U.S. authorities issued a warning on Wednesday to airlines flying to the United States to watch out for militants who may have hidden bombs in their shoes, U.S. government sources said.
The warning came from the Department of Homeland Security, the sources said, and it is consistent with concerns security agencies have about militants trying to smuggle explosives onto airplanes in shoes, cosmetics or liquids.
The sources said the warning principally applied to flights originating overseas and heading for the United States, rather than domestic flights or planes headed overseas from the United States.
However, some sources said the warnings did not mean that the United States had specific intelligence indicating a plot to use a shoe bomb overseas was in progress. One source said the alert was being issued "out of an abundance of caution." A second source said the alert was "not a big deal."
Wednesday's warning follows one earlier this month in which U.S. authorities warned airlines flying to Russia for the Sochi Winter Olympics to watch out for toothpaste tubes that could hold bomb-making ingredients and could be smuggled through airport security.
Since the Sept. 11, 2001, attacks on the United States, Islamist militants have made two known attempts to blow up U.S.-bound airliners using bombs hidden in their clothes or footwear.
In December 2001, Richard Reid of Britain tried to set off explosives concealed in his shoe on a Paris-to-Miami American Airlines flight. Passengers subdued him before he could light the fuse.
On Christmas Day 2009, Umar Farouk Abdulmutallab of Nigeria tried to detonate an explosive hidden in his underwear as his Delta Air Lines flight descended into Detroit after taking off from Amsterdam. Abdulmutallab's device malfunctioned, burning him, and he was subdued by passengers.
Both Reid and Abdulmutallab are serving long sentences in U.S. prisons.