Intelligence collected by the United States and other countries has indicated terror groups have been working on new shoe-bomb designs. The Department of Homeland Security has warned airlines to be on the lookout for explosives hidden in shoes on flights from overseas to the United States. The warning outlined 25 to 30 cities overseas, all of which have nonstop flights to the United States, an airline industry source said. Johannesburg, Paris, London and Cairo, as well as some cities in the Middle East, were on the list. The warning was meant to give a heads up to airlines about delays, not asking airlines to do anything specific, the industry source said.