Scotland Yard says a parcel bomb found on a cargo plane at a British airport in October was timed to explode over the eastern seaboard of the United States.
The device hidden in a printer cartridge was sent from Yemen and removed from the aircraft at East Midlands Airport.
A second mail bomb also sent from Yemen was intercepted in Dubai.
The bomb intercepted at East Midlands airport was discovered following a tip-off from Saudi intelligence.
Both the intercepted parcels contained a powerful plastic explosive and were addressed to synagogues in Chicago.
A statement from Scotland Yard says forensic examination has indicated the device found at East Midlands would have exploded just under three hours after it was defused.
That would have put the explosion over the eastern coast of the US.