(CNN) -- 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 said.
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.
Terrorism experts say that airlines continue to be a target of terrorists wishing to make a spectacular impact with an attack. The focus on security since 2001 has shifted from hijackings to bombs, especially those that might be hidden in luggage.
In December 2001, just weeks after the 9/11 attacks, passengers aboard an American Airlines flight from Paris to Miami thwarted another passenger's attempt to detonate explosives hidden in his sneakers. Richard Reid, a British citizen, pleaded guilty to the crime and is serving a life sentence.
A failed attempt to blow up an overseas flight heading to Detroit on Christmas Day in 2009 involved a bomb concealed in a passenger's underwear.