Paris, AP—Police hunted Thursday for two heavily armed men, one with possible links to Al-Qaeda, in the methodical killing of 12 people at a satirical newspaper that caricatured the Prophet Muhammad. The prime minister said the possibility of a new attack “is our main concern” and announced several overnight arrests.

Tensions in Paris were high as France began a day of national mourning. France’s top security official abandoned a top-level meeting after just 10 minutes to rush to a shooting on the city’s southern edge in which at least one police officer wounded.

It was not immediately clear if Thursday’s shooting was linked to the attack on Charlie Hebdo, which left two police among the dead.

France’s prime minister, Manuel Valls, said the two suspects in the Charlie Hebdo shootings were known to intelligence services and the fear that they could carry out another attack “is our main concern.” Valls told RTL radio there had been several detentions overnight.

One of the suspects, Cherif Kouachi, had a history of funneling jihadi fighters to Iraq and a terrorism conviction from 2008. He and his brother, Said, should be considered “armed and dangerous,” French police said in a bulletin early Thursday, appealing for witnesses after a fruitless search in the city of Reims, in French Champagne country.

A third man, Mourad Hamyd, 18, surrendered at a police station in a small town in the eastern region after learning his name was linked to the attacks in the news and social media, said Paris prosecutor’s spokeswoman Agnes Thibault-Lecuivre. She did not specify his relationship to the Kouachi brothers.

France raised its terror alert system to the maximum and bolstered security with more than 800 extra soldiers to guard media offices, places of worship, transport and other sensitive areas. A nationwide minute of silence was planned for noon.

Fears had been running high in Europe that jihadis trained in warfare abroad would stage attacks at home. The French suspect in a deadly attack on a Jewish museum in Belgium had returned from fighting with extremists in Syria; and the man who rampaged in the south of France in 2012, killing three soldiers and four people at a Jewish school in Toulouse, received paramilitary training in Pakistan.

One witness to Wednesday’s attack said the gunmen were so methodical he at first mistook them for an elite anti-terrorism squad. Then they fired on a police officer.

The masked, black-clad men with assault rifles stormed the offices near Paris’ Bastille monument in the Wednesday noontime attack on the publication, which had long drawn condemnation and threats—it was firebombed in 2011—for its depictions of Islam, although it also satirized other religions and political figures.

The staff was in an editorial meeting and the gunmen headed straight for the paper’s editor, Stephane Charbonnier, widely known by his pen name “Charb,” killing him and his police bodyguard first, said Christophe Crepin, a police union spokesman.

Shouting “Allahu akbar!” as they fired, the men spoke in fluent, unaccented French as they called out the names of specific employees.

Eight journalists, two police officers, a maintenance worker and a visitor were killed, said prosecutor François Molins. He said 11 people were wounded, four of them seriously.

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.

“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 grâce,” said the witness, who refused to allow his name to be used because he feared for his safety.

“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.

One police official, speaking on condition of anonymity because the investigation was ongoing, said the suspects were linked to a Yemeni terrorist network. Cédric Le Bechec, a witness who encountered the escaping gunmen, quoted the attackers as saying: “You can tell the media that it’s Al-Qaeda in Yemen.”

After fleeing, the attackers collided with another vehicle, then hijacked 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.

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.” Charbonnier was the artist.

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.'”

In a somber address to the nation Wednesday night, French President François Hollande pledged to hunt down the killers, and pleaded with his compatriots to come together in a time of insecurity and suspicion.

“Let us unite, and we will win,” he said. “Vive la France!”

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 the Islamic State of Iraq and Syria (ISIS) group have repeatedly threatened to attack France, which is conducting airstrikes against extremists in Iraq and fighting Islamic militants in Africa. Charbonnier was specifically threatened in a 2013 edition of the Al-Qaeda magazine Inspire, which also included an article titled “France the Imbecile Invader.”

Cherif Kouachi, now 32, 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 US prison at Abu Ghraib near Baghdad and “really believed in the idea” of fighting the US-led coalition in Iraq.

A tweet from an Al-Qaeda representative who communicated Wednesday with The Associated Press said the group was not claiming responsibility for the attack, but called it “inspiring.”