Washington -- Al Qaeda leader Ayman al-Zawahiri's death at the hands of a US drone strike brings an end to his "trail of murder and violence against American citizens," President Joe Biden said Monday evening.
Zawahiri, 71, was a key architect behind multiple assaults on the US, and was "deeply involved" in the planning of the September 11, 2001 terror attacks, Biden said.
"People around the world no longer need to fear the vicious and determined killer. The United States continues to demonstrate our resolve and our capacity to defend the American people against those who seek to do us harm," Biden said from the Blue Room Balcony of the White House.
Here's what you need to know about Zawahiri and the US' strike against him.
How did Zawahiri rise to power?
Born in 1951, Zawahiri grew up in an upper-class neighborhood in Cairo, Egypt, the son of a prominent physician and grandson of renowned scholars.
His grandfather, Rabia'a al-Zawahiri, was an imam at al-Azhar University in Cairo. His great-uncle, Abdel Rahman Azzam, was the first secretary of the Arab League.
Zawahiri was imprisoned for his involvement in the 1981 assassination of Egyptian President Anwar Sadat.
"We want to speak to the whole world. Who are we? Who are we?" he said in a jailhouse interview.
By that time, Zawahiri, a young doctor, was already a committed terrorist who conspired to overthrow the Egyptian government for years and sought to replace it with fundamentalist Islamic rule. He proudly endorsed Sadat's assassination after the Egyptian leader made peace with Israel.
What was his relationship with Osama bin Laden?
Zawahiri left Egypt in 1985 and made his way to Peshawar, Pakistan, where he worked as a surgeon treating the fighters who were engaged with Soviet troops in Afghanistan.
That is where Zawahiri met bin Laden, a prominent Mujahedeen leader and who also had left behind a privileged upbringing to join the fight in Afghanistan. The two became close, linked by their common bond as "Afghan Arabs."
After reuniting in Afghanistan, bin Laden and Zawahiri appeared together in early 1998 announcing the formation of the World Islamic Front for the Jihad Against the Jews and the Crusaders -- formally merging the Egyptian Islamic Jihad and al Qaeda.
At one point, he acted as bin Laden's personal physician.
"We are working with brother bin Laden," he said in announcing the merger of his terror group in May 1998. "We know him since more than 10 years now. We fought with him here in Afghanistan."
Osama bin Laden sits with Ayman al-Zawahiri on November 10, 2001.
Together, the two terror leaders signed a fatwa, or declaration: "The judgment to kill and fight Americans and their allies, whether civilians or military, is an obligation for every Muslim."
What role did Zawahiri play in al Qaeda's attacks against the US?
The attacks against the US and its facilities began shortly after bin Laden and Zawahiri's fatwa, with the suicide bombings of US embassies in Kenya and Tanzania that killed more than 200 people and wounded more than 5,000 others.
Then, there was the attack on the USS Cole in Yemen in October 2000, when suicide bombers on a dinghy detonated their boat, killing 17 American sailors and wounding 39 others.
The culmination of Zawahiri's terror plotting came on September 11, 2001, when nearly 3,000 people were killed in the attacks on the twin towers of the World Trade Center and Pentagon. A fourth hijacked airliner, headed for Washington, crashed in a Pennsylvania field after passengers fought back.
Before and after the September 11 attacks, Zawahiri appeared on numerous video and audiotapes calling for attacks against Western targets and urging Muslims to support his cause.
Some Egyptians traced Zawahiri's anger toward the United States to what many Afghan Arabs felt was the CIA's betrayal to support their cause after the Soviets left Afghanistan and the country slipped into tribal anarchy.
Others date Zawahiri's wrath to 1998, when US officials pushed for the extradition of a number of Egyptian Islamic Jihad members from Albania to stand trial in Egypt for terrorism.
Zawahiri's brother, Mohammad, told CNN in 2012, "Before you call me and my brother terrorists, let's define its meaning. If it means those who are bloodthirsty merciless killers, then this is not what we are about," he said.
"We only try to regain some of our rights that have been hijacked by Western powers throughout history."
When did Zawahiri start leading al Qaeda?
Zawahiri became al Qaeda's leader after US forces killed bin Laden in 2011.
He was constantly on the move once the US-led invasion of Afghanistan began after the September 11 attacks. At one point, he narrowly escaped a US onslaught in the rugged, mountainous Tora Bora region of Afghanistan, an attack that left his wife and children dead.
Zawahiri "was not a charismatic leader in the mold of bin Laden," CNN National Security Analyst Peter Bergen said Monday. "He didn't prove to be a very competent leader of al-Qaeda. But the reason I think that he was killed in Afghanistan over the weekend was he was beginning to take a lot more risks."
"According to the United Nations, he'd released kind of an unprecedented number of videos. Every time you record a video, there's the chain of custody of that video, getting it out there, somebody maybe taking the video," Bergen continued.
"So he was becoming more prominent. And, I think, it seems to me that may well have been the reason that he was detected."
In a briefing by a United Nations panel of experts last week it was noted that Zawahiri's apparent increased comfort and ability to communicate has coincided with the Taliban's takeover of Afghanistan and the consolidation of power of key al-Qaeda allies within their de-facto administration.
The last known public address by Zawahiri was an audio message released on July 13 by the media arm of al Qaeda.
How did the US kill Zawahiri?
The US undertook "a precision counterterrorism operation" in Afghanistan targeting Zawahiri, who was sheltering in a safehouse in Kabul, a senior administration told reporters Monday.
According to the official, "a precise tailored airstrike" using two hellfire missiles was conducted at 9:48 p.m. ET on Saturday, July 30 -- 6:18 a.m. Kabul time -- via unmanned air strike and was authorized by Biden following weeks of meetings with his Cabinet and key advisers.
No American personnel were on the ground in Kabul at the time of the strike.
CNN's Kevin Liptak, Kylie Atwood, Natasha Bertrand and Donald Judd contributed to this report.