(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.
Ahmedy 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.
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.
Brothers trained with terror groups02:04
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.
The attack at the Paris office of the Charlie Hebdo left 12 dead and shocked France.
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 forBoumeddiene, 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.
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.
How manhunt for French terrorists ended02:54
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.
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 groupISIS.
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."
CNN's Laura Smith-Spark wrote and reported from London, Ray Sanchez from New York and Faith Karimi from Atlanta, while journalist Hakim Almasmari reported from Sanaa. CNN's Paul Cruickshank, Barbara Starr, Nic Robertson, Nick Paton Walsh, Jim Sciutto, Atika Shubert, Sandrine Amiel, Fred Pleitgen, Christiane Amanpour, Jim Bittermann, Al Goodman, Saskya Vandoorne, Margot Haddad, Radina Gigova, Hande Atay and Tim Lister contributed to this report.