There are countless ways a terrorist may plant bombs, but a recent warning to airlines by the United States government says that terrorists may be hiding explosives inside shoes.

According to two officials well-known with the situation, this recent warning is the result of new intelligence. They added however that despite the warning, there is no specific threat.

The new intelligence gathered by the U.S. government along with other countries unearthed a new tactic by terrorists -- new shoe-bomb designs. This new intelligence has made the Department of Homeland Security start warning airlines to be more careful and have a lookout for explosives which might be hidden in shoes.

An official from a separate intelligence said that the warning was put on hand to enhance more caution. The intelligence official stated, "This threat is not specific or credible enough to require a specific response. DHS often issues these alerts out of an abundance of caution, but this does not necessarily rise to the level of facilitating a response."

While a different official says that there is no specific threat and is even puzzled as to why DHS issued the warning, one more source added the warning about explosives might also include explosives in cosmetics and liquids.

This focus on airline security started in 2001, because in an incident that December, weeks after the 9/11 attacks, passengers on an American Airlines flight from Paris to Miami found another passenger with explosives hidden in his sneakers. Fortunately, they were able to thwart the attempt.

A British citizen named Richard Reid was responsible for the attack. He pleaded guilty and is currently serving a life sentence.

Passengers going through security checkpoints are now required, by the standing Transportaion Security Administration policy, to take off their shoes for an X-ray.

The warning was given for airlines not to worry about delays.