The United States killed al Qaeda boss Ayman al-Zawahiri in a drone strike Sunday, following a more than 20-year effort to assassinate the terrorist, writes dailymail.co.uk.

Labeled by U.S. officials as Osama bin Laden’s number-2, al-Zawahiri, 71, was a key plotter of the September 11 terrorist attacks and took over as the leader of the notorious terror group following bin Laden’s death in 2011.

The strike was carried out early Sunday at an Afghanistan safe house the elderly terrorist had be holed up in, at 6:18 am local time and 9:48 pm Saturday in the U.S. The early morning attack saw al-Zawahiri killed by two hellfire missiles fired from drones deployed by the CIA, as he stood on the balcony of the safe house in downtown Kabul.

The Egyptian-born jihadist apparently proceeded to linger on the landing, as U.S. intelligence had noted he often did, allowing the drone ample time to execute the attack. The mission, officials said, took 6 months to plan - but served as the culmination of a much wider, carefully coordinated campaign to track down and kill the al Qaeda head, who had successfully evaded U.S armed forces up until this point.

The slaying came more than 11 years after bin Laden was killed in Pakistan by U.S. Navy Seals on May 2, 2011, in a more hands on, drone-less strike that came after a nearly decade-long hunt for the 9/11 mastermind.