Abu Yahya al-Libi, the second most senior leader of al Qaeda, has been killed by a U.S. drone strike in Pakistan, a U.S. official said Tuesday.

His 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…

Al Qaeda will have a hard time replacing al Libi, according to the U.S. official who said al Libi was dead. “There is no one who even comes close in terms of replacing the expertise (al Qaeda) has just lost,” said the U.S. official, who spoke on the condition of anonymity.