Key Points
Gunmen have attacked the offices of French satirical magazine Charlie Hebdo in Paris
Twelve people have been killed, four are seriously injured
Four cartoonists, including the magazine's editor-in-chief, are reportedly among those killed
Paris has been placed on the highest level of alert following the attack
President Hollande called it an act of "extreme barbarity"
There has been international condemnation, with UK Prime Minister David Cameron calling the act "sickening"
In 2011, the publication came under attack after naming the Prophet Muhammad as its "editor-in-chief"