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.
Another U.S. official said the Homeland Security Department would not have taken steps to warn carriers about flights bound for the United States had it not been credible.
"This is not just some flip comment on the part of a bad guy," that official said.
Homeland Security Secretary Jeh Johnson said on Thursday in Los Angeles that "concern about shoe bombs have been out there for years" and the agency updates advisories and modifies procedures "so that we remain vigilant."
Intelligence collected by the United States and other countries indicates terror groups have been working on new shoe-bomb designs, sources familiar with the matter told CNN on Wednesday.
The threat apparently is unrelated to recent warnings about toothpaste and cosmetic tubes potentially being used to hide explosives on flights to Russia.
Terrorism experts say airlines continue to be a target of terrorists wishing to make a spectacular impact with an attack.
The focus on security since 2001 has shifted from hijackings to bombs, especially those that might be hidden in luggage.
Following the 9/11 attacks, passengers aboard an American Airlines flight from Paris to Miami thwarted another passenger's attempt to detonate explosives hidden in his sneakers.
Richard Reid, a British citizen, pleaded guilty and is serving a life sentence.
Since, Transportation Security Administration policy requires passengers going through airport security checkpoints to take off their shoes to be X-rayed.