The package was removed from the cargo plane 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.
British police said the UPS cargo aircraft intercepted in England left the country without the package at around 3:20 a.m. GMT on Oct. 29 (11:20 p.m. EDT on Oct. 28), two hours after landing. The device was timed to detonate at 9:30 a.m. GMT. (5:30 a.m. EDT).
The second mail bomb was discovered on a plane transitioning through Dubai in the United Arab Emirates.
The FBI and Homeland Security officials believe Al Qaeda terrorists in Yemen hid both mail bombs inside computer printers and wired them to detonate, using cell-phone technology and powdered explosives that resembled the cartridges' ink powder when X-rayed.
The threat set off a major security alert in several major cities around the world and in the U.S. Suspicious packages were investigated in Philadelphia, New Jersey and Queens.
The Associated Press contributed to this report.