By the CNN Wire Staff
His death marks one of the most significant blows to al-Qaidasince the U.S. military killed Osama bin Laden in a daring nighttime raid in Pakistan a year ago.
Al-Libi was second-in-command behind al-Qaida leader Ayman al-Zawahiri, who took the helm after bin Laden's death.
"There is no one who even comes close in terms of replacing the expertise (al-Qaida) has just lost," said the U.S. official, who spoke on the condition of anonymity.
Al-Libi "played a critical role in the group's planning against the West, providing oversight of the external operations efforts," the official told CNN.
"Zawahiri will be hard-pressed to find any one person who can readily step into Abu Yahya's shoes," the official said. "In addition to his gravitas as a longstanding member of AQ's leadership, Abu Yahya's religious credentials gave him the authority to issue fatwas, operational approvals, and guidance to the core group in Pakistan and regional affiliates."
The Monday strike was the third such deadly attack in as many days and the 21st suspected U.S. drone strike in Pakistan this year. At least six missiles were fired at a militant compound near the town of Mir Ali in the North Waziristan region near the Afghanistan border.
Reports emerged a couple of years ago that al-Libi was slain, but they proved to be incorrect.
After bin Laden's death, Ayman al-Zawahiri became al-Qaida's leader and al-Libi was elevated to No. 2 in the terror organization.
Al-Qaida's leadership has been so thinned by the U.S. drone campaign in Pakistan that these men were the only two real leaders of the organization left, U.S. counterterrorism officials said, according to CNN Security Analyst Peter Bergen. Al-Qaida offshoots in other parts of the world, such as the group's affiliate in Yemen, have meanwhile become more potent and worrisome to the United States.
An Islamic scholar and high-ranking member of the group, al-Libi frequently appeared in Internet videos. He gave many videotaped speeches praising al Qaeda leaders, urging resistance and trying to recruit new members.
"Al-Libi is a key motivator in the global jihadi movement and his messages convey a clear threat to U.S. persons or property worldwide," said a "Wanted" statement posted on the website of the U.S. State Department's "Rewards for Justice" program, which offers rewards for information about suspected terrorists.
"Al-Libi is believed to be in hiding in Afghanistan or Pakistan," said the website, which offered a reward of up to $1 million for the 49-year-old Libyan.
Al-Libi purportedly was among al-Qaida leaders working in Libya since last year to establish a presence there.
In a video message to fellow Libyans distributed on jihadist forums in December, al-Libi said: "At this crossroads you have found yourselves, you either choose a secular regime that pleases the greedy crocodiles of the West and for them to use it as a means to fulfill their goals, or you take a strong position and establish the religion of Allah."
Al-Libi was captured in 2002 and imprisoned at Bagram Air Force Base in Afghanistan, according to the U.S. government, but he escaped in 2005.
In 2008, a statement posted on radical Islamic websites known to carry messages from al-Qaida described how four "military leaders" including al-Libi escaped from the prison, but the statement said then that one of the escapees, Abu Abdallah al-Shami, had been killed in a U.S. airstrike.
The statement said that among those escaping with al-Shami was key al-Qaida figure Omar al-Faruq, who died in a British airstrike after the escape.
Another escapee, Abu Nasir al-Qahtani, was captured in Afghanistan in 2006. Al-Shami's death left al-Libi as the only remaining member of that escape who had not been killed or captured.
President Barack Obama's administration recently justified its use of unmanned drones to target suspected terrorists overseas in a rare public statement, with John Brennan, the president's top counterterrorism adviser, saying the strikes are conducted "in full accordance with the law."
The program uses unmanned aerial vehicles, often equipped with Hellfire missiles, to target suspected terrorists in remote locations overseas, with many such strikes occurring in Yemen and Pakistan, despite some internal opposition within the latter country.
Brennan said the United States "respects national sovereignty and international law" and is guided by the laws of war in ordering those attacks.
The Pakistani border area is widely believed to be the operating base for the Haqqani network and other militant groups that have attacked international troops in neighboring Afghanistan.