( CNN ) -- Abu Yahya al-Libi , al Qaeda 's No . 2 man , was killed in Pakistan on Monday , according to U.S. officials . Al-Libi 's death was `` another serious blow to core al Qaeda , '' White House spokesman Jay Carney said . Al-Libi , 49 , was a well-regarded figure in jihadist circles and had emerged as one of the terrorist network 's most important clerics and propagandists , appearing in countless videos in recent years . He was killed by a CIA drone strike , according to U.S. officials . Drone strikes remain a highly contentious issue between the United States and Pakistan . Who is Abu Yahya al-Libi ? By most accounts , al-Libi was effectively al Qaeda 's deputy leader . A Libyan citizen and an Islamic scholar , al-Libi bolstered his credibility within jihad groups after escaping from U.S. custody in Afghanistan in 2005 . He became the public face of al Qaeda and used his religious training to justify the organization 's actions . As one of the group 's chief ideologues and propagandists , al-Libi appeared in numerous recruitment videos in which he cast himself as a sheikh with the legitimacy to issue fatwas . Other than his appearances in propaganda videos , it 's unclear which plots against the West al-Libi was involved in . A wanted ad from the U.S. State Department described him as a `` key motivator in the global jihadi movement , '' and said that `` his messages convey a clear threat to U.S. persons or property worldwide . '' What does his death mean for al Qaeda ? This is a `` very serious blow '' to al Qaeda , according to Noman Benotman , a former senior member of the Libyan Islamic Fighting Group who spent significant time with al-Libi in the 1990s . No one else within the group rivals his legitimacy as a religious scholar nor has the credibility in the Arab world to provide Islamic justifications for al Qaeda 's global campaign of terrorism , he said . `` There is no one who even comes close in terms of replacing the expertise [ al Qaeda ] has just lost , '' said a U.S. official who spoke on the condition of anonymity . Al Qaeda leader Ayman al-Zawahiri may be the only leader of consequence left , wrote Peter Bergen , CNN national security analyst . However , he said al Qaeda 's regional affiliates remained a threat . For a complete list of terrorists sought by the United States , check out this list on CNN 's Security Clearance blog . What are drone strikes ? In drone strikes , unmanned aerial vehicles , often equipped with hellfire missiles , are used to target suspected terrorists in remote locations . These attacks have been used in Pakistan , despite opposition from the country . This issue has strained relations between Pakistan and the United States . The strike on Monday that killed al-Libi is believed to be the 21st U.S. drone strike in the country this year . Critics of such attacks say the drone strikes have killed innocent civilians and that the U.S. has not provided evidence that the missiles have reached their intended targets . U.S. officials rarely discuss the CIA 's drone program in Pakistan , though privately they have said the covert strikes are legal and an effective tactic in the fight against extremists . What 's the reaction in Pakistan ? There has neither been widespread reaction in Pakistan nor any uproar or outrage over al-Libi 's death . It 's likely that many Pakistanis do not know who al-Libi was , according to Reza Sayah , CNN 's Pakistan-based correspondent . Are drone strikes legal ? Critics say that drone attacks are not a democratic strategy and that the practice violates international laws and national sovereignty . President Barack Obama 's administration has justified its use of drones overseas . In a rare public statement recently , John Brennan , the president 's top counterterrorism adviser , said the strikes are conducted `` in full accordance with the law . '' Brennan said the United States `` respects national sovereignty and international law '' and is guided by the laws of war in ordering those attacks . CNN 's Paul Cruickshank , Peter Bergen , Reza Sayah and the CNN Wire contributed to this report .
Ayman al-Zawahiri , al Qaeda deputy leader , was killed by a drone strike , the United States says . His death was a `` very serious blow '' to al Qaeda , al Qaeda says . White House strikes remain highly controversial because of civilian deaths .