(CNN) French law enforcement officers have been told to erase their social media presence and to carry their weapons at all times because terror sleeper cells have been activated over the last 24 hours in the country, a French police source who attended a briefing Saturday told CNN terror analyst Samuel Laurent.

Amedy Coulibaly, a suspect killed Friday during a deadly kosher market hostage siege, had made several phone calls about targeting police officers in France, according to the source.

It was one of a flurry of developments Saturday, including reporting in a French-language magazine that brothers Cherif and Said Kouachi had been under watch by the French, but despite red flags, authorities there lost interest in them.

L'Express national security reporter Eric Pelletier shared with CNN details of his story, for which he talked to multiple French officials.

Tipped off by U.S. intelligence agencies that Said Kouachi may have traveled to Yemen in July, France placed him under surveillance in November 2011 but terminated the scrutiny in June 2014 when French security services deemed him no longer dangerous, officials told Pelletier.

The surveillance of his brother Cherif terminated at the end of 2013 when his phone calls suggested he had disengaged with violent extremism and was focused on counterfeiting clothing and shoes.

A U.S. official told CNN's Barbara Starr that Said Kouachi's 2011 travel lasted three or more months and that he is believed to have trained with al Qaeda in Yemen during that period.

French intelligence officials believe there is a strong probability Cherif Kouachi also traveled to Yemen for a short trip in 2011, separately from his brother, Pelletier's sources told him.

A Yemeni journalist and researcher, Mohammed al-Kibsi, told CNN that he had met and spoken with Said Kouachi in Yemen in 2011 and 2012.

But al-Kibsi, who said he met the man twice, said Said Kouachi was in Yemen most of 2011. Kouachi first went there in 2009 and stayed until mid-2010 before leaving briefly and returning at the end of that year, according to al-Kibsi.

Kouachi entered Yemen multiple times with an officially issued visa, a senior Yemeni national security official told CNN.

"Said was not being watched during the duration of his stay in Yemen because he was not on the watch list," said the official, adding that, at the time, Yemen's Western allies had not raised concerns about Kouachi. The official did not specify when the visits took place.

Kouachi, who was studying Arabic grammar, and underwear bomber Umar Farouk AbdulMutallab previously were roommates for one to two weeks in Yemen's capital, Sanaa, living in the same small apartment, al-Kibsi said. AbdulMutallab is serving a life sentence for trying to bring down a Northwest airlines flight over Detroit on Christmas in 2009 with an underwear bomb.

Kouachi's residence was very near to the famous Al-Tabari School and he and AbdulMutallab used to pray together there, said al-Kibsi by telephone Saturday. It wasn't clear when they were roommates, but AbdulMutallab was arrested after the 2009 bombing attempt.

There has been no official confirmation of the claim that he and AbdulMutallab were associates.

The Kouachi brothers, who authorities say carried out Wednesday's deadly attack in the offices of satirical magazine Charlie Hebdo , were killed Friday in a shootout with French security forces outside of Paris.

France, meanwhile, continues to cope with three days of terror that left 17 people dead. Thousands gathered on the streets for vigils Saturday and hundreds of thousands were expected at massive rallies Sunday, along with heads of state and other dignitaries.

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Suspect's significant other reportedly in Turkey

The alert came amid word that the lone remaining suspect wanted in connection with a terrorism spree -- Hayat Boumeddiene -- entered Turkey on January 2, a Turkish prime ministry source told CNN.

Boumeddiene was tracked by Turkish authorities to a location near the Turkey-Syria border, according to an official in the Turkish Prime Minister's office.

Boumeddiene arrived at the Istanbul airport on a flight from Madrid with a man. During routine screening of passengers, the couple were flagged by Turkey's Risk Assessment Center and a decision made to maintain surveillance on their movements, the official said. The official in the Turkish Prime Minister's office would not elaborate as to when Boumeddiene was tracked to the border province.

That means Boumeddiene may not have been in France at the time of Thursday's deadly shooting of a policewoman in Paris, as authorities originally believed. Authorities offered no immediate explanation of the discrepancy, but have said she is wanted in connection with a terrorist attack.

French authorities on Saturday asked security officials in Spain to look into the possibility that she transited through Spain on her way to Turkey, a source close to the Spanish officials said.

'Nation relieved'

The attack at the Paris office of the Charlie Hebdo left 12 dead and shocked France.

