French police forces backed by helicopters have descended on a town northeast of Paris after a possible sighting of two suspects in an attack on satirical magazine Charlie Hebdo which killed 12 people on January 7.
Reports said two heavily armed men robbed a gas station near the town of Villers-Cotterets, firing shots, before fleeing in a grey Renault Clio -- the same make and color as a car hijacked by the suspects in the Charlie Hebdo attack.
The security operation in the town of Villers-Cotterets unfolded as a policewoman was killed earlier on January 8 on the southern outskirts of Paris.
The female officer died after she and a city employee were shot and seriously wounded near Porte de Chatillon, on the southern edge of Paris.
A man wearing a bulletproof vest reportedly opened fire with an automatic rifle before fleeing by car.
It was not immediately clear whether there was any link to the attack on Charlie Hebdo, in which masked gunmen shouting Islamist slogans killed 12 people, including two police.
Separately, a source close to the case said investigators found a dozen Molotov cocktails and two jihadist flags in one of the cars used initially in Charlie Hebdo massacre, which was later abandoned.
Several people have been detained in the hunt for the suspects -- Cherif Kouachi, 32, who was convicted in 2008 for involvement in a network sending fighters to Iraq, and his brother Said, 34.
French Prime Minister Manuel Valls announced the arrests as France observed a day of mourning for the victims of the worst attack of its kind in France in decades. Valls voiced concern the attackers could strike again, and authorities are "urgently trying to track down" the culprits.
Asked about a possible new attack, Valls told RTL radio, "That question is entirely legitimate, that's obviously our main concern, and that is why thousands of police and investigators have been mobilised to catch these individuals."
The attackers shouted "Allahu akbar" (God is great) and said they wanted to "avenge the prophet" during the attack on the magazine, which had drawn repeated threats for its caricatures of the Prophet Muhammad.
Earlier on January 8, a judicial source told French media that seven men and women close to the brothers were being questioned by police.
Paris prosecutor's spokeswoman Agnes Thibault-Lecuivre said that a third man, Mourad Hamyd, 18, surrendered at a police station in a small town in the eastern region of Champagne after learning his name was linked to the attacks in the news and social media.
She did not specify his relationship to the Kouachi brothers.
Meanwhile, a minute of silence was observed across France at noon on January 8 to pay tribute to the victims of the attack, which killed cartoonists whose names were known to millions of citizens and increased tension in a country with a large Muslim population.
Police conducted raids on homes in the northeastern city of Reims as part of a nationwide manhunt for the Kouachi brothers.
Raids were also carried out in Strasbourg and some small towns near Paris.
Former President Nicolas Sarkozy told journalists after discussing the crisis with President Francois Hollande on January 8 that "war has been declared against civilization and the responsibility of civilization is to defend itself."
The attack sparked strong international condemnation and triggered a wave of demonstrations and expressions of outrage and solidarity with the victims.
On the evening of January 7, tens of thousands of people took part in rallies around France and in other European cities -- including London, Madrid, Amsterdam, New York, and Berlin -- in support of freedom of speech and democracy.
Prosecutors also said that Muslim places of worship in two French towns were fired upon overnight, but nobody was hurt.
Three blank grenades were thrown at a mosque shortly after midnight in the city of Le Mans, west of Paris. A bullet hole was also found in a window of the mosque.
In the Port-la-Nouvelle district near Narbonne in southern France, several shots were fired in the direction of a Muslim prayer hall shortly after evening prayers.
An explosion at a kebab shop near a mosque in the eastern town of Villefranche-sur-Saone on January 8 also caused no casualties. Local prosecutors have described it as a "criminal act."
In Britain, the government said it had increased border security, including at ports and checkpoints it operates on French soil in response to the January 7 attack.
With reporting by AFP, AP, Reuters and the BBC