Al-Zawahiri, in 2001, had overseen the 9/11 attacks – which had killed 2,977 people – along with Osama bin-Laden, who was killed by the US in 2011 in Pakistan.
After the attack on Monday, Biden said in a tweet:
"This mission was carefully planned, rigorously minimising the risk of harm to other civilians. And one week ago, after being advised the conditions were optimal, I gave the final approval to go get him and the mission was a success."
But how was the mission planned? What missile was used to kill Zawahiri? Here’s what we know.
How US Intelligence Tracked Down Zawahiri
A senior US administration official told reporters that 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, AFP reported.
Further, speaking on the condition of anonymity, an official provided the following details on the operation:
The US government had been aware of a network that supported Zawahiri
Since last year’s withdrawal, officials had been watching for indications of Al Qaeda's presence in the country
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
Intelligence officials in early April started briefing senior administration officials. Jake Sullivan, National Security Advisor, subsequently briefed President Joe Biden.
Once al-Zawahiri arrived at the Kabul safe house, officials were not aware of him ever leaving it. He was identified multiple times on the balcony – where he was ultimately struck.
The construction and nature of the safe house was investigated for US to ensure that they could confidently conduct an operation to kill al-Zawahiri without threatening the structural integrity of the building and minimising the risk to civilians and al-Zawahiri's family.
On 1 July, Biden was briefed on a proposed operation in the White House Situation Room by members of his cabinet including CIA Director William Burns.
Senior inter-agency lawyers examined intelligence and confirmed that the al-Qaeda chief was a lawful target.
The strike was ultimately carried out at 9:48 pm ET on 30 July by a drone firing so-called "hellfire" missiles.
What Is a Hellfire Missile? Why Was There No Explosion?
Zawahiri was killed by two missiles fired at his Kabul home. However, there was no explosion and no one else was harmed, US officials said.
This points to the use of the Hellfire R9X, a warhead-less missile – believed to be equipped with six razor-like blades extending from the fuselage that slices through its target but does not explode.
The R9X first appeared in March 2017 when Al-Qaeda senior leader Abu al-Khayr al-Masri was killed by a drone strike while travelling in a car in Syria.
The mysterious weapon has been dubbed the "flying ginsu," after a famous 1980s television commercial for Japanese kitchen knives that would cut cleanly through aluminium cans and remain perfectly sharp.
The munition of choice for targeting leaders of extremist groups has also been dubbed as the "ninja bomb," which successfully avoids civilian casualties.
(With inputs from AFP.)