American intelligence officials tracked and intercepted three suspicious packages in mid-September that they now suspect were sent to the United States as a dry run for the mail bomb scheme intercepted last week, a US official said.
The official also disclosed that both mail bombs, one recovered in Dubai and the other in Britain last Friday, were wired to detonators that used cell phone technology.
It still was unclear whether those detonators would have been set off by telephone calls or by an internal alarm.
Before the September shipments reached their destinations in Chicago, US authorities seized and searched the boxes.
They removed "papers, books and other materials" that now appear to have been sent by the Yemeni militant group al Qaida in the Arabian Peninsula (Aqap) to test logistics of the air cargo system, the official said.
"We received information several weeks ago that potentially connected these packages to Aqap. The boxes were stopped in transit and searched. They contained papers, books, and other materials, but no explosives," said the official, who was familiar with details of the shipments and spoke on condition of anonymity to discuss classified intelligence.
The apparent dry run was disclosed first in a report by ABC News.
The official said authorities, already aware of the militants' interest in striking at aviation, "obviously took notice" and considered the likelihood that the militants might have extended their threat to the cargo system.
"When we learned of last week's serious threat, we recalled the incident and factored it in to our government's very prompt response," the official said.
The threat last week came in the form of explosive devices hidden in the cartridges of computer printers.
Investigators centred on the Yemeni al-Qa'ida faction's top bomb maker, who previously designed a bomb that failed to go off on a crowded US-bound passenger jet in the United States last December.Reuse content