Pakistani intelligence agency's relations with militant groups is being blamed for the brutal assault at Peshawar's Army Public School that left 141 people dead, including 132 children.
According to the Guardian, western intelligence officials also believe the ISI has close relations with the Afghan Taliban and the Haqqani network, an insurgent faction which has repeatedly struck international targets in Afghanistan.
The spy agency is also said to have links with sectarian groups and militant groups within Pakistan that have been fighting the Indian security forces in Kashmir.
It is believed that ISI's continuing relationships with terror groups in an attempt to fight the Indian army has led to the carnage.
While a few blame the attack on the military-run school in Peshawar on ISI's secret relationship with militants, others argue that there is hardly any connection between the Pakistani Taliban and the spy agency.
Michael Kugelman of the Woodrow Wilson centre in Washington says "the military formally and institutionally considers the TTP as an enemy of the state as it has killed many soldiers over the years."
Meanwhile, Ajai Sahni, an Indian security analyst, said that the ISI's use of certain militant groups as strategic assets makes concerted action against others impossible.
