US intelligence officials feared that al Qaeda terrorists in Yemen were plotting to attack the United States and actually intercepted what they now believe were "dry run" shipments to Chicago in mid-September, according to several people briefed on the plot and a senior US official.
The senior US official told ABC News that the "dry run" involved a carton of household goods including books, religious literature, and a computer disk, but no explosives, shipped from Yemen to an address in Chicago by "someone with ties to al Qaeda in the Arabian Peninsula."
Another person briefed on the incident said it is now believed the terrorists sent the package "so they could track how long it took and whether there would be any problems for the package getting through the system."
Senior administration officials told ABC News that, after the September shipment was discovered, U.S. intelligence agencies had specific concerns about the Yemen-based group's interest in Chicago, noting not only the destination of the September shipment, but also a photograph of the Chicago skyline in a magazine recently published by the terror group's propaganda arm.
US intelligence "intercepted the packages in transit," the senior intelligence official said, searched them, and then allowed them to continue to Chicago.
"The dry run is always important to al Qaeda," said Dick Clarke, a former White House counterterrorism official and now an ABC News consultant. "In this case they wanted to follow the packages using the tracking system to know exactly when they got to a point, how long the timer had to be set for, so the bomb would go off at the right point, which presumably was over Chicago."
The US official said the CIA feared the packages "were intended to probe the security system for air cargo but there was nothing in them that could have been used to hide a bomb."
While officials believed air cargo might be used for an attack, "no one in the US government had specific timing or date" for the real bombs, the senior US official said.
The White House said it only learned of the actual air cargo plot late Thursday night when Saudi intelligence provided "a tip" about the bombs being shipped.
Homeland Security Secretary Janet Napolitano said the tip involved the FedEx and UPS tracking numbers which made it possible for the US to stop the shipments at transfer points in Dubai and the United Kingdom.
"We were able to identify where they were emanating from by package number, where they were located," Napolitano told ABC News.
The senior US official said the tracking numbers were not known to the US until after the packages had left Yemen.
The timing is significant because one of the bombs, the one shipped by FedEx, was moved to Dubai on two separate Qatar Air passenger jets.
The UPS shipment was moved on an all-cargo flight through Germany and on to England where it was to have been sent to the United States.
US and British authorities say they now believe that the bombs, hidden in desktop printers, were designed to be detonated on board the aircraft carrying them.
"If one cargo plane is taken down by a bomb," said Brad Garrett, an ABC News consultant and former FBI agent, "you could literally shut down cargo transport across the world."