Anonymous declared war on Islamic extremists Friday and promised to take revenge for the attack on French satirical magazine Charlie Hebdo.

In a video posted on YouTube, the group of hackers said they would track down websites and social media networks linked to terrorists, and take them down.

"We, Anonymous around the world, have decided to declare war on you the terrorists," it said.

The video is described as a message for "al Qaeda, the Islamic State and other terrorists," and promises to avenge the killing of 12 people in Wednesday's attack.

"We intend to take revenge in their name, we are going to survey your activities on the net, we are going to shut down your accounts on all social networks," Anonymous said.

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Cherif and Said Kouachi, the brothers wanted for the massacre at Charlie Hebdo's offices in Paris, were killed Friday by French security forces in a town where they had been surrounded, according to a local official.

Said spent several months in Yemen in 2011, receiving weapons training and working with Al Qaeda in the Arabian Peninsula, U.S. officials said Friday.

His younger brother Cherif was sentenced to three years in prison for being part of a jihadist recruitment ring in Paris that sent fighters to join the conflict in Iraq.

An ISIS radio broadcast Thursday praised the attackers, calling them "brave jihadists." But the broadcast did not say whether the two had any connection to the militant group.

The attack has prompted an outpouring of solidarity, with thousands joining rallies in defense of freedom of speech and republishing many of the magazine's most controversial cartoons -- including of the Prophet Mohammed.

Anonymous has hacked websites belonging to government departments, companies and other organizations. The loose collective is also known for supporting the Occupy movement.