The US Department of Homeland Security has warned airlines to watch for explosives hidden in the shoes of passengers flying into the United States, officials say.
The alert is based on new intelligence indicating that a shoe bomb may be used to blow up a US-bound jetliner, said two law enforcement officials who described the bulletin on the condition of anonymity.
Officials said the threat was not specific to a particular airline, flight, country or time.
It was not related to the Winter Olympics in Sochi, Russia.
The alert was issued "out of an abundance of caution," said a homeland security official.
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Airport screeners at international airports were instructed to step up scrutiny of passengers boarding flights for the US.
Screeners will increase uses of swabs that can detect traces of explosive powder on shoes, bags and hands. They also are likely to pull aside more passengers for pat-downs and full-body screening, officials said.
In December 2001, three months after the September 11 attacks, British national Richard Reid first made the world aware of such threats when the al-Qaida operative boarded an American Airlines flight from Paris to Miami with explosives packed in his sneakers.
Passengers and crew members subdued Reid when he attempted to ignite the explosives. They failed to detonate and Reid was arrested after the plane made an emergency landing at Logan International Airport in Boston.
Reid pleaded guilty to eight counts of criminal terrorism in federal court in 2002 and is serving a life sentence in prison.
