Before the packages reached their destinations, U.S. authorities seized and searched the boxes. They now appear to have been sent by the Yemeni militant group al Qaeda in the Arabian Peninsula (AQAP) to test the logistics of the air cargo system, a U.S. 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.
Special Section: Terrorism in the U.S.
The theory is that AQAP mailed the innocuous packages to monitor delivery times, to determine when planes would be in the air and when the packages would be on the ground, reports CBS News homeland security correspondent Bob Orr. The information could have been useful in setting timers on future mail bombs.
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.
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The apparent dry run was first disclosed Monday night by ABC News.
The official said authorities, already aware of the militants' interest in striking at aviation, "obviously took notice" this past weekend 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 (September) 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 toner cartridges of computer printers. Investigators have centered on the Yemeni al Qaeda faction's top bomb maker, who had previously designed a bomb that failed to go off on a crowded U.S.-bound passenger jetliner last Christmas.
This time, authorities believe that master bomb maker Ibrahim al-Asiri packed four times as much explosives into the bombs hidden last week on flights from Yemen. The two bombs contained 300 and 400 grams of the industrial explosive PETN, according to a German security official, who briefed reporters Monday in Berlin on condition of anonymity in line with department guidelines.
By comparison, the bomb stuffed into a terrorist suspect's underwear on the Detroit-bound plane last Christmas contained about 80 grams.
"It shows that they are trying to again make different types of adaptations based on what we have put in place," said John Brennan, President Barack Obama's counterterrorism adviser. "So the underwear bomber, as well as these packages, are showing sort of new techniques on their part. They are very innovative and creative."
The U.S. and its allies Monday further tightened scrutiny of shipments from Yemen. U.S. counterterrorism officials warned police and emergency personnel to be on the watch for mail with characteristics that could mean dangerous substances are hidden inside.
Germany's aviation authority extended its ban on air cargo from Yemen to include passenger flights. Britain banned the import of larger printer cartridges by air on Monday as it also announced broader measures to halt air cargo from Yemen and Somalia.
A Yemeni government statement Tuesday expressed "sorrow and astonishment" at Germany's decision and said such a "rushed and exaggerated reaction to suspicious packages will harm Yemen's efforts in combating terrorism and serves no one but al Qaeda terrorists who always sought to ... hurt Yemen's interests, reputation and relations with regional and international friends and partners."
U.S. and British officials said they believed the targets were planes, not the two Chicago-area synagogues named on the addresses. Exactly how the bombs would have worked, however, remains a focus of investigators.
Activating a bomb by cell phone while a plane is in midair is unreliable because cell service is spotty or nonexistent at high altitudes. Further complicating the plot, it be would unlikely for terrorists in Yemen to know which planes the bombs had been loaded onto and when they were airborne.
With U.S.-bound cargo out of Yemen temporarily frozen, Transportation Security Administration chief John Pistole said Monday the U.S. would provide Yemen with new screening equipment for cargo. Yemen has promised to step up its security at airports.
Nobody, including the Internet-savvy al Qaeda group in Yemen, has taken credit for the failed attack. Jihadist Web sites contained numerous messages praising the attempted bombing but nothing official from the group's leadership.
Though al Qaeda's core is based in the lawless tribal regions of Pakistan, offshoots have sprung up in other countries, including Yemen and Algeria. The Yemen group is the most active affiliate and has become a leader in recruiting and propaganda, especially in the West thanks to its English-speaking, U.S.-born spokesman, Anwar al-Awlaki.
On Tuesday, Yemeni prosecutors charged al-Awlaki in absentia with plotting to kill foreigners.
The U.S. is providing some $300 million in military, humanitarian and development aid to Yemen this year, according to State Department counterterrorism coordinator Daniel Benjamin. About half of that is for military equipment and training, including some 50 special-operations trainers for Yemeni counterterror teams.
State Department spokesman P.J. Crowley said Monday that the U.S. would not reduce that aid in response to the failed attack.
The FBI, Pentagon and CIA all have people on the ground in Yemen, working with counterterrorism officials in Yemen. A military and intelligence campaign, financed and directed by the U.S., to target al Qaeda has had mixed results. Brennan said Yemeni cooperation is better than it has ever been but still could be better.