A French government source told CNN that authorities believe other terrorist sleeper cells have been activated and could strike at any time.

A rally in Paris today is expected to draw nearly a million people and will include many world leaders. Security is incredibly tight as an estimated 50,000 police and military personnel have been deployed

USA Today:

Authorities tightened security in France's capital city Saturday amid fears of new attacks after a series of deadly terror assaults rattled the nation.

The French government maintained its terror alert system at the highest level in Paris, and a heavy security presence was easily visible on city streets as well as at houses of worship, landmarks and government buildings.

"There is risk (of another attack)," said French Interior Minister Bernard Cazeneuve. "We are creating a defense zone, to deploy a reaction force should it become necessary."

French law enforcement officers have been told to erase their presence on social media and to carry their weapons at all times because terror sleeper cells have been activated over the last 24 hours in the country, CNN reported citing an unnamed French police source.

The two brothers — Cherif, 32, and Said Kouachi, 34 — who attacked the French satirical newspaper Charlie Hebdo earlier this week may have been part of a sleeper cell, Morten Storm, a Danish national who spied on al-Qaeda for Danish, British and American intelligence, told ABC News.

"What I can say from my experience or from my knowledge is that these people here have managed to deceive French intelligence to believe that, while they were once extremists, they no longer were," Storm said. "So they managed to get under the radar and… they finally woke up again, like a sleeper cell, and (committed) this atrocity."

Might the attacks have been a signal to other sleeper cells to awaken? If they were sent by AQAP, it's a possibility. The terrorist who took over the Jewish grocery store, Amedy Coulibaly claims that AQAP had "directed" the attack and the Cherif brothers both trained at AQAP camps in Yemen. There is every reason to believe that there are more terror cells in France connected to AQAP and it wouldn't be surprising if they have now become active and are preparing to attack specific targets.