A frustrated manhunt for two heavily armed brothers suspected in France’s worst terrorist attack in generations shifted to the cottages and country lanes of rural France as fresh details emerged that one of the brothers had tried to meet with al-Qaeda’s affiliate in Yemen.

U.S. officials said the older of the two, Said Kouachi, 34, is believed to have traveled to Yemen in 2011 in an effort to link up with al-Qaeda’s affiliate there at a time when that group was eclipsing the terror network’s core leadership in Pakistan as the principal threat to the United States.

U.S. officials said Kouachi may have received small-arms training and picked up other skills while in Yemen, but they described the years that followed that 2011 visit as a “kind of hole” in the timeline, with significant gaps in authorities’ understanding of the brothers’ activities and whereabouts.

Those blank spots have led U.S. and other officials to seek to determine whether one or both brothers traveled to Syria or another conflict zone, or whether they managed to lower their profile in France to such a degree that scrutiny of them subsided.

Now Said and his younger brother, Chérif Kouachi, 32, are France’s most wanted men, believed to be armed with Kalashnikov rifles and rocket-propelled grenade launchers and on the loose somewhere in the French countryside. In a massive show of force Thursday, armored vehicles rolled past the ancient stone fences and sugar-beet fields of Aisne, an agricultural district 44 miles north of the capital. Black-clad troopers wearing bulletproof Kevlar gear and carrying assault rifles cordoned off a large area of farmland as they went door-to-door, field-to-field, forest-block-to-forest-block.

The two men, French authorities say, are homegrown Islamist extremists and the perpetrators of Wednesday’s bloody assault in a Paris newspaper office that left 12 dead and 11 wounded. Late Thursday, however, authorities were at least partially suspending search efforts in some areas as night fell and amid confusion over whether the suspects had ditched their gray Renault Clio after apparently robbing a gas station in the northern city of Villers-Cotterets earlier in the day.

Despite the exasperating nature of the manhunt, French officials vowed to bring the men to justice and announced that they had taken nine people into custody in relation to the case. Authorities would not release their names, but French media said that those picked up in the dragnet included a sister of the men as well as her companion and the wife of Said Kouachi.

Authorities gave no details on any possible connection to the main suspects in Wednesday’s raid: the Kouachi brothers, the Paris-born sons of Algerian immigrants.

“We will show these terrorists through the firm defense of the values of the republic that we are not afraid and that we remain united,” said Bernard Cazeneuve, France’s interior minister.

Nevertheless, even as thousands poured into Paris’s Place de la Republique for a second night to honor the dead — including some of France’s best-known cartoonists at a publication that had lampooned Islam along with other targets — this nation of 66 million remained on high alert.

Many spoke of unity, with the Eiffel Tower shrouded in black Thursday evening, its lights doused in honor of the fallen. The slogan “Je suis Charlie” — I am Charlie — became ubiquitous in offices, on sidewalks and in public squares nationwide.

And in a nation that is home to Western Europe’s largest Muslim population as well as the continent’s strongest anti-immigrant and extreme far-right movements, there were also fears of rising religious and political tensions in the aftermath of the attack. On Thursday, a man was arrested in the city of Poitiers after painting the words “Death to Arabs” on the gates of a mosque. In the city of Caromb, a car belonging to a Muslim family was shot at. In two other French cities, small explosives went off near mosques.

No injuries were reported in any of the incidents, but they immediately ignited concerns about further ideological clashes, violent or otherwise.

“I’m afraid this is going to open a boulevard for the far right,” said Diane Tribout, 28, a public servant who joined a candlelight vigil in the Place de la Republique on Thursday, where crowds chanted, “Charlie isn’t dead!”

“On the streets of Paris, you might not see it as obviously, but I know that in small towns and villages all across France, this tragic event is going to be used to fuel anger and rage,” Tribout said.

Marine Le Pen, the head of the far-right National Front, which has surged in opinion polls here well before Wednesday’s attack, spoke out Thursday, calling her party the only one that had challenged the notion of “Islamic fundamentalism on our territory.”

She added her voice to those blasting the current and former governments of France for the security lapses that they say allowed the attacks to take place. Authorities knew both suspects, raising questions about why they fell so faroff the radar of the French security services.

Le Pen was additionally infuriated by the decision of those organizing a national vigil this Sunday to withhold an invitation to her party, which in opinion polls is now commanding the support of more than a quarter of the nation. Citing the omission, she insisted, “There is no longer any national unity.”

Strong and growing anti-immigrant movements across Europe appeared to be roused by Wednesday’s attack, in which hooded gunmen speaking fluent French burst into the newspaper’s weekly staff meeting and sprayed the room with gunfire, leaving behind what one witness described as “absolute carnage.” The far right was using the attack as a rallying cry.

In Germany, organizers of a growing movement of anti-immigrant marchers that drew record numbers in the city of Dresden on Monday, called the attack a vindication of their efforts. In Britain, Nigel Farage, the leader of the anti-immigrant United Kingdom Independence Party, which also has been growing in strength, called the attacks the product of a secret “fifth column” of foreigners in Europe.

“We’ve got people living in these countries holding our passports that hate us,” he told Britain’s Channel 4 News.

But many in France said that the far right would not succeed in leveraging the attack for its own purposes, saying the nation was pulling together in tragedy, not being drawn apart.

“In the last 24 hours, what I have seen is a sense of national responsibility, a sense of unity,” said Jean-Charles Brisard, a Paris-based terrorism and security expert. “We know they want to use this to tear us apart, to create division. But France will not allow that.”

Still, a day after the attack, France’s capital was a mix of mourning, anger and hair-trigger tensions — raised even further after the slaying of a policewoman in a Paris suburb Thursday morning. Authorities said there was no immediate link with Wednesday’s terrorist attack. But it underscored one of the main concerns among France’s shaken leaders: that the violence may not be over.

French Prime Minister Manuel Valls said “a main concern” was whether the brothers — or possibly others — could carry out another attack.

“There is no such thing as zero risk,” Valls told RTL radio.

Miller reported from Washington. Virgile Demoustier in Paris contributed to this report.