U.S. officials said on Saturday that Atiyah abd al-Rahman, a Libyan, was killed in Pakistan. One official said he was killed in a strike by an unmanned drone on 22 August.
The killing is likely to be particularly highly prized by Washington as U.S. strategists would have been concerned about Rahman's potential influence in Libya's turmoil following the overthrow of Muammar Gaddafi, analysts say.
Rahman, in his 40s and from the coastal Libyan town of Misrata, built a reputation in al Qaeda as a thinker, organiser and trusted emissary of the Pakistan-based central leadership to its offshoots.
In particular he played a key role in managing ties between the core leadership and al Qaeda in Iraq and helped negotiate the formation in 2007 of al Qaeda in the Islamic Maghreb (AQIM) with a group of Algerian Islamist guerrillas.
He was also one of the first al Qaeda leaders to provide a response to the uprisings in the Arab world, urging the group's supporters to cooperate with the revolts even if the rebellions were not Islamist-inspired.
"It's immensely important that he's been killed," said Anna Murison, who monitors Islamist violence for Exclusive Analysis, a London-based risk consultancy.
She said he was widely trusted throughout the organisation and Islamists from very varied backgrounds listened to him.
"Al Qaeda as an idea will live on, but al Qaeda core as an organisation looks pretty much finished as there are so few people who can now move up into those senior ranks," she said.
She said he was one of only four people in al Qaeda's leadership with a global profile in the small but passionate transnational community of violent Islamist militants. She rates these as al Qaeda's current leader Ayman al-Zawahri, Egyptian plotter Saif al-Adl, and the other Libyan in al Qaeda's central leadership, the theologian Abu Yahya al-Libi.
Rahman rose to the number two spot when al-Zawahri took the reins of al Qaeda after Osama bin Laden was killed in May in a U.S. raid in Pakistan.
Noman Benotman, a former Libyan Islamist and now an analyst at Britain's Quilliam think tank, said his death was a heavy blow to al Qaeda as he was its main organiser and manager.
"This was the one man al Qaeda could not afford to lose," Benotman said. "He was the CEO of al Qaeda who was at the heart of the management process of al Qaeda worldwide.
Benotman said that in the last two years he "more or less single-handedly" kept al Qaeda together. "He was a strong decision maker, an excellent debater and a skilled peacemaker between various Islamist groups."
FOUGHT IN ANTI-SOVIET AFGHAN WAR
Benotman said Rahman, whose real name was Jamal Ibrahim Ishtawi, was a graduate of the engineering department of Misrata University and left Libya to go to Afghanistan in 1988 and join the Islamist groups then fighting Soviet occupation.
He said Rahman was a personal acquaintance of his but was never a member of the Libyan Islamic Fighting Group, an Islamist guerrilla organisation that waged a failed insurgency to topple Gaddafi in the 1990s and of which Benotman was a leader.
Rahman was one of al Qaeda's earliest members and worked for the anti-Western militant group in Algeria and Mauritania as well as Afghanistan, Benotman said.
In a statement posted on militant online forums on Feb. 23, Rahman acknowledged that the revolutions in Egypt and Tunisia were not the "perfections for which we hoped", but they were happy occasions nonetheless.
He dismissed the notion that al Qaeda had a "magic wand" to gather large armies and lead the charge to overturn governments and rescue besieged Muslims, according to a translation by the Site Intelligence Group, a U.S. monitoring company.
Rather, he wrote, "al Qaeda is a simple part of the efforts of the jihadi Ummah (nation), so do not think of them to be more than they are. We all should know our abilities and to try to cooperate in goodness, piety and jihad in the Cause of Allah; everyone in his place and with whatever they can and what is suitable to them