Amedy Coulibaly, who killed a policewoman in southern Paris then four Jewish shoppers in a hostage drama, probably received help from someone else, Manuel Valls said, pledging "the hunt will go on".
Interior Minister Bernard Cazeneuve promised to boost security at Jewish institutions, telling parents of a Jewish school to the south of Paris that soldiers would also be posted as reinforcements.
The Jewish community has been particularly shaken by Friday's attack on the kosher supermarket in eastern Paris, which came just two days after two other gunmen - Said Kouachi and his brother Cherif Kouachi - stormed the offices of Charlie Hebdo satirical weekly, slaughtering 12 people.
This measure "must become widespread" but "it must be done discerningly and intelligently", he said.