Hooded gunmen stormed the Paris offices of a weekly satirical magazine known for lampooning radical Islam, killing at least 12 people, including two police officers in the worst militant attack on French soil in decades. One of the men was captured on video outside the building waving his arms and shouting "Allah!". After dozens of shots rang out, two assailants were seen calmly leaving the scene. One police officer was seen being shot as he lay wounded. 23 Heartbreaking Cartoons From Artists Responding To The Charlie Hebdo Shooting via @broderick @buzzfeednews \u2014 Madhuchhanda Bose (@madhuchhanda11) January 8, 2015 \u00a0 A police union official said the assailants - Interior Minister Bernard Cazeneuve said there were three - remained at liberty and there were fears of further attacks. The government declared the highest state of alert, increasing security at transport hubs, religious sites, media offices and department stores as the search for the assailants got under way. Charlie Hebdo (Charlie Weekly) is well known for courting controversy with satirical attacks on political and religious leaders and has published numerous cartoons ridiculing the Prophet Mohammad. Jihadists online have repeatedly warned that the magazine would pay for its ridicule. The last tweet on its account mocked Abu Bakr al-Baghdadi, the leader of the militant Islamic State, which has taken control of large swathes of Iraq and Syria and called for "lone wolf" attacks on French soil. President Francois Hollande, who will address the nation on television at 8:00 pm (1900 GMT), rushed to the scene of what appeared to be a carefully-planned attack. Sirens could be heard across the city. "An act of indescribable barbarity has just been committed today in Paris," he said. "Measures have been taken to find those responsible, they will be hunted for as long as it takes to catch them and bring them to justice." \u00a0 Check out the photo posted on Twitter during the Paris terror attack: Devant Charlie Hebdo pic.twitter.com\/FPrRNIyhx9 \u2014 Soren Seelow (@soren_seelow) January 7, 2015 \u00a0\u00a0An amateur video broadcast by French television stations shows two hooded men outside the building. One of them sees a wounded policeman lying on the ground, rushes over to him and shoots him dead at point-blank range with a rifle. The two then walk over to a black saloon car. One casually picks up a shoe left on the ground, and then they drive off. In another clip on Television station iTELE, the men are heard shouting in French: "We have killed Charlie Hebdo. We have avenged the Prophet Mohammad." Here is the initial reaction on Twitter - watch video: Attaque \u00e0 #CharlieHebdo : "C'est un attentat terroriste", d\u00e9clare Fran\u00e7ois Hollande \u2014 Le Monde (@lemondefr) January 7, 2015 \u00a0 A witness quoted by 20 Minutes daily newspaper said one of the assailants cried out before entering the car: "Tell the media that it is al Qaeda in Yemen!" The gunmen fled eastwards towards the Paris suburbs, dumping their car in a residential area, police said. They then hijacked another car before running over a pedestrian and disappearing. "There is a possibility of other attacks and other sites are being secured," police union official Rocco Contento said. Check out the photo posted on Twitter during the Paris terror attack: Les tireurs de #ChalieHebdo face \u00e0 une voiture de police. Ils ont fait feu, les policiers ont r\u00e9pliqu\u00e9 puis recul\u00e9 pic.twitter.com\/Ld1sxkRLvW \u2014 Elise Barthet (@EliseBarthet) January 7, 2015 By late afternoon the police presence in central Paris was significantly heavier. A Reuters reporter saw groups of armed policeman patrolling around the Grands Magasins department stores in the shopping district and there was an armed gendarme presence outside the Arc de Triomphe. Speaking in French, U.S. Secretary of State John Kerry, who has close ties to France, condemned the attack. [Live] Suivez en direct l'attaque meurtri\u00e8re \u00e0 "Charlie Hebdo" \/\/ pic.twitter.com\/GMBEbilHDW \u2014 Les D\u00e9codeurs (@decodeurs) January 7, 2015 The White House said France had been one of the stalwart allies of the United States in the fight against Islamic State, while acknowledging it was not yet clear who was responsible for the attack in Paris. Another 20 people were injured in the attack, including four or five critically. Police union official Contento described the scene inside the offices as "carnage". Ten members of Charlie Hebdo staff died in the attack. Sources at the weekly said the dead included co-founder Jean "Cabu" Cabut and editor-in-chief Stephane "Charb" Charbonnier. "Two black-hooded men entered the building with Kalashnikovs (rifles)," witness Benoit Bringer told TV station iTELE. "A few minutes later we heard lots of shots." In a video shot by journalist Martin Boudot from a rooftop near the magazine's offices, a man can be heard screaming "Allah"; then followed the sound of three or four shots. A firebomb attack gutted the headquarters of Charlie Hebdo in November 2011 after it put an image of the Prophet Mohammad on its cover in what it described as a Shariah edition. France last year