Peshawar attack mastermind Umar Mansoor
In a video released on a website used by the Taliban, a man identified as Umar Mansoor explains the reasons behind the attack YouTube

A 36-year-old father of three has been identified as the man behind a Taliban attack in a school in Peshawar which killed 141 people, of whom 132 were children.

Umar Mansoor - who also goes by the nickname "Nary," a word in the Pashto language meaning "slim" - released a triumphalistic video in the aftermath of the massacre, the bloodiest attack in Pakistan's history.

In the video, released on a website frequently used as a conduit for Taliban messages, Mansoor explains the reasons behind the attack.

"If our women and children die as martyrs, your children will not escape," he said. "We will fight against you in such a style that you attack us and we will take revenge on innocents."

It is believed the brutal assault was retaliation for the Pakistani army offensive against the Taliban in Waziristan, after the military allegedly killed civilians in remote areas close to the border with Afghanistan.

Shortly after the massacre, Taliban spokesman Muhammad Umar Khorasani said: "We selected the army's school for the attack because the government is targeting our families and females. We want them to feel the pain."

Others have alleged that the attack also aimed to send a message to the global recognition of Pakistani schoolgirl activist Malala Yousafzai, who was awarded the 2014 Nobel Peace Prize for advocating education for women and children.

Six Pakistani Taliban interviewed by Reuters confirmed the mastermind was Mansoor. Four of them said he is close to Mullah Fazlullah, leader of the fractious group who ordered the attempted assassination of Malala.

"He strictly follows the principles of jihad," one said. "He is strict in principles, but very kind to his juniors. He is popular among the juniors because of his bravery and boldness."

Other insurgents said Mansoor received a high school education in the capital Islamabad and later studied in a madrassa, a religious school.

"Umar Mansoor had a tough mind from a very young age, he was always in fights with other boys."

Mansoor has two brothers and worked as a labourer in the city of Karachi before joining the Taliban soon after it was formed, in late 2007, said one commander.

"[Mansoor] likes to play volleyball," another one explained. "He is a good volleyball player. Wherever he shifts his office, he puts a volleyball net up."

The Taliban video describes him as the "amir", or leader, of Peshawar and nearby Darra Adam Khel. Mansoor deeply opposes talks with the government, the commanders said.

"He was very strict from the start when he joined," a commander said. "He left many commanders behind if they had a soft corner [of their heart] for the government."

Mansoor has now become Pakistan's Public Enemy number one. Following the school attack, the Pakistani government lifted a moratorium on the death penalty and announced it would hang the men who planned the slaughter.

peshawar mourners
Osama Taher's mother is surrounded by relatives as she mourns her son at her house in Peshawar Zohra Bensemra/Reuters