The latest warning to airlines about shoe-bomb threats is a product of heightened U.S. concern about al-Qaeda’s Yemen-based affiliate, whose leaders include a technically-savvy bombmaker, U.S. security sources said on Thursday.

U.S. Homeland Security authorities on Wednesday issued a new warning about shoe bombs to airlines which fly from overseas to the United States out of concern that Ibrahim Hassan al-Asiri, a Saudi believed to have been behind failed plots to attack airliners with explosives hidden in shoes or clothing, may have come up with new bomb design innovations to evade airport security measures, the sources said.

The sources said the warnings were sent to airlines flying to the U.S. from around thirty airports in Europe, the Middle East and Africa.