(CNN) — A heartbroken France will unite in grief Sunday amid intense security after three days of terror left 17 people dead last week.
The nation was on high alert after security forces fatally shot a man police said killed four hostages at a kosher supermarket in eastern Paris on Friday.
The same day, police killed Said and Cherif Kouachi, the brothers police said raided the Paris offices of satirical magazine Charlie Hebdo and left 12 people dead.
In addition to the magazine victims and the hostages, a policewoman was killed Thursday in Montrouge south of Paris.
As more details on the days of horror emerged, France remained on high alert.
French law enforcement officers have been told to erase their social media presence and carry weapons at all times because terror sleeper cells have been activated in the country, a police source said.
The source told CNN that the cells were activated in the past 24 hours,
As France struggles to cope in the aftermath of the terror attacks, thousands are expected at massive rallies in Paris on Sunday.
Dignitaries and world leaders will attend in what government officials are calling a “unity rally” in defiance of the terror rampage.
French officials announced “exceptional measures” to protect not only the throngs expected to gather near the Place de la Republique in central Paris, but also a veritable who’s who of foreign leaders that will test security forces of a nation rocked by the violence.
Leaders expected to stand with French President François Hollande are Russian Foreign Minister Sergey Lavrov, Turkish Prime Minister Ahmet Davutoglu, Jordan’s King Abdullah II, Palestinian Authority President Mahmoud Abbas and Israeli Prime Minister Benjamin Netanyahu.
Targeting police officers?
Amedy Coulibaly, the suspect killed the 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 a L’Express magazine report that the Kouachi brothers had been under watch by the French, but despite red flags, authorities there lost interest in them.
L’Express national security reporter Eric Pelletier talked to multiple French officials.
Tipped off by U.S. intelligence agencies that Said Kouachi may have traveled to Yemen, France placed him under surveillance in November 2011 but terminated the scrutiny last year when it 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.
Said Kouachi’s is believed to have trained with al Qaeda in Yemen, a U.S. official told CNN’s Barbara Starr.
French intelligence officials believe there is a strong probability Cherif Kouachi also traveled to Yemen for a short trip in the same year, separately from his brother, Pelletier’s sources told him.
The Kouachi brothers were killed Friday in a shootout with French security forces outside of Paris
Suspect left for Turkey before attacks
The lone remaining suspect wanted in connection with the terrorism rampage — Hayat Boumeddiene — entered Turkey on January 2, a Turkish prime ministry source told CNN. She arrived at the Istanbul airport on a flight from Madrid with a man.
That means she 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.
During routine screening of arriving passengers, the pair were flagged by Turkey’s Risk Assessment Center at the airport and a decision made to maintain surveillance on their movements. The pair checked into a hotel in Istanbul and engaged in “tourist type” activity for a couple of days. When her name was made public by French investigators after the Paris terror attacks, Turkish authorities alerted their French counterparts as to her movements. Turkish agents tracked her to what the official said was her last confirmed location near the Syrian border.
She had a return ticket to Madrid for January 9, but she failed to take her return flight from Istanbul on January 9, according to an official in the office of Turkey’s Prime Minister.
Arson attack at German newspaper
Meanwhile, an incendiary device was hurled at a German newspaper that reprinted the Charlie Hebdo cartoons. No one was in the building at the time of the attack early Sunday.
The arson attack at the Hamburger Morgenpost occurred about 2 a.m. local time, the newspaper said on its website.
The device was thrown into the archive section of the building, setting it on fire. It’s unclear if the arson attack is connected to the Charlie Hebdo attack in Paris last week.
The German paper reprinted Charlie Hebdo cartoons depicting the Prophet Mohammed after the attack on the satirical magazines offices in Paris.
CNN’s Ray Sanchez wrote and reported from New York and Faith Karimi from Atlanta, while journalist Hakim Almasmari reported from Sanaa. CNN’s Nic Robertson, Al Goodman and Tim Lister contributed to this report.