VIDEO

The covert fleet, which shuttles warheads from missile silos, bomber bases and submarine docks to nuclear weapons labs across the country, is operated by the Office of Secure Transportation, a troubled agency within the U.S. Department of Energy so cloaked in secrecy that few people outside the government know it exists.

The $237-million-a-year agency operates a fleet of 42 tractor-trailers, staffed by highly armed couriers, many of them veterans of the Iraq and Afghanistan wars, responsible for making sure nuclear weapons and components pass through foggy mountain passes and urban traffic jams without incident.

The transportation office is about to become more crucial than ever as the U.S. embarks on a $1-trillion upgrade of the nuclear arsenal that will require thousands of additional warhead shipments over the next 15 years.

The increased workload will hit an agency already struggling with problems of forced overtime, high driver turnover, old trucks and poor worker morale — raising questions about its ability to keep nuclear shipments safe from attack in an era of more sophisticated terrorism.

"We are going to be having an increase in the movements of weapons in coming years and we should be worried," said Robert Alvarez, a former deputy assistant Energy secretary who now focuses on nuclear and energy issues for the Institute for Policy Studies in Washington. "We always have to assume the worst-case scenario when we are hauling nuclear weapons around the country."

That worst case would be a terrorist group hijacking a truck and obtaining a multi-kiloton hydrogen bomb.

"The terror threat is significant," said one high-level Energy Department official, who spoke on condition of anonymity because he was not authorized to discuss the program publicly. "If you are in one of the communities along the route, you have something to worry about."