– The unmarked eighteen-wheelers ply the nation’s interstates and two-lane highways, logging 3 million miles a year hauling the most lethal cargo there is: nuclear bombs.

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 semitrailer trucks, 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.”

The Los Angeles Times/Tribune Washington Bureau reviewed government documents dating back two decades and interviewed dozens of government officials, former military officers and arms control advocates to examine the agency. The picture that emerges is an organization hampered by an insular management, a crisis of morale among the rank-and-file and outdated equipment.

Despite these problems, the agency asserts that it has maintained a high level of security and has never lost a weapon, though it has been involved in several accidents.

The agency issued a statement touting its safety record: “For more than 40 years — even after driving the equivalent distance of a trip to Mars and back — no cargo has ever been damaged in transit,” it said.

The United States has 4,018 nuclear warheads.

About 450 are in underground silos in Wyoming, Colorado, Montana, Nebraska and North Dakota. An additional 1,000 or so are on submarines, which dock at bases in Washington and Georgia. Hundreds more bombs are assigned to the U.S. strategic bomber fleet, which is based in Louisiana, North Dakota and Missouri. And a reserve stockpile sits in bunkers near the transportation office headquarters at Kirtland Air Force Base in New Mexico.

Each weapon requires routine inspection, testing and maintenance. But the workers who perform those services don’t travel to the weapons. The weapons go to them.

The result is an unwieldy system that requires some of the most dangerous and vulnerable components of the nation’s defense system to be routinely shipped on long-distance journeys from one end of the country to the other — and the shipments, with the coming modernization effort, are only expected to multiply.

More serious than the inefficiencies in moving so many parts is the vulnerability inherent in placing nuclear bombs on the highways, several experts said.

“Transportation is the Achilles heel of nuclear security and everyone knows that,” said Bruce Blair, a retired Air Force missile officer, Princeton University researcher and founder of Global Zero, a nonprofit group that seeks elimination of nuclear weapons.

The danger is not a traffic accident — even a fiery crash is not supposed to explode a warhead — but a heist.

“In an age of terrorism, you’re taking a big risk any time you decide to move nuclear material into the public space over long distances via ground transport,” Blair said. “Bad things happen.”

The high-security trailers that carry the weapons present potential intruders with formidable obstacles, including shock-delivering systems, thick walls that ooze immobilizing foam, and axles designed to explode to prevent a trailer from being towed away, according to independent nuclear weapons experts.

“The trucks will kill you,” a scientist involved in the matter said.

The courier job itself is mundane and tiring, involving long hours on the road under a constant state of high alert. Workers often put in 75 hours a week, according to numerous reviews of the agency.

Matt Hill joined the transportation office after 13 years in the Marine Corps and three deployments to Iraq. But life on the road meant long weeks away from his family. The pay, about $73,000 a year, was less than he made in the Marines.

Couriers have been quitting, many of them the experienced veterans crucial to maintaining safety, Hill said. In February 2016, after just three years on the job, Hill quit too.

“The senior agents are all leaving,” he said. “People at the top won’t listen.”