Al-Qaeda chief Ayman al-Zawahiri had a $25 million bounty on his head.

Context: Sheltered in Kabul, Al Zawahiri was killed on the balcony of his safe house, a compound owned by a senior Taliban leader.

Zawahiri was deeply involved, US President Biden said, in the "planning of 9/11, one of the most responsible for the attacks that murdered 2,977 people on American soil. For decades, he was the mastermind of attacks against Americans."

• "I authorized a precision strike that would remove him from the battlefield, once and for all," Biden said.

• "Now, justice has been delivered and this terrorist leader is no more. People around the world no longer need to fear the vicious and determined killer," he said, according to CNN.

• "We make it clear again tonight, that no matter how long it takes, no matter where you hide, if you are a threat to our people, the United States will find you and take you out," Biden said.

Interesting choice of weapon: Al-Zawahiri was killed by two missiles fired at his safe house by a US drone.

• The drones are believed to have used Hellfire R9X, a warhead-less missile, for the operations, reports said.

• A modified version of the widely used Hellfire missile, the R9X has been specially built to kill terrorists without an explosion.

• This design choice minimises damage and drastically reduces the likelihood of civilian casualties.

Called 'the flying Ginsu' or 'Ninja bomb' as well, the missile is designed to plunge more than 100 pounds of metal.

• For this purpose, it is also equipped with six long, razor-like blades that are stowed inside the fuselage and deployed seconds before the missile impacts the target.

• Using these blades, the missile slices through its target, but does not explode.

Gone through the grind: The weapon was under development as early as 2011.

• It appeared for the first time in March 2017 when the US eliminated al-Qaeda deputy leader (directly below al-Zawahiri) Abu al-Khayr al-Masri while he was travelling in a car in Syria.

• The photos of al-Masri's vehicle that emerged a few days later showed a large hole through the roof, with the car's metal and interior shredded and the front and rear parts intact.

• "A missile with similar capabilities was considered as a “Plan B” to kill al Qaeda leader Osama bin Laden in his compound in Abbottabad, Pakistan, that year, according to several of the officials," the Washington Post said in a report on the mystery weapon in May 2019.

Bottom line: Ayman al-Zawahiri's killing marks another milestone in America's seemingly never-ending war on terror.