Terrorists may be planning to hijack cargo planes overseas and crash them into targets in America, says the authorities in Washington. The warning has come from a single source, and is not yet corroborated, but US officials are taking it seriously. Local and state authorities - and those responsible for safety at nuclear plants, bridges and dams - have been warned of the potential threat. It comes as US diplomatic missions in Saudi Arabia were closed following "credible evidence" of a threat. The US is also warning its journalists in Afghanistan that Taleban insurgents may be planning to kidnap them in order to put pressure on America to release prisoners.