Michael Winter

USA TODAY

Terrorist groups may have devised shoe bombs that avoid detection, and the warning to airlines Wednesday stemmed from a credible threat linked to al-Qaeda's offshoot in Yemen, security sources say.

Intelligence is focused on a Saudi militant named Ibrahim Hassan al-Asiri, the master bombmaker for al-Qaeda in the Arabian Peninsula, the unidentified officials told CNN and Reuters. Described as technically savvy, he may have improved the weapons after failed attempts to blow up airliners using explosives hidden in shoes or clothing, including the Christmas 2009 attempt over Detroit.

The Department of Homeland Security on Wednesday warned U.S. and overseas airlines flying into the United States about a possible shoe-bomb threat, though it mentioned no specific plot.

The warning is not related to the Winter Olympics or a previous alert about possible explosives in toothpaste tubes.

Sources told Reuters that alerts went to airlines flying into the United States from about 30 airports in Europe, the Middle East and Africa. Extra scrutiny was encouraged at Amsterdam's Schiphol airport, along with Heathrow and Gatwick in London and the airport in Manchester, England.