France’s deadliest terrorist attack in modern memory unfolded with chilling precision here Wednesday as gunmen speaking fluent French burst into a satirical newspaper’s weekly staff meeting and raked the room with bullets, leaving behind what one witness described as “absolute carnage.”

The assault claimed a dozen lives, including the provocative paper’s well-known editor and two police officers, while traumatizing a nation that had long feared such an assault but was nonetheless shocked by the ferocity and military-style professionalism with which it was carried out.

After shooting dead their final victim, the exultant killers calmly fled the scene, sparking a manhunt that extended across this capital city and deep into its suburbs. Police named three suspects, and one of them was reported Thursday to have surrendered.

Authorities also said Thursday that “several arrests” had been made since the attack.

France raised its security alarm to the highest level and mobilized teams on foot, by air and in vehicles seeking the three masked assailants, who carried out the assault shouting the Arabic call of “Allahu Akbar,” or “God is great,” amid the gunfire, according to video posted by France’s state-run broadcaster.

Editor's note: This video contains graphic content. Videos shot near the Paris office of Charlie Hebdo captured two gunmen fleeing the area. One shows the gunmen shooting a police officer. (The Washington Post)

By early Thursday, police had surrounded an apartment building in the city of Reims, about a two-hour drive from Paris, with media reporting that a swarm of heavily armed officers was preparing to raid the site. But they pulled back around 2 a.m., apparently without making any arrests.

According to police and other officials, two of the suspects are French brothers in their early 30s, Said and Cherif Kouachi. Both are from the Paris region. The third is 18-year-old Hamyd Mourad from Reims. There were conflicting reports on whether the teenager was also a French national.

Later Thursday morning, news outlets citing French judicial officials reported that Mourad had turned himself in at a police
station in Charleville-Mezieres, about 140 miles northeast of Paris near the Belgium border.

Wednesday’s mass killing added Paris to a list of European capitals, including London and Madrid, that have experienced major terrorist attacks since the Sept. 11, 2001, attacks in the United States.

The assault came at a time of heightened anxiety across Europe about the threat of radical Islamist groups as thousands of young men and women from across the continent have poured into Syria to join the fight there. Many have come home radicalized by the experience.

There was no indication Wednesday that any of the assailants had battlefield experience. But experts said the men were well prepared for their mission, and there were widespread reports that one of the alleged suspects, Cherif Kouachi, had been convicted of recruiting fighters to battle American forces in Iraq.

Wednesday’s raid was “a terrorist attack, without a doubt,” said French President François Hollande, who later declared Thursday as a national day of mourning.

“Journalists and police officers have been assassinated in cowardly fashion,” Hollande said after visiting the scene. “France is in a state of shock.”

The attack coincided with a staff meeting at the weekly Charlie Hebdo newspaper and left its well-known editor, Stéphane Charbonnier, and other prominent cartoonists among the dead.

[Read: What is Charlie Hebdo?]

Edouard Perrin, a former writer for the newspaper who was in the office across the hall at the time of the attack, said he took cover when the shooting started and was among the first to enter after the killers fled.

“When we got inside, it was an absolute carnage, in the proper sense of the word,” he said.

In addition to the dead, he said, “there were survivors. We carried out CPR on them. I touched one person lying on the ground. The body had no pulse.”

Later, at the sealed offices and on nearby streets, forensic experts looked for DNA or other possible clues to aid in the rapidly expanding hunt. Others pored over security-camera video and cellphone images posted online.

Across Paris, meanwhile, security patrols were stepped up at media outlets, transportation hubs and other key sites.

The attack is likely to raise calls for tougher crackdowns on suspected extremists in a country that has faced decades of internal tensions over its Muslim population, which at 5 million is the largest in Europe.

In recent years, France has thrust itself to center stage in the war against Islamist extremism. In 2013, French forces joined those loyal to Mali’s government to push back an onslaught by Islamist militants. France was also the first nation to join the U.S.-led effort against the Islamic State in Syria and Iraq, conducting bombing raids.

In just the past several weeks, France has been particularly on edge. Before Christmas, a man yelling “God is great” in Arabic was shot after stabbing three police officers in a suburb of Tours in central France.

Also, 23 people were injured in Nantes and Dijon after men, in two separate incidents, drove vehicles into crowds. French officials deployed between 200 and 300 more military personnel on the streets last week, in addition to 780 already on the ground.

But the mood in Paris on Wednesday was less angry and fearful than mournful and resolved.

As dusk fell, a somber crowd of thousands of Parisians converged on the Place de la Republique to show solidarity with the attack’s victims. Many bore handmade signs with the words “Je suis Charlie” — “I am Charlie” — and mourners spelled out the words in votive candles. The crowd periodically broke out in rhythmic chants of “Charlie!” — but was otherwise largely silent.

