Police counterterrorism teams backed by helicopters tracked two heavily armed brothers suspected in the newsroom massacre at a satirical French weekly that spoofed religious and political leaders, 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 both 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.

They 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 suspects — Cherif Kouachi, 32, and Said Kouachi, 34.

Both men were on the U.S. no-fly list, a senior U.S. counterterrorism official said Thursday. European and U.S. intelligence officials close to the investigation told Reuters Thursday that the elder Kouachi had travelled to Yemen in 2011 to train with Al-Qaeda in the Arab Peninsula, one of the group's most active affiliates. 

A French security official said American authorities had shared intelligence indicating that Said Kouachi had travelled to Yemen several years ago for training and French authorities were seeking to verify the accuracy of the intelligence. The official spoke on condition of anonymity because of the sensitivity of the matter.

Witnesses said the attackers claimed allegiance to al-Qaeda in Yemen during the bloody attack Wednesday.

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

9 people detained, questioned

The worst spasm of terror violence in more than a half-century stunned the nation. 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.

The attack struck a chord beyond French borders and similar tributes were held around Europe and elsewhere.

"Terror is no match for freedom and ideals we stand for," U.S. President Barack Obama wrote in a condolence book during a visit Thursday to the French Embassy. He concluded his lengthy entry with "Vive la France!"

French President Francois Hollande — joined by residents, tourists and Muslim leaders — called for tolerance after the country's worst terrorist attack in decades.

"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," Hollande said.

France Newspaper Attack

Cherif Kouachi, left, served time in prison after attempting to join militant fighters in Iraq. Far less is known about his older brother Said, right, though both were known to French counterterrorism police. (Prefecture de Police de Paris/Associated Press)

Nine people, members of the brothers' entourage, have been detained for questioning in several regions. 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 in a statement.

The minister confirmed reports the men were identified by an ID left in an abandoned getaway car, a slip that contrasted with the seeming professionalism of the attack.

'He was ready to die for his ideas'

The Kouachi brothers — the Paris-born offspring of Algerian parents — were well known to French counterterrorism authorities. Cherif Kouachi, a former pizza deliveryman, had appeared in a 2005 French TV documentary on Islamic extremism and was sentenced to 18 months in prison in 2008 for trying to join up with fighters battling in Iraq.

Eight journalists, two police officers, a maintenance worker and a visitor were killed in the attack and 11 people were wounded, four of them critically. The publication had long drawn threats for its depictions of Islam, although it also satirized other religions and political figures.

Charlie Hebdo Paris shooting

French anti-terrorism police converged on an area northeast of Paris after two brothers suspected of being behind the attack were spotted at a gas station in Villers-Cotterets in the region. (Christian Hartmann/Reuters)

Charlie Hebdo had caricatured the Prophet Muhammad, and a caricature of ISIS's leader was the last tweet sent out by the irreverent newspaper, minutes before the attack. Its feed has since gone silent.

Charlie Hebdo planned a special edition next week, housed in the offices of another paper.

"The paper will continue because they haven't won," Patrick Pelloux, a Charlie Hebdo columnist said tearfully to iTele TV.

Editor Stéphane Charbonnier, known as Charb, who was among those slain, "symbolized secularism ... the combat against fundamentalism," his companion, Jeannette Bougrab, said on BFM-TV.

"He was ready to die for his ideas," she said.

Witnesses to the massacre have said the attackers claimed allegiance to al-Qaeda in Yemen. On videos they were heard saying they were avenging the prophet, one witness, Cedric Le Bechec, wrote on Facebook that the attackers yelled, "Tell the media that it's al-Qaeda in Yemen," as they were fleeing.

Jarring France further, two mosques in France were firebombed and a police officer killed in Montrouge, on the southern edge of Paris. Cazeneuve told reporters that "at this stage" there was no known link between the killing and Wednesday's attack on Charlie Hebdo.

Suspects believed spotted at gas station

Police searched apartment in Reims, in the Champagne region, where the interior minister said Said Kouachi lived, with technicians gathering samples.

The hunt moved further north after a report that two men resembling the suspects robbed a gas station in Villers-Cotterets early Thursday. The focus then enlarged to Crepy-en-Valois, where heavily armed security forces with air cover and a giant black rapid intervention truck moved through rural streets and among old stone buildings.

One French police official, speaking on condition of anonymity because the investigation was ongoing, said the suspects were linked to a Yemeni terrorist network.

Charlie Hebdo Paris shooting

The manhunt for the brothers has widened across France. (Michel Spingler/Associated Press)

The governor of a southern province in Yemen told The Associated Press on Thursday that four French citizens had been deported from Yemen in the last four months. Gov. Ahmed Abdullah al-Majidi said he didn't have their names and there was no confirmed link between those deportations and the Charlie Hebdo attack.

Authorities around Europe have warned of the threat posed by the return of Western jihadists trained in warfare. France counts at least 1,200 citizens in the war zone, headed there, returned or dead, and officials have said France is a preferred target. Both the Islamic State group and al-Qaeda have issued threats to France — home to Western Europe's largest Muslim population.

France is taking part in airstrikes in Iraq in a bid to defeat ISIS, intervened to rout out al-Qaeda extremists from northern Mali, a former French colony.

The French suspect in a deadly 2014 attack on a Jewish museum in Belgium had returned from fighting with extremists in Syria; and the man who rampaged in southern France in 2012, killing three soldiers and four people at a Jewish school in Toulouse, received paramilitary training in Pakistan.

A journalist who took refuge on the building roof during the attack said that when he went into Charlie Hebdo's offices he was confronted with life and death.

"On the one side the living and the other the dead," Edouard Periin told iTele. The dead and dying, he said, were on the left.

With files from Reuters