The shoes used in a failed attempt to blow up a passenger plane in December 2001 are displayed at a 9/11 exhibit in 2011.

The U.S. government has warned airlines to pay particular attention to the possibility of terrorists attempting to hide explosives in shoes, a result of new intelligence, according to two people familiar with the situation.

The officials stressed there is no specific threat or known plot.

Intelligence collected by the United States and other countries has indicated terror groups have been working on new shoe-bomb designs, the sources said Wednesday.

That knowledge prompted the Department of Homeland Security to warn airlines to be on the lookout for possible explosives hidden in shoes on flights from overseas to the United States, they added.

A separate intelligence official underscored the warning was issued out of a sense of heightened caution.

“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,” the intelligence official said.

A fourth official similarly stressed there is no specific threat, and said there is already some puzzlement about why DHS issued the warning.

Source: CNN.com.