The WSJ today reported that the bomber whose plot to blow up a jetliner was foiled was a double agent, providing “vital information” to U.S. and Arab intelligence agencies, this according to officials who celebrated a successful infiltration of al Qaeda’s “most dangerous branch.”

The news came one day after the Central Intelligence Agency, working with foreign security services, had foiled a bomb plot by al Qaeda’s Yemeni branch to bring down a U.S. jetliner with an advanced version of the underwear bomb used in the 2009 Christmas Day failed attempt.

The newest plot offers a freighting illustration of al Qaeda’s ability to learn from past mistakes, as the bomb that was found had two detonators, one being a crucial backup in case the other failed, according to a U.S. official.

The double agent was given the bomb and instructions for carrying out the attack, but instead of following his directions, after he left Yemen he contacted intelligence authorities, and turned over the bomb and fresh intelligence about al Qaeda.