Senior U.S. officials met Sunday to develop a U.S. response to the al-Qaida faction linked to the powerful explosives addressed to synagogues in Chicago.
Investigators were still piecing together the potency and construction of two bombs they believed were designed by the top explosives expert working for al-Qaida in the Arab Peninsula, the Yemen-based militant faction thought to be behind the plot. Yemeni authorities hunted suspects linked to the group, but on Sunday they released a woman engineering student arrested earlier, saying someone else had posed as her in signing the shipping documents.
But authorities admitted Sunday how close the terrorists came to getting their bombs through, and a senior U.S. official said investigators were still trying to figure out if other devices remained at large.
"We're trying to get a better handle on what else may be out there," deputy national security adviser John Brennan told NBC's "Meet the Press" on Sunday. "We're trying to understand better what we may be facing." He told CNN's "State of the Union" that "it would be very imprudent ... to presume that there are no others (packages) out there."
Brennan said authorities are "looking at the potential that they would have been detonated en route to those synagogues aboard the aircraft as well as at the destinations. But at this point we, I think, would agree with the British that it looks as though they were designed to be detonated in flight." He made those remarks on CBS' "Face the Nation."
Britain's prime minister, David Cameron, had raised the possibility that the bombs were aimed at blowing up the planes carrying them, but Brennan and other officials had previously concentrated more on the threat to the American synagogues where the bombs were addressed.
One of the explosive devices found inside a shipped printer cartridge in Dubai had flown on two airlines before it was seized, first on a Qatar Airways Airbus A320 jet to Doha and then on an as-yet-undisclosed flight from Doha to Dubai. The number of passengers on the flights were unknown, but the first flight had a 144-seat capacity and the second would have moved on one of a variety of planes with seating capacities ranging from 144 to 335.
Such a plot aimed at blowing up jets in mid-flight is not new for al-Qaida. A mid-1990s scheme hatched by now-imprisoned terrorist mastermind Khalid Sheikh Mohammed aimed to bring down a dozen jets simultaneously, but the plan was shelved in favor of the "flying bomb" approach used during the 9/11 attacks.
After masterminding the attempt last December to blow up a U.S.-bound airliner with explosives hidden in a militant's underwear, the Yemen terror affiliate appears to have nearly pulled off its own audacious plot capitalizing on weak points in the world's aviation security and cargo systems.
The U.S. has tried to kill or capture the group's leaders, but the American response to the thwarted attacks was still being developed Sunday. Brennan headed up a meeting of national security and intelligence officials at the White House to determine the U.S. response in concert with a Yemeni government that has been reluctant to give free rein to the American military in taking on the militants.
A Yemeni official said Sunday his government is aiming for a "surgical" response with the help of the U.S. against the cell that carried out the plot. The official spoke on condition of anonymity because of the sensitivity of the talks.
As the two countries decide a course of action, new details have emerged about the events leading up to the near-disaster. U.S. officials said a call from Saudi intelligence with information about packages containing explosives led to frantic search in Dubai and England.
"It was a race against the clock to find those packages, to neutralize them," Brennan told CNN on Sunday.
German Interior Minister Thomas de Maiziere said Sunday that German Federal Police were tipped off to the suspicious package on board the cargo plane Friday. The package was flown from Yemen to Cologne-Bonn airport, where UPS has its hub. From there it was transferred to a plane bound for Britain's East Midlands airport.
De Maiziere said that by the time German officials received the information, the package was already en route to Britain. The Germans then alerted their British colleagues, who had also been contacted by the Saudis.
The cargo plane landed in the dead of night at East Midlands Airport in central England on what seemed like a routine trans-Atlantic run. The plan was to stop at East Midlands - a relatively small airport that that handles both passengers and cargo - then continue to Philadelphia and Chicago in the United States.
There was almost no movement at the airport when the flight landed shortly after 3 a.m., and British officials removed cargo from the plane for an extensive search. As a standard precaution, a cordon was put in place outside the cargo area of the airport, even though there was very little traffic in those pre-dawn hours.
But the search came up empty. Even a computer printer cartridge later found to contain plastic explosives was cleared, and the cordon was removed at around 10 a.m., restoring traffic flow.
The incident seemed almost over - but then officials in Dubai told their British counterparts that a suspicious computer printer cartridge had been found to contain lethal PETN explosives.
The Dubai officials told British police precisely how to pinpoint the explosive, which was carefully placed to pass through an X-ray machine undetected.
The cordon went back up, the search teams went back in and this time they found the deadly explosive, judged capable of blowing up a plane mid-flight.
What happened in Dubai was even more troubling. The bomb had traveled on two commercial passenger planes.
The package with the second bomb arrived in Qatar Airways' hub in Doha, Qatar, on one of the carrier's flights from the Yemeni capital San'a. It was then shipped on a separate Qatar Airways plane to Dubai in the United Arab Emirates, where it was discovered by authorities late Thursday or early Friday.
Police in Yemen had arrested a young engineering student on suspicion of mailing the bombs but released her Sunday. The Yemeni official said authorities concluded that the woman's identity had been stolen and "she wasn't the individual who signed the shipping manifesto."
UK Home Secretary Theresa May said Sunday the mail bomb plotters would not have been able to control where the bombs detonated because cargo planes often change their routes at the last minute. She said it was unclear if the bomb found at a UK airport would have exploded over Britain or the United States.
"As I understand it, with these freight flights sometimes the routing can change at the last moment so it is difficult for those who are planning the detonation to know exactly where - if it is detonated to a time, for example - exactly where the aircraft will be," she said.
She said the device was capable of downing an aircraft if it was detonated while the plane was in flight.
Forensic analysis indicates that the same bombmaker had a hand in the devices used in the failed bombing on a Detroit-bound airliner last Christmas and the attack on Saudi Arabia's counterterrorism chief last year. All three bombs contained the industrial explosive PETN. The latest bombs have been described as sophisticated and professional.
U.S. intelligence officials believe the suspected bombmaker is a 28-year-old Saudi named Ibrahim Hassan al-Asiri. The militant's own brother died in the attack against the Saudi counterterrorism chief.
U.S. intelligence is also looking at U.S.-born cleric Anwar al-Awlaki. He has been linked to the Christmas attack and has inspired other terrorists with his violent message. He's believed to be hiding out in Yemen.
The Yemeni official said that while more than one source has indicated that al-Awlaki blessed this operation, Yemen doesn't believe he was involved in the operational planning.
Schreck reported from Dubai. Matt Apuzzo and Kimberly Dozier in Washington, Melissa Eddy in Berlin and Gregory Katz in London also contributed to this report.