reinforced its anti-terrorism laws and was already on alert after calls from Islamist militants to attack its citizens and interests in reprisal for French military strikes on Islamist strongholds in the Middle East and Africa. "I am extremely angry. These are criminals, barbarians. They have sold their soul to hell. This is not freedom. This is not Islam and I hope the French will come out united at the end of this," said Hassen Chalghoumi, imam of the Drancy mosque in Paris's Seine-Saint-Denis northern suburb. GUNMEN FLED Dozens of police and emergency services were at the site as police secured a wide perimeter around the shooting site, where a Reuters reporter saw a car riddled with bullet holes. German Chancellor Angela Merkel was among European leaders condemning the the shooting. "This abominable act is not only an attack on the lives of French citizens and their security. It is also an attack on freedom of speech and the press, core elements of our free democratic culture." The New York-based Committee to Protect Journalists described the attack as a brazen assault on free expression. The scale of the violence is appalling," said CPJ Deputy Director Robert Mahoney. "Journalists must now stand together to send the message that such murderous attempts to silence us will not stand." Late last year, a man shouting "Allahu Akbar" ("God is greatest") injured 13 by ramming a vehicle into a crowd in the eastern city of Dijon. French officials say several attacks were prevented in recent weeks and Valls has said France had "never before faced such a high threat linked to terrorism". While there was no early claim for the shooting, supporters of Islamic State and other jihadist groups hailed the attack on Internet sites, suggesting the image of Mohammad was the reason for it. The last major attack in Paris was in the mid-1990s when the Algerian Armed Islamic Group (GIA) carried out a spate of attacks, including the bombing of a commuter train in 1995 which killed eight people and injured 150. French weekly has history of angering Muslims with cartoons (AP) The French newspaper Charlie Hebdo's staple is to be provocative - poking fun at popes, presidents as well as the Prophet Muhammad. The satirical weekly attacked Wednesday by gunmen, killing at least 12, has a history of drawing outrage across the Muslim world with crude cartoons of Islam's holiest figure. The magazine's offices were firebombed in November 2011 after it published a spoof issue that ''invited'' Muhammad to be its guest editor and put his caricature on the cover. A year later, the magazine published more Muhammad drawings amid an uproar over an anti-Muslim film. The cartoons depicted Muhammad naked and in demeaning or pornographic poses. As passions raged, the French government defended free speech even as it rebuked Charlie Hebdo for fanning tensions. The small-circulation weekly leans toward the left and takes pride in making acerbic commentary on world affairs through cartoons and spoof reports. ''We treat the news like journalists. Some use cameras, some use computers. For us, it's a paper and pencil,'' the Muhammad cartoonist, who goes by the name Luz, told The Associated Press in 2012. ''A pencil is not a weapon. It's just a means of expression.'' Les dessinateurs \u00ab horrifi\u00e9s \u00bb apr\u00e8s l'attentat contre leurs \u00abcopains\u00bb de #CharlieHebdo pic.twitter.com\/6ye5XfH4Pm \u2014 Le Monde (@lemondefr) January 7, 2015 Chief editor Stephane Charbonnier, who publishes under the pen name ''Charb,'' has also defended the Muhammad cartoons. ''Muhammad isn't sacred to me,'' he told The AP in 2012. ''I don't blame Muslims for not laughing at our drawings. I live under French law. I don't live under Quranic law.'' Islam is not alone in being singled out by Charlie Hebdo's satire. Past covers include retired Pope Benedict XVI in amorous embrace with a Vatican guard; former French President Nicolas Sarkozy looking like a sick vampire; and an Orthodox Jew kissing a Nazi soldier. The magazine occasionally publishes investigative journalism, taking aim at France's high and mighty. Charlie Hebdo has come under pressure ever since its 2011 Muhammad issue. Its website has been hacked, and Charbonnier has needed police protection. Riot police guarded the magazine's offices after the 2012 issue hit the stands. Obama spokesman: US stands with French in time of suffering President Barack Obama's top spokesman says the United States is determined to help the French apprehend those responsible for the attack on a satirical Paris newspaper that left at least 12 people dead. Press secretary Josh Earnest says U.S. officials have been in close contact with the French after the bloody attack today on the Charlie Hebdo weekly. Earnest tells CNN the French have been ''stalwart allies'' in the U.S. fight against Islamic State extremists. The spokesman also says, ''We know they are not going to be cowed by this terrible act.'' Earnest also says Washington will work hard to protect Americans at home and abroad. He says, ''We obviously are trying to monitor what we consider to be a very important threat, which is the threat of foreign fighters.''