A U.S. government warning this week to airlines about possible shoe bombs represents a credible threat linked to al Qaeda, officials told CNN.
Recent intelligence points to tactics believed to be tied to al Qaeda in the Arabian Peninsula and its master bomb-maker Ibrahim al Asiri, a federal law enforcement official said.
Counterterrorism officials don't believe there is an active plot in the works. But the law enforcement source said the United States periodically receives information on attempts by those believed to have been trained by Asiri to try to develop bombs that could defeat screening systems.
It's not a new threat, but one viewed as ongoing because they know the al Qaeda unit based in Yemen is constantly trying to improve its bombs.