A former CIA official says in a new book that President Barack Obama ordered a barrage of drone strikes in Yemen in 2013 that killed the al-Qaida operatives behind the most serious plotting against American interests in years.

When the U.S. closed 20 diplomatic facilities across the Middle East in August of 2013, officials said it was in response to intercepted communications about an unspecified plot. They said little about why they later deemed the threat abated. But former CIA official Michael Morell says the reason was that that many of the key operatives involved in the plot were killed by U.S. air strikes.

Morell, who retired later in 2013 as deputy CIA director after years in leadership posts, offers the most detailed account of the episode to date in a book obtained by The Associated Press.