– Police SWAT teams backed by helicopters tracked two heavily armed brothers with Al-Qaida sympathies suspected in France's worst terrorist attack in generations, homing in Thursday on a region north of Paris as the nation mourned the dozen slain.

Authorities fear a second strike by the suspects, who U.S. counterterrorism officials said were on the U.S. no-fly list, and distributed their portraits with the notice "armed and dangerous." More than 88,000 security forces were deployed on the streets of France.

Officials also extended France's maximum terror alert from Paris to the northern Picardie region, focusing on several towns that might be possible safe havens for the two — Cherif Kouachi, 32, and Said Kouachi, 34.

U.S. officials said that Said Kouachi is believed to have traveled to Yemen in 2011 in an effort to link up with Al-Qaida'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 his 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 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 the two are France's most-wanted men and are 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 bullet-resistant 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 Islamic extremists and the perpetrators of Wednesday's bloody assault in a Paris newspaper office that left 12 dead and 11 wounded.

Late Thursday, authorities were at least partly 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.

In all, 90 people, many of them witnesses to the grisly assault on the satirical weekly Charlie Hebdo, were questioned for information on the attackers, Interior Minister Bernard Cazeneuve said.

A third suspect, 18-year-old Mourad Hamyd, surrendered at a police station Wednesday evening after hearing his name linked to the attacks. His relationship to the Kouachi brothers was unclear.

The worst spasm of terror violence in more than a half-century stunned France. The lights of the Eiffel Tower went out Thursday night in a tribute to the dead from the elegant iron lady that symbolizes France to the world. At noon, the Paris Metro came to a standstill and a crowd fell silent near the Notre Dame Cathedral.

French President Francois Hollande — joined by residents, tourists and Muslim leaders — called for tolerance. "France has been struck directly in the heart of its capital, in a place where the spirit of liberty — and thus of resistance — breathed freely," he said.

In Washington, President Obama paid his respects at the French Embassy, declaring "terror is no match for freedom," and filling nearly a page in a condolence book.

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 attack. But it underscored one of the main concerns among France's shaken leaders: that the violence may not be over.

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

"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 Paris. "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."

The Associated Press contributed to this report.