PARIS (CBSNewYork/AP) — Scattered gunfire and explosions shook France on Thursday as its frightened yet defiant citizens held a day of mourning for 12 people slain at a Paris newspaper.

France’s prime minister said the possibility of a new attack “is our main concern” and announced several overnight arrests. Tensions ran high in Paris, and police patrolled places of worship and schools. Britain increased its security checks at ports and borders.

MORE: Full Coverage From CBS News | PHOTOS: Paris Terror Attack

France’s prime minister, Manuel Valls, said the two suspects still at large in the Charlie Hebdo slayings — Cherif Kouachi, 32, and his 34-year-old brother, Said Kouachi — were known to intelligence services.

Cherif Kouachi was convicted of terrorism in 2008 for being involved in a network sending radical fighters to Iraq. He and his brother, Said, should be considered “armed and dangerous,” French police said in a bulletin Thursday.

As CBS2’s Dick Brennan reported, more than 80,000 security forces flooded towns in northern France, and officials said a police helicopter might have spotted the suspects in the woods.

Officers asked a local village mayor if there were any caves nearby, then went to investigate.

Two men resembling the suspects also robbed a gas station in northeast France on Thursday morning, and police swarmed the site while helicopters hovered above. But officials said later the newspaper attackers were not there.

Two explosions hit near mosques early Thursday, raising fears the deadly attack at Charlie Hebdo would ignite a backlash against France’s large and diverse Muslim community. No one was injured in the attacks, one in Le Mans, southwest of Paris, and another near Lyon, southeast of the capital.

But France’s top security official abandoned a top-level meeting after just 10 minutes to rush to a shooting on the city’s southern edge that killed a policewoman. The shooter remained at large and it was not immediately clear if her death was linked to Wednesday’s deadly attack.

A third man, Mourad Hamyd, 18, surrendered at an eastern police station after learning his name was linked to the attacks in the news, said the Paris prosecutor spokeswoman Agnes Thibault-Lecuivre. She did not specify his relationship to the Kouachi brothers.

France raised its terror alert system to the maximum and bolstered security with more than 800 extra soldiers guarding media offices, places of worship, transport and other sensitive areas.

Authorities hope now that a nation on alert will give police advantages in the search.

“Every French citizen knows their face right now, so that’s going to be No. 1 obviously,” David Katz, a former agent with the U.S. Drug Enforcement Agency, told CNN. “If they commit another crime, that’s another avenue of capturing them.”

The masked, black-clad men with assault rifles launched the attack around noon Wednesday. The publication had long drawn condemnation and threats for its depictions of Islam and the Prophet Muhammad, although it also satirized other religions and political figures.

The gunmen headed straight for the paper’s editor, Stephane Charbonnier, widely known by his pen name Charb, killing him and his police bodyguard first, said Christophe Crepin, a police union spokesman. Shouting “Allahu akbar!” as they fired, the men then called out the names of specific employees.

Eight journalists, two police officers, a maintenance worker and a visitor were killed, and 11 people were wounded, four of them critically.

Two gunmen strolled out to a black car waiting below, one of them calmly shooting a wounded police officer in the head, according to video and a nearby witness.

A witness to Wednesday’s attack said one of the men claimed to be acting on behalf of “al Qaeda in Yemen” during the shooting.

French investigators also have evidence Said Koachi travelled to Yemen in 2011, and linked up and trained with the terrorist affiliate Al-Qaeda in the Arabian Peninsula (AQAP.)

Investigators said Said returned home intending to use his training to carry out an attack. But they cannot explain the three year gap between Said’s homecoming, and the massacre of 12 people at Charlie Hebdo.

Al Qaeda in the Arabian Peninsula (AQAP), the branch of the terror network considered to represent the most immediate threat to the U.S. and other Western nations, is based in Yemen, CBS News reported.

“Al Qaeda in Yemen is a very dangerous group,” former CIA Deputy Director Mike Morell told “CBS This Morning” on Thursday. “I think it’s actually the most dangerous terrorist group out there. They said just a few days ago that they want to conduct attacks against the West.”

Both al Qaeda and the Islamic State group have repeatedly threatened to attack France, which is conducting airstrikes against extremists in Iraq and fighting Islamic militants in Africa.

Charbonnier was specifically threatened in a 2013 edition of the al Qaeda magazine Inspire. A caricature of Islamic State’s leader was the last tweet sent out by the newspaper, minutes before the attack. Its feed has since gone silent.

Meanwhile, it is not known that the Kouachi brothers were on U.S. no-fly and terror watch lists. Authorities in the U.S. are checking databases and evaluating targets here, fearing terror groups can influence potential attackers, CBS2’s Brennan reported.

“The new wrinkle is that groups like ISIS that, while they have a less sophisticated external planning operations node, have a much more sophisticated planning arm that is extolling people to do this without having contact with the mothership,” said John Miller, the NYPD’s deputy commissioner of intelligence and counterterrorism.

World Rallies In Support Of Victims

On Thursday evening, President Barack Obama went to the French Embassy in Washington, D.C. to sign the book of condolence, while Paris, the City of Lights, became a city in mourning. The Eiffel Tower was dimmed and the bells of the Notre Dame Cathedral pealed in the rain, as many mourners held a symbol of defiance, saying the pen is mightier than any gun.

Protesters around the world and in the U.S. have also demonstrated against the deadly terror attack.

In New York City, several hundred people gathered in Union Square to show their support for the victims.

PHOTOS: Vigil For Victims Of Paris Attack

The mourners Wednesday night chanted “We are not afraid” and held signs in English and French saying “We are Charlie.” They also lit candles at a makeshift memorial to the victims.

“I think it’s amazing how quickly people rose up and are pushing back on this,” said Brian Borkowsky, of Ditmas Park, Brooklyn.

Other demonstrations took place in San Francisco; Seattle; London; Berlin; Madrid; Brussels; Nice, France; and elsewhere.

Online, the declaration “Je Suis Charlie” or “I Am Charlie” replaced profile pictures on Facebook while Twitter users showed themselves with the slogan on signs with words of support for the 12 victims.

The “Je Suis Charlie” slogan grew into a trending hashtag on Twitter and spread to Instagram, along with an image of a machine gun with the words “Ceci n’est pas une religion,” or “This is not a religion.”

Morell called the shooting “an attack on our very way of life.”

“This was an attack on freedom of speech, this was an attack on the way the West lives and that’s different from what we’ve seen before. The response you see on the street and around the world is a reflection of that,” he said. “As 9/11 in New York, as the subway bombings in London and now this in Paris, this tells everybody that this can happen anywhere.”

Charlie Hebdo’s surviving staff has moved to a temporary office. Unintimidated, they say they will publish next week. And with all the support, they will up the normal print run, from 60 thousand, to a million.

NYPD: Security Stepped Up After Paris Attack

Earlier in the day Wednesday, the NYPD said it was putting out a cadre of heavily armed officers in highly visible locations, including the French Consulate.

Police Commissioner Bill Bratton said there is no credible threat against New York, but said an NYPD detective assigned to Paris has been relaying information on the attack, allowing police here at home to monitor the incident.

“We will always err on the side of caution and fortunately we have the resources to do quite a lot, even without a direct threat,” Bratton said.

Mayor Bill de Blasio said he called Paris Mayor Anne Hidalgo on Wednesday morning to offer his condolences and to express New York City’s “unwavering support for the people of Paris.”

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