The suspicion of a car in a garage belonging to an American nuclear weapons plant has prompted staff inside the facility to seek a safe haven for them briefly.
The accident took place at the Pantex plant in Amarillo, Texas, just before noon (1800 GMT) when a routine inspection identified "possible car doubts", the National Nuclear Security Administration said in a statement.
"All staff are placed in safe shelters as a precautionary measure," the statement said.
Police attended the facility, which closed all the roads leading to it for an hour, while officials were conducting their investigations.
"After the search of the vehicle, it was confirmed that it contained no prohibited materials or explosives, and the emergency procedures were resolved without incident," the NNSA said.
PANTEX operates US government contractors, along with six other facilities that are the main centers for the assembly, dismantling and maintenance of US nuclear arsenal.
A letter on the machine's answering machine says the work has returned to normal after what was initially described as a "security incident."
