The manhunt for the two suspects in the killing of 12 people at a satirical magazine in Paris is concentrating on an area northeast of the capital after the men were spotted at a gas station in the region.
Two police sources told Reuters that the men were spotted at a petrol station in the region, were seen armed and in a Renault Clio car at a petrol station on a secondary road in Villiers-Cotterets some 70 kilometers (43 miles) from the French capital.
Bruno Fortier, the mayor of neighboring Crépy-en-Valois, told Reuters helicopters were circling his town and police and anti-terrorism forces were deploying en masse.
"It's an incessant waltz of police cars and trucks," he told Reuters.
Earlier Thursday, a female police officer, who has yet to be named, was killed and a roadsweeper injured in Montrouge south of Paris after an assailant opened fire before fleeing.
French Interior Minister Bernard Cazeneuve told reporters before the officer's death was confirmed that authorities were doing their utmost to identify and arrest the attacker, and he cautioned against jumping to conclusions, The Associated Press reported.
France began a day of mourning for the journalists and police officers shot dead on Wednesday morning by black-hooded gunmen using Kalashnikov assault rifles. French tricolour flags flew at half mast throughout the country.
Overnight, French police released photos of the two the French nationals calling them "armed and dangerous": brothers Cherif and Said Kouachi, aged 32 and 34, both of whom were already under watch by security services.
The journal Charlie Hebdo is well known for lampooning Islam and other religions as well as political figures.
Prime Minister Manuel Valls said France faced a terrorist threat "without precedent" and confirmed the two brothers were known to security services. But he added it was too early to say whether authorities had underestimated the threat they posed.
"Because they were known, they had been followed," he told RTL radio, adding: "We must think of the victims. Today it's a day of mourning."
A total of seven people had been arrested since the attack, he said. Police sources said they were mostly acquaintances of the two main suspects. One source said one of the brothers had been identified by his identity card, left in the getaway car.
Late on Wednesday an 18-year-old man, Hamyd Mourad, turned himself into police in Charleville-Mézières, some 230 kilometres northeast of Paris near the Belgian border as anti-terrorism police carried out searches in Paris and the northeastern cities of Reims and Strasbourg. French media quoted friends as saying he was in school class at the moment of the attack.
Cherif Kouachi served 18 months in prison on a charge of criminal association related to a terrorist enterprise in 2005. He was part of an Islamist cell enlisting French nationals from a mosque in eastern Paris to go to Iraq to fight Americans in Iraq and arrested before leaving for Iraq himself.
The brothers were on the U.S. no-fly list, a senior U.S. counterterrorism official told the AP. Officials did not specify what else they knew about the suspects.
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Across France, people observed a minute's silence in memory of the victims - among them some of France's most prominent and best-loved political cartoonists - and to support freedom of speech.
Concerns have been raised that, because of the alleged killers' Islamist ties, France's sizeable Muslim community could be subject of reprisal attacks. Early Thursday morning, an explosion rocked a kebab shop next to a mosque in Villefrance-sur-Saone in eastern France, Reuters reported.
Marine Le Pen, leader of France's right-wing Front National, has told media that if she came to power, she would hold a referendum to reintroduce the death penalty.
"I personally believe that the death penalty should exist in our legal arsenal," Le Pen told television channel France 2.
In the U.K, security has been increased at ports and border points after the deadly attack in Paris. However, officials say there is no specific new threat to the country.
—Reuters and The Associated Press contributed to this report.