting Man on Pavement)
One attacker says: "You wanted to kill me?" before coldly shooting the officer dead.
Prosecutors said 11 people were also injured in the attack, with four in critical condition.
One man who witnessed the attack described a scene like "in a movie".
"I saw them leaving and shooting. They were wearing masks. These guys were serious," said the man who declined to give his name. "At first I thought it was special forces chasing drug traffickers or something." (Caught on Camera the Gunmen Escaping After Charlie Hebdo Attack)
Authorities said the gunmen's professional and calculating manner suggested they had been well-trained.
An employee at a nearby daycare centre said he was walking with children when panic erupted.
"People leaned out of the window and yelled at me to get off the pavement," said 56-year-old Jean-Paul Chevalier.
The attack came at a time of heightened fears in France and other European capitals over fallout from the wars in Iraq and Syria, where hundreds of European citizens have gone to fight alongside the radical Islamic State group.
- 'Cowardly, evil' assault -
US President Barack Obama led the global condemnation of what he called the "cowardly, evil" assault.
Pope Francis condemned the "horrible attack" saying such violence, "whatever the motivation, is abominable, it is never justified."
British Prime Minister David Cameron called it "sickening", German Chancellor Angela Merkel said the attack was "despicable" and Russian President Vladimir Putin as well as the Arab League condemned the violence.
Saudi Arabia, the home of Islam's holiest sites, condemned "this cowardly terrorist attack which is incompatible with Islam".
The imam of Drancy mosque in the northern suburbs of Paris, Hassen Chalghoumi, visited the scene, calling the shooters "barbarians, they lost their soul, sold their soul to hell".
"They want terror, they want fear. We must not give in, we must all be strong. I hope the French will come out in solidarity and not against the Muslim minority in Europe."
- 'National unity' -
The attacks revived fears of a return to the dark days of the 1980s and 1990s when France, which is home to Europe's largest Muslim population, was hit by a wave of extremist violence.
In 1995, a bomb in a commuter train blamed on Algerian extremists exploded at the Saint Michel metro station in Paris, killing eight and wounding 119.
Al-Qaeda-inspired gunman Mohamed Merah killed seven people in and around the southern city of Toulouse in 2012. His victims included three French soldiers and four Jews -- three children and a rabbi.
Hollande called for "national unity", adding that "several terrorist attacks had been foiled in recent weeks".
Charlie Hebdo's offices were fire-bombed in November 2011 when it published a cartoon of Mohammed under the title "Sharia Hebdo". (Charlie Hebdo Attacked in Past for Satirizing Prophet Mohammed)
Despite being taken to court under anti-racism laws, the weekly continued to publish controversial cartoons of the Muslim prophet.
In September 2012, Charlie Hebdo published cartoons of a naked Mohammed as violent protests were taking place in several countries over a low-budget film, titled "Innocence of Muslims", which was made in the United States and insulted the prophet.
Editor-in-chief Stephane Charbonnier, known as Charb and who had lived under police protection after receiving death threats, was among Wednesday's fatalities. (Don't Blame Muslims for Not Laughing at Our Cartoons, Editor Had Said)
Others included Jean Cabut, known across France as Cabu, Georges Wolinski and Bernard Verlhac, better known as Tignous.
The attack took place on the day the latest edition of Charlie Hebdo was published.
It featured an eerily premonitory cartoon of an armed militant noting "Still no attacks in France. Wait! We have until the end of January to send greetings" -- a reference to France's tradition of wishing someone a Happy New Year before January 31.
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