The communication cards had been removed from the cell phones attached to the bombs, meaning the phones could not receive calls, officials said, making it likely the terrorists intended the alarm or timer functions to detonate the bombs.

“The cell phone probably would have been triggered by the alarm functions and it would have exploded midair,” said a U.S. official briefed on the investigation of the bombs taken off cargo planes Friday in England and the United Arab Emirates. This person, like other officials in this story, spoke on condition of anonymity to discuss the case.

The official also said Tuesday that each bomb was attached to a syringe containing lead azide, a chemical initiator that would have detonated PETN explosives packed into each computer printer toner cartridge. Both PETN and a syringe were used in the failed Christmas Day bombing of a Detroit-bound airliner linked to an al-Qaida branch in Yemen.

The Obama administration, which has been monitoring intelligence on possible mail plots since at least early September, was preparing new security rules for international cargo in response to the attempted attack.

Security officials are considering requiring that companies provide information about incoming cargo before planes take off, one U.S. official said. Currently, the U.S. doesn’t get that information until four hours before a plane lands.

A second official said the U.S. will also expand its definition of high-risk cargo, meaning more cargo will be screened from countries known as hotbeds of terrorism.

President Barack Obama stressed the need for stronger security for air cargo in a telephone conversation Tuesday with Ali Abdullah Saleh, Yemen’s president, the White House said.

Investigators believe al-Qaida mailed three innocent-looking packages from Yemen to Chicago in mid-September to watch the route they took.

One of those packages contained a copy of British author George Eliot’s 1860 novel “The Mill on the Floss.” Authorities were investigating whether it was a subtle calling card from Anwar al-Awlaki, the U.S.-born Yemeni cleric who has inspired a string of attempted attacks against the West.

The militant cleric is now a fugitive, targeted by a U.S. kill or capture list. Yemeni authorities put him on trial in absentia Tuesday, charging him as a new defendant in the October killing of a French security guard.

Al-Awlaki became well versed in English literature while in prison in Yemen from 2006 to 2007 and later posted online book reviews slamming Shakespeare and praising Charles Dickens. Beyond that, however, there was no immediate connection between al-Awlaki and the book found in the package mailed in September, one U.S. official said.

Shipping carriers allow Internet users to monitor packages from point to point through the international cargo system.

While a test run would have given al-Qaida a sense of the shipping routes, there was no guarantee the route would be the same a month — or even a day — later, officials at UPS and FedEx said Tuesday. Routes change based on the weather, cargo volume and plane schedules, they said.

Neither company lets customers see precisely which planes their packages are on. Sometimes they are packed on cargo planes, sometimes on passenger planes. There is no way for customers to track their packages in real time while in flight, officials with both companies said.

Still, knowing the time shipments were logged in leaving Europe and the time they were scanned arriving in Chicago would have given al-Qaida operatives a large enough time window to allow them to have rigged their bombs to blow up somewhere along the way.

The packages sent last week were addressed to two Chicago-area synagogues. Because the addresses were out of date and the names on the packages included references to the Crusades — the 200-year wars waged by Christians largely against Muslims — officials do not believe the synagogues were the targets.

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Associated Press writer Adam Goldman contributed to this report.