It's very likely, French Prime Minister Manuel Valls said.
"We are doing everything we can to dismantle what appears to be a network. ... No doubt there was complicity and networks and maybe finance also," he told CNN's Christiane Amanpour.
Said and Cherif Kouachi, blamed for Wednesday's attack on Charlie Hebdo magazine, are dead. So is Amedy Coulibaly, who authorities say killed a police officer, carried out an attack at a kosher market and might have shot a jogger
It's not clear how many people might have helped them plot last week's attacks. But Valls said they couldn't have done it without help.
"I don't really believe in the idea of the lone wolf," he said.
Investigators looking into woman who fled France
A key player who could help unravel details is Hayat Boumeddiene, Coulibaly's partner and alleged accomplice.
Early reports said she was by his side during the hostage siege at the kosher market. But now, investigators believe she was far from Paris when the attacks unfolded.
Boumeddiene arrived in Turkey from Madrid on January 2 -- five days before the first attack in Paris, Turkey's Anadolu news agency reports.
Boumeddiene stayed at an Istanbul hotel and then traveled to Syria on Thursday, Andalou said. The last place authorities spotted Boumeddiene was somewhere near Turkey's border with Syria
Police have searched an apartment outside Paris as they try to track her down.
There investigators discovered ISIS flags, automatic weapons and detonators at an apartment rented out by Coulibaly, Boumeddiene's partner and suspected co-conspirator, France's RTL Radio reported
, citing authorities.
Authorities are looking into whether Boumeddiene helped prepare the attacks before leaving France, Valls told CNN.
And there's another concern, a U.S. intelligence official told CNN: Now that she is believed to have escaped to Syria, Boumeddiene could be used in propaganda videos to gloat about the attacks.
Soldiers and police deploy
France was on its highest alert level Monday as people went back to work and children returned to schools following the attacks.
At least 10,000 soldiers and 8,000 police officers will be deployed across the country, French media reported.
"We must remain vigilant because the threat is still very much present," Valls told CNN affiliate BFMTV.
'France without Jews of France is not France,' PM says
About 4,700 officers will be tasked with securing 717 Jewish schools, Valls said.
Four of the victims were killed Friday at the kosher market, the latest in a string of anti-Semitic attacks that have led many Jewish families to leave the country
A Muslim employee is credited with helping hide some hostages
in a walk-in freezer, which he turned off. A photo from the freezer shows a baby was among the hostages.
"The situation can only get worse. It does not look like the problem is going away," high school student Gary Uzan, 17, told CNN on Monday. "I will go to Israel. It is better for us. It is safer."
Terrorist's wife 'astonished'
The wife of Cherif Kouachi has condemned her husband's actions.
"She condemns every violent act," her attorney, Christian Saint-Palais, said in a statement. "She was astonished when she learned her husband was involved in the Charlie Hebdo attack."
The statement -- which did not include the wife's name -- expressed her sympathies for the families of the victims.
Historic, peaceful march
About 40 world leaders joined in the march in the French capital.
Police said no incidents had been reported despite the record number of people involved in the Paris rally.
Still, 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.
Investigators are looking into whether the attackers last week were part of a larger cell and, if so, when it may strike again, Valls told CNN's Christiane Amanpour.
U.S. on alert after ISIS message
The threats go well beyond France.
The New York City Police Department and other U.S. law enforcement agencies responded to a threat from ISIS after someone released again a September message that tells followers to "rise up and kill intelligence officers, police officers, soldiers, and civilians" -- specifically naming the United States, France, Australia and Canada as targets.
According to a New York police memo obtained by CNN
, department employees were told to "remain alert and consider tactics at all times while on patrol," especially in light of the attacks in France last week. The FBI and Department of Homeland Security issued a similar bulletin to law enforcement across the United States.
In a statement Monday, U.S. Secretary of Homeland Security Jeh Johnson said the Transportation Security Administration is increasing the number of random searches of passengers and carry-on luggage at U.S. airports. Johnson said his agency had implemented new measures, including stepped-up efforts to share "information about terrorist threats and individuals of suspicion" with France and "other key counterterrorism allies" and to boost security outside U.S. government buildings.
Early Sunday, a firebomb was hurled at a German newspaper that reprinted Charlie Hebdo cartoons depicting the Prophet Mohammed, though authorities haven't said whether the arson attack was connected to the Charlie Hebdo massacre. No one was injured in the attack.