The Transportation Security Administration on Thursday reminded officials charged with securing mass transit around the country to be aware of security issues as the country enters the busy holiday travel season. There was no specific threat behind the warning.
The reminder came in a bulletin that the agency already issued earlier this year, Transportation Security Administrator John Pistole said.
The intelligence community does not currently know of a specific threat to mass transit, according to a counterterrorism official who spoke on condition of anonymity because he was not authorized to speak publicly about intelligence.
"It's something that we've seen in reporting over time that terrorists, around the world, clearly, are interested because of the accessibility, the open architecture, of both buses and rail, and so we try again to work with our state and local counterparts as best we can," Pistole told an audience at an event hosted by the Homeland Security Policy Institute at George Washington University.
Buses and rail systems have been targeted by terrorists in the U.S. and overseas. Evidence found in Osama bin Laden's compound after he was killed in May showed that al-Qaida was looking at the structures of trains and train tracks, seeking the best spot for a derailment that would kill the most people.
In March, two U.S. airmen were killed at the Frankfurt, Germany, airport when a man opened fire on them at close range with a handgun after the attacker got into an argument with them outside their military bus. And in October, a Taliban suicide bomber rammed a vehicle loaded with explosives into an armored NATO bus on a busy thoroughfare in Kabul, Afghanistan, killing 17 people, including a dozen Americans.
Generally, the TSA sends warnings to state and local law enforcement and others charged with securing transportation systems. They are often reminders to be on the lookout for suspicious activity and are not always prompted by intelligence about a specific threat or plot.