Paris - One suspect in the fatal attack on the Paris offices of the satirical magazine Charlie Hebdo has turned himself in, as French police continued their search early on Thursday for two other suspects in the city of Reims, north-east of Paris.The 18-year-old suspect in custody turned himself in to police in Charleville-Mzires, about 70km north-east of Reims, French news agency AFP reported, after the attack which killed 12 people, most of them journalists.The teenager went to police after seeing his name circulating on social media, AFP said, quoting unnamed sources close to the investigation. Several people linked to the two other suspects in the attack were also in custody, AFP said.French police published pictures of two brothers said to have been behind the attack. They are 34-year-old Said Kouachi and 32-year-old Cherif Kouachi.They are likely "armed and dangerous," the police said in their appeal for witnesses.The two brothers are from Paris with French citizenship and have links to organisations recruiting jihadist fighters, according to local media quoting investigators.The attack on the Paris headquarters of Charlie Hebdo on Wednesday left 12 people dead in what authorities say was the worst terrorist attack in France in decades.Two masked gunmen with automatic rifles stormed the building in central Paris at midday, killing two police and staffers, including the editor and three well-known cartoonists at the magazine. The terrorists cried "Allah is great" and "We have avenged the prophet," witnesses said.One witness, Corinne Ray, told the newspaper l'Humanite the attackers said they belonged to the terrorist group al-Qaeda.The publication often prints cartoons of the Islamic prophet Muhammad, and has been targeted in the past.Video footage of the midday attack taken from an office across the street from Charlie Hebdo showed the two gunmen leaving the building, one of them shooting a policeman before they drove away in a small black car.The dead include editor Stephane Charbonnier, known as Charb; Jean Cabut; Georges Wolinksi; and Bernard Velhac. Economics columnist Bernard Maris also was among the victims, according to Radio France. Eight of the dead were journalists, according to Le Monde. Two policemen also died.Of the 11 reported wounded, four were in critical condition.French President Francois Hollande called for a national day of mourning and thousands of Parisians gathered for a silent vigil to oppose the attack on free expression. Similar events were organized nationwide involving many more thousands.Peaceful protesters gathered near the site of the shooting in the early evening to mourn the victims and condemn the attack on press liberty.Online and in the streets, people adopted the slogan "Je suis Charlie" (I am Charlie) to express support for the weekly magazine.Charlie Hebdo's headquarters were firebombed in November 2011 after it published a spoof issue that featured the prophet Muhammad as editor. No one was injured in that attack.In January 2013 it published a 64-page issue described as part one in a series of cartoons depicting the life of the prophet Mohammed. Its latest issue features a cartoon of novelist Michel Houellebecq, who recently published a book about a futuristic France in which an Islamic government rules.A few minutes before the attack, Charlie Hebdo tweeted a cartoon of Islamic State leader Abu Bakr al-Baghdadi with the caption: "Best wishes, in fact."France has thwarted several attempts to carry out terrorist attacks over the last few weeks, Hollande told journalists, saying that the government will beef up security at department stores, media outlets, places of worship and on public transportation."Our best weapon is the unity of our citizens," Hollande said in an evening address to the nation, adding that the perpetrators would be chased down, judged and punished. Flags across France will flag at half-mast for three days.