French police say they have located the two heavily armed men said to be responsible for the killing of 12 people at a satirical newspaper in Paris, AFP news agency has reported.

The Paris-based agency said on Thursday that the police have confirmed the two men were in the Aisne district in northern France, but would not say whether they had been apprehended.

The manager of a petrol station near Villers-Cotteret in Aisne said he “recognised the two men suspected of having participated in the attack against Charlie Hebdo”, AFP said, quoting sources close to the manhunt.

The latest developments come hours after Manuel Valls, France’s prime minister, announced several arrests overnight and said the possibility of a new attack “is our main concern”.

Tensions in Paris were high on Thursday as France began a day of national mourning, after the controversial Charlie Hebdo magazine was attacked by three armed men on Wednesday. Twelve people were killed and 11 others were injured in the attack.

Meanwhile, in another incident on Thursday, a police officer was killed on the outskirts of Paris.

It was not immediately clear if that shooting was linked to the previous day’s attack at the Paris-based satirical magazine, where two other police were among the dead.

Police have identified and released the photographs of two brothers suspected to be involved in the attack.

The two brothers have been identified as 32-year-old Said Kouachi and 34-year-old Cherif Kouachi. The two men, along with another person, who is believed to be a teenager, carried out Wednesday’s attack before escaping in a car.

The getaway car was later found abandoned in northern Paris from where the hooded gunmen hijacked a Renault Clio.

Earlier, Interior Minister Bernard Cazeneuve said that seven people had been detained in the hunt for the two suspects.

Witnesses said the attackers spoke fluent, unaccented French, shouted “Allahu Akbar!” as they opened fire in the noon-time attack on the Charlie Hebdo headquarters, located near Paris’ iconic Bastille monument.

Charlie Hebdo’s depictions of Islam, including the Prophet Muhammad, had drawn condemnation and threats before. It was firebombed in 2011 – although it also satirised other religions as well as political figures.

The attack triggered global outrage and condemnation. French President Francois Hollande said it was a “terrorist act of exceptional barbarism”, adding that other attacks have been thwarted in France in recent weeks. Fears have been running high in France and other countries in Europe that fighters returning from conflicts in Syria and Iraq will stage attacks at home.

The security alert in the country was raised to the highest level and protective measures at houses of worship, stores and media offices immediately reinforced. Schools across the French capital have also been closed. However, security concerns did not deter thousands of people from packing the Republique Square near the site of the shooting to honour the victims.

Those gathered waved pens and papers reading “Je suis Charlie (I am Charlie)”.

Similar rallies were held in London’s Trafalgar Square as well as Madrid, Berlin and Brussels.

There was no immediate claim of responsibility for the attack.

The lawyer of the magazine confirmed that four cartoonists working with the publication, including the editor Stephane Charbonnier, known as “Charb”, were among the dead.

The other cartoonists killed were known as Cabu, Tignous and Wolinski.

France’s Muslim leadership also sharply condemned the shooting as a “barbaric” attack and an assault on press freedom and democracy.

“This extremely grave barbaric action is also an attack against democracy and the freedom of the press,” the French Muslim Council said in a statement.

The body represents France’s Muslim community, which is Europe’s biggest and estimated to number between 3.5 million and 5 million people. Al Jazeera