"The nation is relieved tonight," Interior Minister Bernard Cazeneuve said after the two standoffs concluded

Photos: Manhunt for terror suspects in France Photos: Manhunt for terror suspects in France Helicopters fly over a printing shop in Dammartin-en-Goele, France, where there was a standoff Friday, January 9, between police and two men suspected in the Charlie Hebdo shootings earlier this week. Cherif and Said Kouachi, the two brothers wanted in the case, were killed by security forces, authorities said. Hide Caption 1 of 25 Photos: Manhunt for terror suspects in France French authorities released photographs of the Kouachi brothers, warning that both could be armed and dangerous. A third suspect, Hamyd Mourad, surrendered to police earlier this week, according to the news agency Agence France-Presse. Hide Caption 2 of 25 Photos: Manhunt for terror suspects in France Police officers leave after storming the printing shop in Dammartin-en-Goele on January 9. Hide Caption 3 of 25 Photos: Manhunt for terror suspects in France Smoke rises from the building in Dammartin-en-Goele, a town about 40 kilometers (25 miles) northeast of Paris. Hide Caption 4 of 25 Photos: Manhunt for terror suspects in France Police take position on a roof during the standoff. Hide Caption 5 of 25 Photos: Manhunt for terror suspects in France A bird flies overhead as police snipers take aim from a roof. Hide Caption 6 of 25 Photos: Manhunt for terror suspects in France French gendarmes stand guard as a nearby school is evacuated in Dammartin-en-Goele on January 9. Hide Caption 7 of 25 Photos: Manhunt for terror suspects in France Children wait inside a school before being picked up by their parents. Dammartin-en-Goele residents were told to stay inside during the standoff, and schools were put on lockdown, the mayor's media office told CNN. Hide Caption 8 of 25 Photos: Manhunt for terror suspects in France A police officer checks a car in Dammartin-en-Goele. Hide Caption 9 of 25 Photos: Manhunt for terror suspects in France French authorities block access to a road leading to Dammartin-en-Goele on January 9. Hide Caption 10 of 25 Photos: Manhunt for terror suspects in France Armed security forces fly in Dammartin-en-Goele on January 9. Hide Caption 11 of 25 Photos: Manhunt for terror suspects in France Police vans line up in Dammartin-en-Goele on January 9. Hide Caption 12 of 25 Photos: Manhunt for terror suspects in France Police officers communicate inside a school in Dammartin-en-Goele. Hide Caption 13 of 25 Photos: Manhunt for terror suspects in France Police and military forces take positions in Dammartin-en-Goele. Hide Caption 14 of 25 Photos: Manhunt for terror suspects in France A police officer stands guard in Fleury, France, on Thursday, January 8. Hide Caption 15 of 25 Photos: Manhunt for terror suspects in France French soldiers patrol the Gare du Nord railway station in Paris on January 8. Hide Caption 16 of 25 Photos: Manhunt for terror suspects in France Police walk down a street in Corcy, France, on January 8. Hide Caption 17 of 25 Photos: Manhunt for terror suspects in France On January 8, police tape and vehicles block off the entrance to a gas station north of Paris where the two suspects were reportedly seen the night before. Hide Caption 18 of 25 Photos: Manhunt for terror suspects in France French police officers gather January 8 in Longpont, France, about 10 kilometers (6.2 miles) from where the suspects were reportedly spotted. Hide Caption 19 of 25 Photos: Manhunt for terror suspects in France French police patrol a street in Corcy on January 8. Hide Caption 20 of 25 Photos: Manhunt for terror suspects in France A police officer stands guard in front of Paris' Elysee Palace on January 8. Hide Caption 21 of 25 Photos: Manhunt for terror suspects in France Police stand guard in front of an apartment building in the Croix-Rouge suburb of Reims, France, early on January 8. Forensics officers were looking for evidence related to the three suspects. Hide Caption 22 of 25 Photos: Manhunt for terror suspects in France Sharpshooters are seen outside during the operation in Reims. Hide Caption 23 of 25 Photos: Manhunt for terror suspects in France Forensics officers are seen through a window in Reims. Hide Caption 24 of 25 Photos: Manhunt for terror suspects in France French national police arrive at a police station in Charleville-Mezieres, France, on Wednesday, January 7. Hide Caption 25 of 25

But the French government's work is not over.

There's still a lot of healing to do, and questions to answer on how this happened and how to prevent future attacks. Meanwhile, police continue the hunt for Boumeddiene , Coulibaly's partner.

France will remain at a heightened security as investigations continue, Cazeneuve said after an emergency security meeting.

All necessary measures will also be taken to ensure the safety of people who attend a massive unity rally planned in Paris on Sunday, he said. Extra steps will also be taken to protect religious institutions.

Cazeneuve and other officials outlined the extraordinary security measures, including snipers, plainclothes and anti-terror officers as well as parking and transit restrictions, that will be in place for the rally.

European leaders including Britain's David Cameron, Germany's Angela Merkel and Spain's Mariano Rajoy will join French President François Hollande at the unity march. Russian Foreign Minister Sergey Lavrov and Turkish Prime Minister Ahmet Davutoglu will attend, according to Russia's Foreign Ministry and Turkish semi-official news agency Anadolu.

Palestinian Authority President Mahmoud Abbas will take part in Sunday's march, the official Palestinian news agency WAFA said.

