A drone strike in northern Pakistan has killed al Qaeda deputy leader Abu Yahya al-Libi, according to U.S. officials, in what might just be the biggest single success in the controversial, covert airstrike campaign's eight-year history. Who is Libi, and how did the Obama administration reportedly catch up to him? Here, a brief guide:

What was Libi's role in al Qaeda?
Libi was a religious scholar from Libya who had been considered al Qaeda's rising star. He became the terrorist network's second-in-command when the former chief deputy, Egyptian-born Ayman al-Zawahiri, took over as leader following Osama bin Laden's death last year. Libi served both as a virtual ambassador for the organization through his frequent online video statements, and as a gatekeeper who relayed instructions from al Qaeda's leadership in Pakistan and its operatives in Yemen, Iraq, and elsewhere.

How did he become so important?
It wasn't through victories in the battlefield. As a young jihadist — during the Soviet war in Afghanistan in the '80s and in an Islamist insurgency against Libya's Moammar Gadhafi in the '90s — Libi was always more interested in religious study than fighting, experts say. He moved to Aghanistan when the Taliban seized control of the country in 1996, and fled into Pakistan's tribal lands just over the border after the U.S. invaded following the Sept. 11, 2001, terrorist attacks. He was captured in a raid, then escaped from the U.S. detention center at Bagram Air Base in 2005, and released a 54-minute video mocking the U.S., which helped establish him as one of al Qaeda's top propagandists.

Will he be hard for al Qaeda to replace?
Very. His death amounts to a "cataclysmic blow" to al Qaeda, security analysts tell CNN. Libi was widely respected as a religious scholar, and he used his pull to justify al Qaeda's call to jihad. His religious training gave him the authority to issue fatwas that other militants would respect and follow. He was also a charismatic leader in his own right. "There is no clear successor to take on the breadth of his responsibility," White House spokesman Jay Carney said. Losing Libi puts al Qaeda "closer to its ultimate demise than ever."

How did the U.S. find him?
That information is tightly guarded, but behind the shadowy drone operations, which Pakistan insists are illegal, "lies an unseen intelligence operation which fed U.S. commanders knowledge of Libi's whereabouts," says Adam Brookes of BBC News. Tracking down Libi probably involved satellites and spy drones, as well as listening stations to intercept militants' communications. Then, two missiles were fired by a Central Intelligence Agency drone on Monday — the first hit a suspected militant compound in the volatile tribal region of North Waziristan. Then a second missile struck as more militants arrived at the scene.

Are U.S. officials sure Libi is dead?
They say they have confirmed his death from sources in Pakistan. Local intelligence sources told Reuters that Libi was seriously wounded in the strike, and died after being taken to a private hospital. However, this isn't the first time Libi has been reported killed. Rumors of his death circulated in December 2009 after a similar strike in South Waziristan. But this time the U.S. seems certain it got Libi.

Sources: BBC News, CNN, New York Times, TIME