Washington ? American intelligence officials tracked and intercepted three suspicious packages in mid-September that they now suspect were sent to the U.S. as a dry run for the mail bomb scheme unleashed last week, a U.S. official said Monday.

The official also disclosed that both mail bombs, one recovered in Dubai and the other in Britain on Friday, were wired to detonators that used cell phone technology. It still was not clear 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, U.S. 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 to test the 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.