Abu Yahya al-Libi, who officials in Pakistan and the US say was the target of a US drone strike on June 4 in Pakistan's North Waziristan region, is said to be one of the most senior figures in al-Qaeda.
Libi, reportedly born in 1963, made repeated appearances on al-Qaeda videos and wrote prolifically, becoming one of the group's most prominent media warriors. Administration officials said that he was killed in the drone strike along with more than a dozen other people.
According to his profile on a US Department of State "most wanted" website, Libi was "a key motivator in the global jihadi movement and his messages convey a clear threat to US persons or property worldwide".
It also described Libi as an "Islamic scholar".
The US has offered a $1 million reward for information leading to Libi's arrest, but in a public service announcement had offered "up to $5 million" for such intelligence at one point.
Rewards for the information leading to the capture of suspected terrorists go as high as $25 million.
Jarret Brachman, a counter-terrorism expert who published a biography of Libi for Foreign Policy magazine in 2009, wrote that details about Libi's life were "sparse" and that most of what has been gleaned from him came from his colleagues as well as previously published interviews with al-Qaeda.
According to Brachman, Libi studied chemistry at Sebha University in Libya, with some sources reporting that he completed his studies.
In the late 1980s or early 1990s, Libi travelled to Logar province in Afghanistan and became part of the Libyan Islamic Fighting Group (LIFG), fighting against Soviet Union forces which occupied the country for a decade.
He did not remain in Aghanistan, instead taking up a job as the Taliban's "webmaster" in Karachi in about 2001. He was arrested in Pakistan the following year and taken to the US-operated prison at Bagram airbase in Afghanistan.
Libi was among a number of prisoners who managed to escape from the facility in 2005 in a breakout described to the New York Times newspaper by one official as "embarrassing and amazing at the same time".
Libi has been on the run ever since, but has managed to produce a number of videos online, offering "courses" on "the workship of jihad" and "seeking knowledge".
He also lashed out the Pakistani government (which had executed his arrest in 2002) as taking "a traitorous position" for declaring "its total support and the backing of America".
Pakistani officials mistakenly said that Libi had been killed in a US drone strike in 2009, but it was later confirmed that the victim was another al-Qeada operative, Saleh al-Somali.
Some US officials describe Libi, whose real name is Mohamed Hassan Qaid, as number two to Ayman al-Zawahri, the Egyptian doctor who took over the leadership of al-Qaeda after Osama bin Laden's death.
But for a time, Libi, a Libyan citizen (his chosen last name means "Libyan"), was himself considered a possible successor to bin Laden.
Zawahiri, however, was named as al-Qaeda's leader over a month after bin Laden was killed in May 2011.