PARIS -- French Prime Minister Manuel Valls says there were "several arrests" overnight in the hunt for two suspects in the deadly shooting at a French satirical newspaper. Another suspect turned himself in, authorities said.
In an interview with RTL radio Thursday, Valls said preventing another attack "is our main concern," as he explained why authorities released photos of the two men still at large, along with a plea for witnesses to come forward.
Valls says the two men where known and followed by French secret services.
The TV channel France 24 said 4 people were being held, at least 2 of whom are relatives of the suspects.
Later, there were reports seven people were in custody in all, including the one who surrendered.
French Interior Minister Pierre Henry Brandet said Thursday the manhunt could be long, perhaps lasting several days.
As France began a day of mourning Thursday, police were hunting for the two heavily armed men in the military-style, methodical killing of 12 people at the office of a satirical newspaper that caricatured the Prophet Muhammad. Police said the two had possible links to al Qaeda.
French brothers Said and Cherif Kouachi, in their early 30s, should be considered armed and dangerous, according to a police bulletin released early Thursday.
Mourad Hamyd, 18, surrendered at a police station in Charleville-Mezieres, a small town in France's eastern Champagne region, said Paris prosecutor's spokeswoman Agnes Thibault-Lecuivre. She did not offer details on Hamyd's relationship to the men, but said he turned himself in because he heard his name on the news in connection with the attack.
By then, heavily armed police had already moved into the nearby city of Reims, searching for the suspects without success. Video from BFM-TV showed police dressed in white apparently taking samples inside an apartment. It was not immediately clear who lived there.
One police official, speaking on condition of anonymity because the investigation was ongoing, told the Associated Press the brothers, French citizens in their early 30s, were linked to a Yemeni terrorist network. A witness of Wednesday's shootings at the offices of Charlie Hebdo said one of the attackers told onlookers, "You can tell the media that it's al Qaeda in Yemen."
Cherif Kouachi was sentenced to 18 months in prison after being convicted of terrorism charges in 2008 for helping funnel fighters to Iraq's insurgency. He said he was outraged at the torture of Iraqi inmates at the U.S. prison at Abu Ghraib near Baghdad.
CBS News' Elaine Cobbe reports that, according to witnesses, two armed and masked men walked into the headquarters of the Charlie Hebdo magazine and opened fire in the entrance hallway, killing people as they saw them. The gunmen reportedly sought out members of the newspaper's staff by name during the rampage through the 2nd floor office, which lasted between five and 10 minutes, according to witnesses.
Shouting "Allahu akbar!" as they fired, the men also spoke fluent, unaccented French in the military-style noon-time attack near Paris' Bastille monument. The publication's depictions of Islam have drawn condemnation and threats before -- it was firebombed in 2011 -- although it also satirized other religions and political figures.
It was France's deadliest terrorist attack in half a century. President Francois Hollande called the assault "an act of exceptional barbarism."
France raised its terror alert system to the maximum -- Attack Alert -- and bolstered security with more than 800 extra soldiers to guard media offices, places of worship, transport and other sensitive areas.
There was no immediate claim of responsibility for the shootings, although supporters of the Islamic State of Iraq and Syria (ISIS) praised the attack. Fears had been running high in France and elsewhere in Europe that jihadis returning from conflicts in Syria and Iraq would stage attacks at home.
Eight journalists, two police officers, a maintenance worker and a visitor were killed, said prosecutor Francois Molins. He said 11 people were wounded -- four of them seriously.
Artist Corinne Rey told the French newspaper L'Humanite that she punched in the security code to the Charlie Hebdo offices after she and her young daughter were "brutally threatened" by the gunmen.
Among the dead: the paper's editor, Stephane Charbonnier.
The staff was in an editorial meeting and the gunmen headed straight for Charbonnier -- widely known by his pen name Charb -- killing him and his police bodyguard first, said Christophe Crepin, a police union spokesman.
Rey said the assault "lasted five minutes. I hid under a desk."
Two gunmen strolled out to a black car waiting below, one of them calmly shooting a wounded police officer in the head as he writhed on the ground, according to video and a man who watched in fear from his home across the street.
The witness, who refused to allow his name to be used because he feared for his safety, said the attackers were so methodical he first thought they were members of France's elite anti-terrorism forces. Then they fired on the officer.
"They knew exactly what they had to do and exactly where to shoot. While one kept watch and checked that the traffic was good for them, the other one delivered the final coup de grace," he said.
"Hey! We avenged the Prophet Muhammad! We killed Charlie Hebdo," one of the men shouted in French, according to video shot from a nearby building.
After fleeing, the attackers collided with another vehicle, then carjacked another car before disappearing in broad daylight, Molins said.
The other dead were identified as cartoonists Georges Wolinski and Berbard Verlhac, better known as Tignous, and Jean Cabut, known as "Cabu." Also killed was Bernard Maris, an economist who was a contributor to the newspaper and was heard regularly on French radio.
Le Bechec, the witness who encountered the gunmen in another part of Paris, described on his Facebook page seeing two men "get out of a bullet-ridden car with a rocket-launcher in hand, eject an old guy from his car and calmly say hi to the public, saying 'you can tell the media that it's al-Qaeda in Yemen.'"
Charlie Hebdo has been repeatedly threatened for its caricatures of the Prophet Muhammad and other sketches. One cartoon, released in this week's issue and titled "Still No Attacks in France," had a caricature of a jihadi fighter saying "Just wait -- we have until the end of January to present our New Year's wishes." Charb was the artist.
Schools closed across Paris, although thousands of people later jammed Republique Square near the site of the shooting to honor the victims, waving pens and papers reading "Je suis Charlie" --"I am Charlie." Similar rallies were held in London's Trafalgar Square as well as Madrid, Barcelona, Berlin and Brussels.
"This is the darkest day of the history of the French press," said Christophe DeLoire of Reporters Without Borders.
Both al-Qaeda and ISIS have repeatedly threatened to attack France, which is conducting airstrikes against extremists in Iraq and fighting Islamic militants in Africa.
CBS News correspondent Bob Orr reports that U.S. intelligence is working with the French authorities in trying to determine who's behind the attack.
"While it is true that Charlie Hebdo magazine has been the subject of violent extremist threats over the past several years, none has been recent nor can they be immediately linked to this attack," a senior U.S. intelligence official told CBS News. "We are working with our intelligence community and foreign partners in identifying the perpetrators of the attack as well as monitoring for threat reporting that might warn of a subsequent attack."