French police on Thursday continued the manhunt for two brothers wanted for the terrorist attack on the magazine Charlie Hebdo, in which 12 people were killed on Monday. CNN reported Cherif Kouachi, 32, and Said Kouachi, 34, are the main suspects in the massacre, and authorities so far have been unable to apprehend them.

"They are still free. They are heavily armed, so we can be afraid of further violence," French Prime Minister Manuel Valls warned on RTL television.

"We are facing an unprecedented terrorist threat, both internally and externally," Valls added, according to the Christian Science Monitor.

A third suspect, 18-year-old Mourad Hamyd, turned himself in to police on Thursday after learning his name was linked to the attacks, said Agnes Thibault-Lecuivre, a spokeswoman for Paris prosecutors, according to Fox News. He was arrested at a police station in a small town about 145 miles northeast of Paris, the Christian Science Monitor detailed.

French authorities upped the country's terror alert system to the maximum level, Fox News reported. They also dispatched more than 800 soldiers to guard media offices, places of worship, transport and other sensitive areas. At the same time, worry of another act of violence and possible sightings of the Kouachi brothers abounded, CNN said.

It remains unclear whether the killing of a female police officer in the Paris suburb of Montrouge is linked to the Charlie Hebdo massacre. The attacker in that incident was dressed in all black and apparently wore a bulletproof vest, recalling images of terrorists who killed a police officer before they stormed the magazine's building.

French media noted that a gas station attendant near Villers-Cotterets in Aisne, the northern Picardy region of France, told police that he saw the suspects Thursday, and that the men had threatened him and stolegas and food before heading toward Paris. Police have not yet confirmed that report, CNN said.

A shocked French public, meanwhile, observed a minute of silence at noon to commemorate the deaths caused by the country's worst terrorist attack in a generation, Le Monde reported.

The next issue of Charlie Hebdo will be published next Wednesday, with a circulation of 1 million copies instead of the usual 60,000.

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