The police organized an enormous manhunt across the Paris region on Wednesday for three suspects they said were involved in a brazen and methodical midday slaughter at a satirical newspaper that had lampooned Islam.
The terrorist attack by masked gunmen on the newspaper, Charlie Hebdo, left 12 people dead — including the top editor, prominent cartoonists and police officers — and was among the deadliest in postwar France. The killers escaped, traumatizing the city and sending shock waves through Europe and beyond.
Officials said late Wednesday that two of the suspects were brothers. They were identified as Said and Chérif Kouachi, 34 and 32. The third suspect is Hamyd Mourad, 18. News reports said the brothers, known to intelligence services, had been born in Paris, raising the prospect that homegrown Muslim extremists were responsible.
Early Thursday, a spokeswoman for the Paris prosecutor said that Mr. Mourad had walked into a police station in Charleville-Mézières, about 145 miles northeast of Paris, and surrendered.
“He introduced himself and was put in custody,” said the spokeswoman, Agnès Thibault-Lecuivre.
The assault threatened to deepen the distrust of France’s large Muslim population, coming at a time when Islamic radicalism has become a central concern of security officials throughout Europe. In the space of a few minutes, the assault also crystallized the culture clash between religious extremism and the West’s devotion to free expression. Spontaneous rallies expressing support for Charlie Hebdo sprung up later in the day in Paris, throughout Europe and in Union Square in New York.
Officials and witnesses said at least two gunmen had carried out the attack with assault weapons and military-style precision. President François Hollande of France called it a display of extraordinary “barbarism” that was “without a doubt” an act of terrorism. He declared Thursday a national day of mourning.
He also raised the nationwide terror alert to its highest level, saying several terrorist attacks had been thwarted in recent weeks as security officials here and elsewhere in Europe have grown increasingly wary of the return of young citizens from fighting in Syria and Iraq.
The French authorities put some schools on lockdown for the day; added security at houses of worship, news media offices and transportation centers; and conducted random searches on the Paris Métro.
The Paris prosecutor, François Molins, said that according to witnesses, the attackers had screamed “Allahu akbar!” or “God is great!” during the attack, which the police characterized as a “slaughter.”
Corinne Rey, a cartoonist known as Coco, who was at the newspaper office during the attack, told Le Monde that the attackers had spoken fluent French and said that they were part of Al Qaeda.
An amateur video of the assailants’ subsequent gunfight with the police showed the men shouting: “We have avenged the Prophet Muhammad. We have killed Charlie Hebdo!” The video, the source of which could not be verified, also showed the gunmen killing a police officer as he lay wounded on a nearby street.
“We strongly condemn these kinds of acts, and we expect the authorities to take the most appropriate measures,” he said, adding, “This is a deafening declaration of war.”
The attack comes as thousands of Europeans have joined jihadist groups in Iraq and Syria, further fueling concerns about Islamic radicalism and terrorism being imported. Those worries have been especially acute in France, where fears have grown that militants are bent on retaliation for the government’s support for the United States-led air campaign against jihadists with the Islamic State group in Syria and Iraq.
Last month, Prime Minister Manuel Valls ordered hundreds of additional military personnel onto the streets after a series of attacks across France raised alarms over Islamic terror.
In Dijon and Nantes, a total of 23 people were injured when men drove vehicles into crowds, with one of the drivers shouting an Islamic rallying cry. The authorities depicted both drivers as mentally unstable. The attacks came after violence attributed to “lone-wolf” attackers in London in 2013, in Canada in October and last month in Sydney, Australia.
In September, fighters in Algeria aligned with the Islamic State beheaded Hervé Gourdel, a 55-year-old mountaineering guide from Nice, and released a video documenting the murder. Mr. Gourdel had been kidnapped after the Islamic State called on its supporters to wage war against Europeans.
President Obama issued a statement condemning the killings. “Time and again, the French people have stood up for the universal values that generations of our people have defended,” he said.
“France, and the great city of Paris where this outrageous attack took place, offer the world a timeless example that will endure well beyond the hateful vision of these killers. We are in touch with French officials, and I have directed my administration to provide any assistance needed to help bring these terrorists to justice.”