Terrorists are still looking for ways to smuggle explosives-laden shoes and other bomb-packed items onto planes., the Department of Homeland Security has warned U.S. and other airlines.
The U.S. government recently obtained intelligence boosting concerns about such threats, sources told ABC News. Though the intelligence is generic in nature and does not indicate any specific threat, federal authorities have a responsibility to communicate any such information to airlines as a precaution so they can take any measures they see fit, the sources said.
"Out of an abundance of caution, DHS regularly shares relevant information with domestic and international partners about relevant threat information as we work to meet our mission of keeping the traveling public safe," a DHS official said in a statement to ABC News. "These types of regular communications are part of that important priority. Our security apparatus includes a number of measures, both seen and unseen, informed by the latest intelligence and as always DHS continue to adjust security measures to fit an ever evolving threat environment."
The latest warning, issued Tuesday via a so-called "tear line" to airlines, is at least the second such warning in as many weeks.
Two weeks ago, DHS issued a similar warning over explosives that could be hidden in toothpaste or cosmetic tubes, but those concerns were tied to potential attacks around the Winter Olympics in Sochi, Russia. At the time, the Transportation Security Administration announced it was temporarily banning travelers from bringing any amount of liquid, gel or aerosol in their carry-on luggage aboard flights between the United States and Russia.
The intelligence that prompted Tuesday's latest warning is not tied to Sochi and comes from a different thread of intelligence.
It's unclear exactly how credible the latest intelligence will turn out to be.
Just three months after the terror attacks on Sept. 11, 2001, Richard Reid tried to detonate a homemade bomb hidden in his shoe aboard American Airlines Flight 63, which was carrying 14 crew members and 183 other passengers from Paris to Miami.
In 2006, British authorities uncovered a plot to use liquid explosives to blow up transatlantic flights headed to the United States and Canada was foiled by U.K. authorities. That prompted U.S. and U.K. authorities to ban most liquids in carry-on luggage.
In recent years, some lawmakers have blasted TSA security measures at U.S. airports.
Just last month, Rep. John Mica , R-Fla., the chairman of a House Oversight and Governmental Affairs subcommittee, held a hearing focused on TSA efforts, saying the agency had "gone awry at great expense and inconvenience."
"Security is incredibly important. We still are at risk. And I believe they'll use aviation again to come after us," Mica said in closing the hearing. "But I don't think this current structure is geared to deal with that. Everything we have done is in reaction. Take off your shoes … It's all reactive. Pretty soon we'll be going through there naked."