Al Qaeda leader Ayman al-Zawahiri was killed in a U.S. strike in Afghanistan over the weekend, the biggest blow to the militant group since its founder Osama bin Laden was killed in 2011.

Zawahiri had been in hiding for years and the operation to locate and kill him was the result of “careful patient and persistent” work by the counter-terrorism and intelligence community, a senior administration official told reporters.

Until the U.S. announcement, Zawahiri had been rumored variously to be in Pakistan’s tribal area or inside Afghanistan.

Speaking on the condition of anonymity, the official provided the following details on the operation:

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* For several years, the U.S. government had been aware of a network that it assessed supported Zawahiri, and over the past year, following the United States’ withdrawal from Afghanistan, officials had been watching for indications of Al Qaeda’s presence in the country.

This year, officials identified that Zawahiri’s family – his wife, his daughter and her children – had relocated to a safe house in Kabul and subsequently identified Zawahiri at the same location.

Read more: Reuters