The FBI and Department of Homeland Security are urging state and local law enforcement agencies to be on the lookout for the possible use of shoes to conceal and smuggle explosive components.
A September incident in Europe involving blasting caps prompted the alert, said the law enforcement advisory, a copy of which CNN obtained Friday.
The advisory said the Department of Homeland Security and FBI don't have any "specific, credible intelligence that terrorists plan to use this concealment tactic" in the United States.
The advisory was distributed "to allow officers on the streets to be alert to items that appear routine but may not be as they go about their daily duties," FBI spokesman Richard Kolko said Thursday.
In the European incident, authorities found electric blasting caps in the hollowed-out soles of shoes that were packed in luggage and crossed international boundaries in a bus, the advisory said.
It did not provide details on the country of the bus' origin or where the blasting caps were discovered.
The advisory, which the department and FBI sent out Wednesday, said security officials "need to be cognizant of the continued interest among terrorists to conceal explosive devices in shoes."
The document noted shoes of passengers are X-rayed at U.S. airports, but other modes of transportation that do not require close screening could be vulnerable to such tactics.
The advisory said security personnel should watch out for shoes with thick soles that could be hollowed out to hide explosives, shoes that appear to have been taken apart and reassembled, people walking in an unusual manner and any wires protruding from shoes.
Terrorists have used shoes in at least one attempted plot. Richard Reid was arrested after trying to ignite shoes containing explosives aboard a trans-Atlantic plane bound for the United States in December 2001. Known as the "shoe bomber," Reid is serving a life sentence at the Supermax federal prison in Florence, Colorado.