A massive manhunt is under way for two suspects in the terror attack on a satirical magazine.

Images have been released of brothers Said Kouachi and Cherif Kouachi, both in their early 30s, who are suspected of being part of the assault that left 12 people dead.

Officials have said the French nationals are linked to a Yemeni terror network.

The youngest of the suspects in the shooting has handed himself in to police after he was named on social media as Hamyd Mourad, 18.

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Sky sources say the man, who has been arrested, is the brother-in-law of the suspects at large. There are reports that he has claimed he was in school at the time of the attack.

On Wednesday night heavily armoured French police raided an apartment in the city of Reims, east of Paris, in the search for the killers.

French officials say seven people were detained overnight.

As tensions remain high in the country, an elite police unit is preparing for an operation near the scene where a female police officer was fatally shot in southern Paris overnight.

Earlier a "criminal" blast was reported at a kebab shop near a mosque in the eastern French town of Villefranche-sur-Saone. No one was injured.

Officials have not said either incident is linked to the attack on Charlie Hebdo.

With people continuing to leave tributes to the victims near the scene of yesterday's shooting at the magazine's offices, thousands of officers have reportedly been deployed in the hunt for the killers.

The publication has been targeted in the past over its portrayal of the Prophet Mohammed.

In Wednesday's attack, masked gunmen armed with Kalashnikov rifles and a rocket-propelled grenade stormed the offices and called out their victims by name before opening fire during a morning editorial meeting.

They were let inside by a female employee who was threatened at gunpoint along with her daughter and forced to punch in a security code.

The editor and a cartoonist, who went by the pen names Charb and Cabu, were among those killed.

Two police officers were among the dead, including one assigned as Charb's bodyguard after he had received death threats and another who was shot in the head as he lay wounded outside the offices.

He has been named as Ahmed Merabet and is believed to be a Muslim.

French President Francois Hollande has declared today a national day of mourning and a nationwide minute of silence is planned for noon (11am UK time).

With the country on maximum terror alert, more than 800 extra soldiers are helping to guard media offices, places of worship and other sensitive areas.

Tens of thousands of people have staged silent protests in France and across the world.

Social media users have used the hashtag #jesuischarlie to show solidarity for the victims of the shooting, with the Charlie Hebdo website also using the image as its masthead.

In the wake of the attack, the British Government is holding an emergency Cobra meeting chaired by the Home Secretary this morning - as British police plan a moment of "solidarity and sympathy" with the victims at 10.30am.

:: Sky News is running a poll on social media today: "Should the media publish satirical religious cartoons?". You can vote on Facebook or Twitter using hashtags #SkyPollYes or #SkyPollNo

:: Watch the Sky News Debate: Is Freedom Of Speech More Important Than Religion? You can join in from 10.30am by tweeting us on @skynews - including the phrase #skydebate - texting on 84501 or emailing news@sky.com.

:: The debate, and a special report on the shootings at 2.30pm, 4.30pm, 8.30pm and 9.30pm, can be watched live on skynews.com, our mobile apps and on channels Sky 501, Virgin Media 602, Freesat 202, Freeview 132.