A U.S. drone killed al Qaeda’s No. 2 man Abu Yahya al-Libi Monday in Pakistan, according to U.S. officials.

The death of al-Libi was “another serious blow to core al Qaeda,” according to White House spokesman Jay Carney. Yahoo News reported he was killed near the Khassu Khel village in North Waziristan in a series of drone attacks over three days.

A $1 million reward was offered for al-Libi’s capture by the State Department. He was imprisoned in 2002 at Bagram Air Force Base, according to Yahoo News, but he escaped in 2005.

At least six missiles were targeted in the last three days at a compound near the Afghan border town of Mir Ali, and an additional 15 militants were killed, according to CNN.

North Waziristan is believed to be the source of the Haqqani network’s operations, and Monday’s attack was the 21st strike in Pakistan this year. The attacks are part of a CIA program that uses unmanned vehicles to drop missiles on remote locations. White House counterterrorism adviser John Brennan said the attacks are “in full accordance with the law.”

Al-Libi was involved in planning operations against the West and according to a U.S. official, he will be hard to replace.

"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.