(CNN) -- Abu Yahya al-Libi, the No. 2 man in al Qaeda and a longtime public face of the terrorist network, is dead, White House spokesman Jay Carney said Tuesday.
Al-Libi's death was "another serious blow to core al Qaeda," said Carney. "His death is part of the degradation taking place in core al Qaeda in the last several years," Carney said.
Earlier Tuesday, U.S. officials said that al-Libi was killed Monday by a CIA drone strike in Pakistan.
Al-Libi's death marks one of the most significant blows to al Qaeda since 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 Qaeda 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 Qaeda) has just lost," said a 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 said.
"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."
Monday's 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.
Some jihadists do not believe the report and will continue to wage jihad, said the SITE Intelligence Group, which monitors Islamist extremism.
"Most jihadists discussing the issue prayed that Libi is still alive and urged fellow supporters to await confirmation," the group said on its website. "Some took the news as fact and prayed that he be accepted by God as a 'martyr.' One jihadist wrote: 'Just as the Sheikh said more than once, our jihad is not dependent on an individual and not even on a group. So, if al-Qaeda is annihilated hero by hero and only its memory remains (!!), jihad will remain standing by the command of Allah.'"
Their skepticism may be well-founded. Reports that emerged a couple of years ago that al-Libi was slain proved incorrect.
Al Qaeda'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 Qaeda 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 videos posted on the Internet. He delivered 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 Qaeda 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 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 Qaeda 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 Qaeda figure Omar al-Faruq, who died in a subsequent British airstrike.
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 defended 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 internal opposition to the practice 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.