A massive manhunt was underway for the assailants who stormed the Paris offices of French satirical newspaper Charlie Hebdo, killing 12 people on Wednesday.A police union official said the assailants remained at liberty and there were fears of further attacks. Media reports suggested that the men were armed with Kalashnikov assault rifles and possibly a rocket launcher. One of the men was captured on video shouting "Allah!" as four shots rang out. Two assailants were then seen calmly leaving the scene.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. 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.A short 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 and strides over to him to shoot him dead at point-blank range. The two then walk over to a black saloon car and drive off. Police said that there may be three suspects in total, who should be considered to be armed and extremely dangerous.In another clip on Television station iTELE, they are heard shouting: "We have killed Charlie Hebdo. We have avenged the Prophet Mohammad."A police official said the gunmen fled towards the eastern Paris suburbs after holding up a car."There is a possibility of other attacks and other sites are being secured," Police union official Rocco Contento said.Sirens could be heard across Paris as Prime Minister Manuel Valls said security would be ramped up at transport hubs, religious sites, media offices and department stores.The White House said US security officials were in contact with their French counterparts."We are in touch with French officials and I have directed my administration to provide any assistance needed to help bring these terrorists to justice," US President Barack Obama said in a statement.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."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.While there was no immediate claim for the shooting, one supporter of Islamic State suggested in a tweet the image of Mohammed was the reason for the attack.