French Prime Minister Manuel Valls has said that the gunman who attacked a kosher supermarket "probably had accomplices" as the country's interior ministry mobilises 10,000 troops to protect the population.
Valls said Amedy Coulibaly, who killed a policewoman in Paris' Montrouge and then four Jewish shoppers in a Hyper Cacher supermarket in Porte de Vincennes, most likely received help from someone else.
"We think that there were actually probably accomplices," he said.
The widow of Coulibaly, Hayat Boumeddiene, crossed into Syria on Thursday (8 January) after travelling to Turkey on 2 January, according to Turkey's foreign minister.
Valls said that the "threat is still present" and the "hunt will go on" after the attacks that left 17 dead, including journalists at the satirical magazine Charlie Hebdo and three police officers.
Coulibaly and the Kouachi brothers were killed by security forces in simultaneous raids.
The deployment of 10,000 security personnel will focus on the most sensitive locations, according to interior minister Bernard Cazeneuve, who also vowed to boost security at Jewish institutions, especially at the country's 717 Jewish schools.
Cazeneuve named a prefect, Patrice Latron, who will coordinate the schools' protection with 4,700 police officers and gendarmes.
The president of Jewish community umbrella group CRIF, Roger Cukierman, said that French President Francois Hollande promised to provide extra protection to Jewish schools and synagogues.
"We have our sadness and our rage, we also have set out a number of urgent measures to take," Cukierman said after a meeting with Hollande.
"They told us that all schools and all synagogues will be protected in measures that, if necessary, extend beyond the police to the army."
France has the largest Jewish population in Europe which amounts to 550,000, according to CRIF.
Communal tensions and anti-Semitic incidents more than doubled last year, according to the interior ministry.
Valls told BFM television on Monday that France is at war against "terrorism, against jihadism, against radical Islam."
Israeli Prime Minister Benjamin Netanyahu, who travelled to Paris to attend Sunday's unity rally against terrorism, has been criticised for encouraging French Jews to move to Israel.
Cukierman said that the CRIF respects the decision of Jews who travel to Israel, but considers it necessary to fight the "enemies of Judaism" in France.