“Charlie is exactly what France needs. They make us laugh and they make us think,” said Dominique Ragu, a cartographer who came to the rally with her daughter and father. “This was an attack on freedom of expression. It was an attack on humor.”

[See: How cartoonists reacted to the Charlie Hebdo massacre.]

At the nearby offices of the leftist newspaper Liberation, the entrance was being guarded by police wielding assault rifles. Inside, staff members were mourning for lost friends but were also defiant.

“We need to be like Charlie. We need to be strong. We need to be irreverent. We need to be impactful,” said Johan Hufnagel, the paper’s deputy editor. “If we change because of these guys, it will mean they will have won.”

The attack targeted the newspaper’s most prominent figures.

One of its designers, Corinne Rey, said two hooded gunmen, speaking perfect French, forced her to type her passcode at the door. It was shortly before 11:30 a.m. Paris time — the time of the newspaper’s editorial meeting attended by key members of the staff.

“I had gone to pick up my daughter at day care,” Rey told the French newspaper L’Humanite. “Two hooded gunmen arrived at the door of the building and brutally threatened us.”

Amateur footage broadcast on France 24 showed panicked employees of Charlie Hebdo scrambling onto the roof at the offices in the densely populated 11th arrondissement of Paris. Another video clip showed black-clad gunmen firing on a police officer on the sidewalk before escaping in a black car.

The assailants, according to French media accounts, later commandeered a vehicle at Porte de Pantin on the northeastern outskirts of Paris before fleeing to the suburbs.

“We heard a ‘boom boom,’ ” said a waiter at the nearby restaurant Le Poulailler who asked to remain anonymous. He described seeing at least two gunmen firing weapons. “We went outside in the alley and saw them shooting at the cops,” he said. “At first we thought it was a movie.”

Christophe Crepin, a police union spokesman, said the dead include 10 members of the newspaper staff, among them the 47-year-old Charbonnier, who was widely known by the pen name Charb.

Other noted staff members killed included economic-affairs columnist Bernard Maris, 68, and renowned cartoonist Jean Cabut, 76, widely known as Cabu.

Two police officers also were killed, including one assigned as the editor’s bodyguard. The other, who encountered the gunmen as they fled, was shot in the head as he writhed wounded on the ground, Crepin said.

At least 20 other people were injured, including four listed in critical condition, police said.

“We killed Charlie Hebdo,” one of the assailants shouted, according to a video made from a nearby building and later broadcast on French television.

“The murderers dared proclaim Charlie Hebdo is dead,” U.S. Secretary of State John F. Kerry said in Washington. “But make no mistake, they are wrong. Today and tomorrow in Paris, in France and across the world, the freedom of expression this magazine represented is not able to be killed by this kind of act of terror.”

In Washington, President Obama denounced the “horrific” shooting and said U.S. officials were ready to provide any assistance to help “bring these terrorists to justice.”

British Prime Minister David Cameron called the attack “sickening,” and German Chancellor Angela Merkel denounced it as “vile.”

There was no immediate claim of responsibility for the Charlie Hebdo attack, but messages of praise appeared on Web sites and other online forums linked to Islamist militants, said the Washington-based Site monitoring group, which tracks extremist posts.

Charlie Hebdo’s iconoclastic style frequently pushed the envelope. The newspaper was already under regular police guard after being targeted in the past. In November 2011, its offices were firebombed a day after it published a caricature of the prophet Muhammad and ironically named him as its “editor in chief” for an upcoming issue.

The attack, however, did little to curb its appetite for Islamic satire. In 2012, the newspaper ignored calls for caution from high-ranking members of the French government and published more images of Muhammad. In one caricature, he was shown being pushed in a wheelchair by an Orthodox Jew in a reference to a hit French movie.

Images of Muhammad have sparked deadly violence and protests in the past. In 2005, a Danish newspaper published cartoons depicting the prophet, touching off months of unrest across the Islamic world.

“It’s a horrible thing that has happened today, and my fear is that this might promote self-
censorship,” said Kurt Westergaard, the Danish cartoonist who penned the most incendiary of the 2005 caricatures of the prophet Muhammad. “These were good people, people who have been critical of anyone in power.”

Only hours before the attack, Charlie Hebdo’s Twitter account carried a cartoon titled “Still No Attacks in France” showing Islamic State leader Abu Bakr al-Baghdadi giving a new year’s greeting.

“Just wait,” a fighter says in the drawing. “We have until the end of January to present our New Year’s wishes.”

Faiola reported from Berlin. Virgile Demoustier in Paris, Karla Adam in London, Souad Mekhennet in Frankfurt, Germany, and Brian Murphy, William Branigin, Katie Zezima and Carol Morello in Washington contributed to this report.