U.S. President Joe Biden announced on Monday that al-Qaida leader Ayman al-Zawahri was killed in a drone strike carried out by the CIA in Kabul, an operation he hailed as delivering "justice" while expressing hope that it brings "one more measure of closure" to families of the victims of the Sept. 11, 2001 attacks on the United States.

Current and former officials began hearing Sunday afternoon that al-Zawahri had been killed in a drone strike, but the administration delayed releasing the information until his death could be confirmed, according to the person who spoke on the condition of anonymity to discuss the matter.

In a statement, Taliban spokesperson Zabihullah Mujahid confirmed that a strike took place and strongly condemned it, calling it a violation of "international principles."

The president said in an evening address from the White House that U.S. intelligence officials tracked al-Zawahri to a home in downtown Kabul where he was hiding out with his family. The president approved the operation last week and it was carried out Sunday.

Al-Zawahri and the better known Osama bin Laden plotted the 9/11 attacks that brought many ordinary Americans their first knowledge of al-Qaida. Bin Laden was killed in Pakistan on May 2, 2011, in an operation carried out by U.S. Navy Seals after a nearly decadelong hunt.

"He will never again, never again, allow Afghanistan to become a terrorist safe haven because he is gone and we’re going to make sure that nothing else happens," Biden said. "This terrorist leader is no more."

The operation is a significant counterterrorism win for the Biden administration just 11 months after American troops left the country after a two-decade war.

The strike was carried out by the Central Intelligence Agency, according to five people familiar with the matter who spoke on the condition of anonymity. Neither Biden nor the White House detailed the CIA's involvement in the strike.

Biden, however, paid tribute to the U.S. intelligence community in his remarks, noting that "thanks to their extraordinary persistence and skill" the operation was a "success."

Al-Zawahri’s loss eliminates the figure who more than anyone shaped al-Qaida, first as bin Laden’s deputy since 1998, then as his successor. Together, he and bin Laden turned the extremist movement’s guns to target the U.S., carrying out the deadliest attack ever on American soil – the Sept. 11 suicide hijackings.

Over the 20-year war in Afghanistan, the U.S. targeted and splintered al-Qaida, sending leaders into hiding. But America’s exit from Afghanistan last September gave the extremist group the opportunity to rebuild. U.S. military officials, including Gen. Mark Milley, chairperson of the Joint Chiefs of Staff, have said al-Qaida was trying to reconstitute in Afghanistan, where it faced limited threats from the now-ruling Taliban. Military leaders have warned that the group still aspired to attack the U.S.

After his killing, the White House sought to underscore that al-Zawahri had continued to be a dangerous figure.

The 2001 attacks on the World Trade Center and Pentagon made bin Laden America’s Enemy No. 1. But he likely could never have carried it out without his deputy. Bin Laden provided al-Qaida with charisma and money, but al-Zawahri brought the tactics and organizational skills needed to forge militants into a network of cells in countries around the world.

Senior Taliban figures were aware of al-Zawahri’s presence in Kabul, according to the official, who added the Taliban government was given no forewarning of the operation.

Inside the Biden administration, only a small group of officials at key agencies, as well as Vice President Kamala Harris, were brought into the process.

'If you're threat, U.S. will find you'

"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, U.S. will find you and take you out," Biden said.

Al-Zawahri, on the FBI’s Most Wanted Terrorist list, had a $25 million bounty on his head for any information that could be used to kill or capture him.

When the 2001 U.S. invasion of Afghanistan demolished al-Qaida’s safe haven and scattered, killed and captured its members, al-Zawahri ensured al-Qaida’s survival. He rebuilt its leadership in the Afghan-Pakistan border region and installed allies as lieutenants in key positions.

More recently, the al-Qaida affiliate in Yemen proved itself capable of plotting attacks against U.S. soil with an attempted 2009 bombing of an American passenger jet and an attempted package bomb the following year.

But even before bin Laden’s death, al-Zawahri was struggling to maintain al-Qaida’s relevance in the changing Middle East.

He tried with little success to coopt the wave of uprisings that spread across the Arab world starting in 2011, urging Islamic hard-liners to take over in the nations where leaders had fallen. But while many gained prominence in the region, they have stark ideological differences with al-Qaida and reject its agenda and leadership.

Nevertheless, al-Zawahri tried to pose as the Arab Spring’s leader. America "is facing an Islamic nation that is in revolt, having risen from its lethargy to a renaissance," he said in a video eulogy to bin Laden.

Al-Zawahri was also a more divisive figure than his predecessor. Many militants described the soft-spoken bin Laden in adoring and almost spiritual terms.

In contrast, al-Zawahri was notoriously prickly and pedantic. He picked ideological fights with critics, wagging his finger scoldingly in his videos. Even some key figures in al-Qaida’s central leadership were put off, calling him overly controlling, secretive and divisive.

Some militants whose association with bin Laden predated al-Zawahri’s always saw him as an arrogant intruder.

There were rumors of Zawahiri's death several times in recent years, and he was long reported to have been in poor health.

A statement from Afghanistan’s Taliban government confirmed the airstrike but did not mention al-Zawahri or any other casualties. It said it "strongly condemns this attack and calls it a clear violation of international principles and the Doha Agreement," the 2020 U.S. pact with the Taliban that led to the withdrawal of American forces.

"Such actions are a repetition of the failed experiences of the past 20 years and are against the interests of the United States of America, Afghanistan, and the region," the statement said.