The alert follows the discovery of bomb detonators - expertly hidden in the hollowed-out soles of this pair of shoes - found aboard a European bus last month.
Intelligence officials say the shoes were not being worn at the time, but instead were being used, as the document says, "to smuggle electric blasting caps across international borders for use in a terrorist attack."
"The terrorists have an interest in explosive devices. They are trying to figure out the best way to push them, to move them through the system," said CBS News counterterrorism analyst Paul Kurtz.
Shoes have been used by terrorists before.
Three months after 9/11, al Qaeda operative Richard Reid tried to blow up an American Airlines jet with a shoe bomb similar to the one shown in this test.
Now at U.S. airports, scanners X-ray all passengers' shoes and carry on bags, searching for explosives.
Still, experts worry that a team of terrorists could beat security by carrying unassembled parts of a bomb past a checkpoint.
"Where one person will carry component A, the next person will carry component B, and they will meet together past the safety point, past the checkpoint and reassemble," explained Mike White, the director of training for Michael Stapleton Associates and a former head of the NYPD bomb squad.
Officials say there is no specific intelligence that terrorists are preparing new attacks against America.
But, the threat remains high - and the bulletin warns law enforcement officials not to assume that routine objects, like shoes, are always what they appear to be.