AQAP has significantly expanded in numbers, strength and territory since one of its top leaders, the U.S.-born cleric Anwar al-Awlaki, was killed in a CIA drone strike last year. White House counterterrorism adviser John Brennan described AQAP as “very, very dangerous” in a speech at New York police headquarters last week, according to an account by CNN.

AQAP has more than 1,000 members in Yemen and “close connections” to al-Qaeda leadership in Pakistan, Brennan said, according to CNN. “We are very concerned about AQAP. It’s the most active operational franchise.”

AQAP has been tied to terrorist plots including the 2010 attempt to mail parcels packed with explosives to addresses in Chicago and the attempt to blow up a Detroit-bound airliner on Christmas Day 2009.

The U.S. military has carried out airstrikes using drones as well as conventional aircraft and ship-based missiles for several years. The CIA joined the hunt last year when it opened a secret drone base at an undisclosed location on the Arabian Peninsula.