WASHINGTON — Al-Qaida's top bombmaker raised his game, officials believe, by following his miss on a crowded U.S.-bound passenger jet last Christmas with four times more explosives packed into 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 contained about 80 grams.
Early forensics on the two bombs packed inside computer printer cartridges point to Ibrahim al-Asiri, the master bomb maker for the Yemen-based group known as al-Qaida in the Arabian Peninsula.
"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."
Al-Qaida's propaganda machine remained unusually silent about the attacks Monday as U.S. counterterrorism officials looked for new ways to root out the Yemen-based group. Its members number about 300 people hidden in an area of rugged, desert twice the size of Wyoming.
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With the U.S. already deeply involved in Yemen's fight against terrorism, it was not immediately obvious how to effectively increase military and intelligence efforts in the impoverished country.
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.
And 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 following the ink cartridge bomb plot.
The Dutch anti-terrorism agency banned all airborne post and freight from Yemen entering the Netherlands.
The exposed plot could fuel calls for the wider use of imaging technology designed to detect explosives, which is not standard, but freight firms are reluctant to bear the full cost.
Tighter international air cargo security rules could deal a blow to trade and the world economy as it recovers from the global recession. According to airlines association IATA, about 35 percent of the value of world trade is carried by air.
Philip Baum, editor of Aviation Security International magazine, said new technology would not eliminate the risk or airline attacks.
"The only thing that has prevented things has either been good luck or people," he told Reuters.
Yemeni authorities on Monday continued to hunt for suspects tied to the mail bomb plot, but a young woman arrested soon after the attacks were thwarted was released. Investigators there said someone had stolen her identity and used it to mail the package.
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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.
The U.S. had been monitoring intelligence on an al-Qaida mail bomb plot for days when a specific tip came in from Saudi Arabia, identifying tracking numbers for the packages. A Yemeni official, speaking on condition of anonymity to discuss the ongoing investigation, identified Jabir al-Faifi, a Saudi militant who had joined al-Qaida in Yemen, as the tipster.
Tipster's role unclear
It's unclear, however, what level of detail al-Faifi provided. He was captured in Yemen last month and was turned over to the Saudis before the packages were mailed, making it unlikely he would have known the tracking numbers.
Nobody, including the Internet-savvy al-Qaida 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. The group claimed credit for the Christmas attack three days later.
Though al-Qaida core is based in the lawless tribal regions of Pakistan, offshoot groups 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.
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-Qaida has had mixed results.
Brennan said Yemeni cooperation is better than it has ever been but still could be better. The failed bombing will put added pressure on Yemen to go even further, perhaps by allowing additional U.S. troops or airstrikes within its border.
The Associated Press and Reuters contributed to this report.