Forensic evidence showed the device, originally sent from Yemen by way of Cologne, Germany, was timed to be detonated about seven hours after the cargo aircraft carrying it left the U.K. for the U.S. The package was removed by police in Britain during transit.
"If the device had not been removed from the aircraft, the activation could have occurred over the eastern seaboard of the U.S.," police said in a statement.
Authorities on both sides of the Atlantic said they only narrowly thwarted the plot, in which terrorists in Yemen hid two powerful bombs inside printers and shipped them to addresses in Chicago. One bomb was intercepted at central England's East Midlands Airport and the other was discovered at a FedEx cargo facility in Dubai.
The Yemen-based al-Qaida in the Arabian Peninsula has claimed responsibility for the bombs and has vowed to send more explosives-packed parcels.
The exact aim of the attack is unclear, but a U.S. official had said evidence points to a plot to blow up cargo planes inside the U.S., either on runways or over American cities.