Police in France have widened their search to a 51-square-mile dense forest in northern France for two brothers suspected of carrying out a mass shooting in broad daylight at the Paris office of satirical magazine Charlie Hebdo that left 12 people dead.

French authorities think that the suspects, identified as Cherif Kouachi, 32, and Said Kouachi, 34, are hiding out in a very large forest known as Forêt de Retz. According to Sky News, two men who may have fit the brothers' description supposedly stole fuel and food from a petrol station in a town 43 miles (70 km) from Paris.

"Each time we arrive at an area where they have been sighted or there is some sort of suspicion of significant police activity, you get the feeling that things have moved on," Joey Jones of Sky News reported. "This is such a fast moving situation, who knows where it will end up."

According to Greg Botelho and Atika Shubert of CNN, helicopters swarmed overhead in northern France's Picardy region, and heavily armed officers searched the countryside and forest for the gunmen.

"About 10 kilometers (6 miles) from the gas station, police blocked a rural country road leading to the French village of Longpont," Botelho and Shubert wrote. "Authorities have not commented in any detail, but pictures showed heavily armed police officers with shields and helmets in the blocked-off area."

French Prime Minister Manuel Valls has placed that region of France on the highest alert level. The entire Ile-de-France region, which includes the city of Paris, is already at that level according to CNN.

CNN's Christiane Amanpour reported that one of the brothers suspected in the Charlie Hebdo attack in Paris went to Yemen back in 2005. French Justice Minister Christiane Taubira did not elaborate on which brother went to that Middle Eastern country.

"One of them went to Yemen because there was a judgment and he went in jail," Taubira said.

According to Fox News, French authorities said that the brothers are the prime suspects in a deadly Islamist terror attack Wednesday morning at the Paris headquarters of Charlie Hebdo, a satirical magazine known in France to mock all religions through its political cartoons.

"The assailants forced their way into the magazine's main offices, killing 12, including the magazine's editor, before fleeing in a getaway car in broad daylight," Fox News reported.

A third suspect, 18-year-old Hamyd Morad, turned himself in to French police after he heard his name linked to the Paris attacks in the news and social media. Agnes Thibault-Lecuivre, a spokesperson for the Paris prosecutor, did not elaborate on his relationship to the Kouachi brothers according to Fox News.

CNN reported that French police have detained nine people so far in connection with the Charlie Hebdo attack. However, French police managed to impound the suspects' getaway car and combed it for evidence, which included an identification card from Said Kouachi.

"They knew exactly what they had to do and exactly where to shoot," a witness said to Fox News, who would not have his name released for safety reasons. "While one kept watch and checked that the traffic was good for them, the other one delivered the final coup de grace."

Fox News reported that French President Francois Hollande addressed the nation in a somber manner Wednesday night, vowing to hunt down the suspects and asking the French people to unite in the face of insecurity and suspicion.

"Let us unite, and we will win," Hollande said. "Vive la France!"