The Pakistani Taliban which claimed responsibility for Tuesday's school rampage that killed at least 141 people is the country's main Islamist rebel group and was also behind the shooting of Nobel Peace laureate Malala Yousafzai.
Considered to have close ties to al Qaeda and listed as a terrorist group by the United States since September 1, 2010, the 'Tehreek-e-Taliban Pakistan' (TTP) is a loose coalition of jihadist militants that has killed thousands of Pakistanis since its formation in 2007.
The organisation, which was formed in reaction to a raid by Pakistani authorities on a radical mosque in Islamabad, declared a holy war against the Pakistani government which it accuses of helping the US in its "war on terror".
An injured Pakistani student lies in bed at a hospital following an attack by Taliban gunmen on a school in Peshawar on Tuesday. (AFP photo)
The group said its attack on Tuesday on the army-run school in the northwestern city of Peshawar came in response to an offensive launched against it by government troops in mid-June.
The TTP's strongholds are located in the country's semi-autonomous tribal zones close to the border with Afghanistan and the regional centre of jihadist activity since Taliban fighters were driven out of Afghanistan in late 2001.
Its first chief was the brutal Baitullah Mehsud who was killed by a US drone strike in 2009, and later replaced by a member of the same tribe, Hakimullah Mehsud.
On October 9, 2012, the group intercepted a bus that was carrying schoolgirl activist Malala, who was just 15 at the time but had already gained fame for campaigning for the right of girls to education.
The group said it had shot her in the face for supporting secularism and for her "campaign against Sharia" or Islamic law.
In November 2013, a US drone strike killed Hakimullah Mehsud. Following a period of infighting, he was replaced by Mullah Fazlullah, who became known as Mullah Radio for his nightly broadcasts to residents of the Swat Valley, which his forces controlled from 2007 to 2009.
Up to that point, all TTP leaders had come from the Mehsud tribe, which represents the group's backbone.
Pakistani officials condemned the drone strike because it took place just as the government was trying to get the rebels to join a peace process.
In mid-June, after fresh negotiations fell through and following repeated requests from Washington as US forces prepare to withdraw from Afghanistan, the Pakistani army launched an offensive against the TTP in North Waziristan.
It has also been riven by more infighting this year.
In May, TTP commander Khan Said Sajna left the group, and in September fighters from the Mohmand tribal zone said announced the creation of a new dissident faction called Jamaat ul-Ahrar.
Meanwhile, leaflets and graffiti in support of the Islamic State (IS) group, which controls territory in Syria and Iraq, have appeared in northwestern Pakistan in a sign of growing support for the newer organisation.
On October 22, TTP said it had fired spokesman Shadidullah Shahid, also known as sheikh Abu Omar Maqbool, for having sworn allegiance to the IS without permission from Mullah Omar, the spiritual leader of the Afghan Taliban.
Videos have also implicated the TTP in preparations of an attack in Barcelona, Spain in early 2008, and in one that was to have hit New York's Times Square on May 1, 2010.
A TTP spokesperson has denied any ties with the man who was detained in connection with the latter attack, however.