A woman raises a pen during a vigil to pay tribute to the victims of a shooting by gunmen at the offices of weekly satirical magazine Charlie Hebdo in Paris, at Trafalgar Square in London January 7, 2015. [Photo/Agencies]

Cherif Kouachi was charged with criminal association related to a terrorist enterprise in 2005 after he was arrested before leaving for Iraq to join Islamist militants. He was sentenced to 18 months in prison in 2008, according to French media.

A police source said anti-terrorism police searching for the suspects had been preparing an operation in Reims, and that there had already been a number of searches at locations across the country as part of the investigation.

A Reuters reporter in Reims saw anti-terrorism police secure a building before a forensics team entered an apartment there while dozens of residents looked on. They did not appear to be preparing a major raid.

A government official said there had been no arrests.

A police union official said there were fears of further attacks, and described the scene in the offices as carnage, with a further four wounded fighting for their lives.

Tens of thousands joined impromptu rallies across France in memory of the victims and to support freedom of expression.

The government declared the highest state of alert, tightening security at transport hubs, religious sites, media offices and department stores as the search for the assailants got under way.