This is still developing, but a senior U.S. counterterrorism official is saying that al Qaeda operative Abu Obaidah al-Masri has died of natural causes.

In September, al-Masri made our list of major al Qaeda figures who are still at large and was described by Rand Corporation terrorism analyst Seth Jones as “probably the most active and most important” figure we included. He played a major role in coordinating attacks on U.S. forces in Afghanistan and is thought to have been one of the architects of the foiled 2006 trans-Atlantic airliner plot. This is a significant loss for al Qaeda.