Islamabad: The mastermind behind the December 2014 massacre at Peshawar's Army Public School, has been killed in a US drone strike in Afghanistan, senior Pakistani security officials claimed. The officials told Dawn on Monday that Umar Mansour was killed, along with another militant leader Qari Saifullah, in the US drone attack in the Bandar area of Afghanistan’s Nangarhar province on Saturday. One of the officials said they had credible reports of Mansour having been killed with Saifullah, who was in charge of suicide bombers. “What we have is pretty credible,” the official said. On May 25, the US State Department had declared Umar Mansour a global terrorist, thus clearing the path for his inclusion in the hit-list. The State DepartmentÂ’s announcement came four days after a drone strike killed Afghan Taliban leader Akhtar Mansour in Balochistan on May 21. Mansour orchestrated the attack on December 16, 2014, on the Army Public School which killed 122 students and 22 teachers. It was one of the worst terrorist attacks which prompted the government to declare an all-out war against terrorist outfits in the country. Mansour and Saifullah belonged to Tehreek-i-Taliban Pakistan's (TTP) Tariq Geedar group and used to operate in Peshawar and adjoining Frontier Region Peshawar and Darra Adamkhel. Mansour fled to Afghanistan following a military operation in Khyber tribal region and continued to operate from there. He was held responsible for the September 2015 attack on a Pakistan Air Force base in Peshawar’s Badaber area, which killed 29 people. He was also behind the January 2016 attack on the Bacha Khan University in Charsadda in which 18 students and faculty members lost their lives.