The Homeland Security Department is warning airlines to be on alert for explosives hidden in shoes.
Homeland Security says it regularly shares relevant information with domestic and international partners, but it declined to discuss specifics of a warning sent to airlines.
Earlier this month Homeland Security warned airlines with flights to Russia to be on the lookout for explosive devices possibly hidden inside toothpaste. The Transportation Security Administration then banned passengers from bringing any liquids in their carry-on luggage on nonstop flights from the U.S. to Russia. That warning became public just days before the opening ceremonies of the Winter Olympics in Sochi.
It is unclear if the latest warning is related to the earlier threats to Russia-bound flights.