Jordan's King Abdullah II and Queen Rania will participate, Jordan's Embassy in Washington said. And Israeli Prime Minister Benjamin Netanyahu will travel to France for the event, according to his office.

In a statement, Netanyahu said he spoke by phone with Celine Shreki, who was a hostage at the kosher market Friday.

"To Celine and all French Jews, and to all European Jews, I would like to say: The State of Israel is not just the place to which you turn in prayer. The State of Israel is also your home," the statement said.

A total of 1,100 French troops are currently deployed in the Paris region, alongside police forces, to increase security following the attacks, the Defense Ministry said. An additional 250 soldiers will be on duty Sunday for the march, the ministry said.

Altogether, nearly 1,900 French troops will take part in providing additional security across the country as part of the France's security alert system, known as Vigipirate.

The precautions may help to ease the nerves of a country left on edge by the wave of violence.

The targeting of the kosher grocery store has shaken Jewish communities in particular. And amid the heightened security concerns, the Grande Synagogue of Paris was closed Saturday for the first time since World War II.

Rabbi Jonas Jacquelin, who serves in a different synagogue, told CNN that an attack on one member of the Jewish community was felt by everyone else.

But, he said, it was important for his synagogue to stay open to demonstrate that the community is not afraid. "We have to show to the world, we have to show to our enemies that all of us are continuing to pray today as we are doing every week and every Shabbat -- nothing can disturb us," he said.

Two sieges

Friday's deadly events started in Dammartin-en-Goele, northeast of Paris, where the Kouachi brothers took refuge in a print shop in an industrial area after two days on the run.

Hours later, after a major police operation locked down the town, the brothers were dead and a man who'd been hiding out in the building was freed unharmed.

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At the scene of the other violent siege that capped an uneasy week in Paris, Jewish and Muslim leaders gathered to pay their respects to the four people who died there. They held hands and left flowers and spoke of unity amid tragedy.

The deadly kosher grocery store standoff unfolded in Porte de Vincennes, eastern Paris.

Four hostages were killed, officials said. Coulibaly was killed after police moved in to end the siege.

The four victims were identified by the French Jewish publication JSSNEWS as Yohan Cohen, 22, Yoav Hattab, 21, Philippe Braham and Francois Michel Saada.

Israeli government sources told CNN that Hollande told Netanyahu that 15 were rescued. The four hostages were killed by the gunman before police stormed the market, sources said.

One of the hostages, identified only as Marie, told CNN affiliate BFMTV that the gunman was heavily armed -- and that she was very happy to be alive.

"As soon as he got inside, he started shooting. He scared us because he told us: I am not afraid to die and he said either I die or I go to jail for 40 years. He knew this was his last day," she said.

Hollande called the Porte de Vincennes deaths an "anti-Semitic" act and urged citizens not to lash out against Muslims.

"Those who committed these acts have nothing to do with the Muslim religion," he said. "Unity is our best weapon."

On Saturday, Hollande and Cazeneuve met with several police officers injured in the raid on the Kosher market.

Ties to Islamist extremists?

While Said Kouachi is suspected of links to al Qaeda in Yemen, Cherif Kouachi has a long history of jihad and anti-Semitism, according to documents obtained by CNN. In a 400-page court record, he is described as wanting to go to Iraq through Syria "to go and combat the Americans."

Cherif Kouachi was a close associate of Coulibaly, a Western intelligence source told CNN.

Al Qaeda in the Arabian Peninsula claimed responsibility for orchestrating the Charlie Hebdo attack, the founder of the magazine The Intercept , Jeremy Scahill, told CNN. CNN has not independently confirmed this claim.

A man claiming to be Amedy Coulibaly, the hostage-taker at the Paris grocery store, told CNN affiliate BFMTV that he belonged to the Islamist militant group ISIS

The Western intelligence source said Coulibaly lived with Boumeddiene, his alleged accomplice in the police shooting.

Boumeddiene exchanged 500 phone calls with the wife of Cherif Kouachi in 2014, according to Paris prosecutor Francois Molins. The wife told investigators that her husband and Coulibaly knew each other well.

French media outlets AFP, iTele and Le Point reported that police released Hamyd Mourad, 18, who turned himself in Wednesday after seeing his name on social media in connection with the Charlie Hebdo attack.

What's next for the magazine?

Charlie Hebdo plans to go on even without its leader and cherished staffers. It's set to publish many extra copies of its latest edition next Wednesday.

On Friday, Charlie Hebdo staff held their editorial meeting at the Libération newspaper offices.

"I don't know if I'm afraid anymore, because I've seen fear. I was scared for my friends, and they are dead," said Patrick Pelloux, a columnist for the magazine.

He and many others are defiant.

"I know that they didn't want us to be quiet," Pelloux said of the slain colleagues. "They would be assassinated twice, if we remained silent."