The explosives, addressed to Chicago-area synagogues, were pulled off airplanes in England and the United Arab Emirates early Friday morning, touching off a tense search for other devices.
It still wasn't clear whether the bombs, which officials said were wired to cell phones, timers and power supplies, could have been detonated remotely while the planes were in the air, or when the packages were halfway around the world in the U.S. But the fact that they made it onto airplanes showed that nearly a decade since the Sept. 11, 2001 attacks, terrorists continue to probe and find security vulnerabilities.
The packages were addressed to two synagogues in the Chicago area. But British Prime Minister David Cameron said Saturday that he believes the explosive device found at the East Midlands Airport in central England was intended to detonate aboard the plane.
British Home Secretary Theresa May added that the bomb was powerful enough to take down the plane. A U.S. official said authorities believe a second device found in Dubai was similarly potent.
Yemeni President Ali Abdullah Saleh told reporters that the United States and United Arab Emirates had provided intelligence that helped identify the woman suspected of mailing the packages.
A Yemeni security official said the young woman was a medical student and that her mother also was detained.
The police action was part of a widening manhunt for suspects believed to have used forged documents and ID cards, Yemeni officials said. One member of Yemen's anti-terrorism unit said the other suspects had been tied to al-Qaida.
The officials, like many in the U.S., spoke on condition of anonymity to discuss the ongoing investigation unfolding on three continents.
Al-Qaida's Yemen branch, known as al-Qaida in the Arabian Peninsula, took credit for a failed bombing aboard a Detroit-bound airliner last Christmas. The bomb used in that attack contained PETN, an industrial explosive that was also used in the mail bombs found Friday.
The suspected bombmaker behind the Christmas Day attack, Ibrahim Hassan al-Asiri, is also the prime suspect in the mail bomb plot, several U.S. officials said. Al-Asiri also helped make another PETN device for a failed suicide attempt against a top Saudi counterterrorism official last year. The official survived, but his attacker died in the blast.
"The forensic analysis is under way," Obama's counterterror chief John Brennan said Friday. "Clearly from the initial observation, the initial analysis that was done, the materials that were found in the device that was uncovered was intended to do harm."
Officials said the plot was discovered thanks to intelligence passed from Saudi Arabia. Without that tip, it's unclear whether anyone would have discovered the bombs before they were airborne -- or on U.S. soil.
The U.S. already had been on the lookout for a such a plot, having received indications that the Yemeni-based al-Qaida faction was interested in "exploring an operation involving cargo planes," a U.S. counterterrorism official said.
U.S. authorities then acted quickly after receiving a tip "that suspicious packages may be en route to the U.S" -- specifically Chicago -- the official said, speaking on condition of anonymity to discuss intelligence matters.
Currently, American officials do not get details about the contents of a U.S.-bound cargo plane until four hours before it's scheduled to land. In the case of long distance flights, those planes would already be airborne. Once a plane lands, officials screen packages that they feel warrant a closer look.
The failed attack should be a "wake up call" that the U.S. needs to step up security on cargo planes, the top Republican on the Senate Intelligence Committee, Kit Bond, said.
U.S. officials were still cobbling together details about the packages but one official briefed on the investigation said authorities believed the plotters may have been associated with two institutions called "Yemen American Institute (for) Languages-Computer-Management" or the "American Center for Training and Development."
It was not immediately clear whether those institutions even exist or whether that information came from false documents or fake addresses.
The U.S. temporarily banned all incoming cargo and mail from Yemen. A UPS employee in Yemen said the office has temporarily halted receiving any packages for delivery.
In Chicago, the leader of a North Side synagogue told members of his congregation at weekend services that a smaller congregation that uses their building was one of the targets of the plot.
Rabbi Michael Zedek of Emmanuel Congregation said he was told by a Jewish leader in Chicago that the smaller Or Chadash congregation was one of the targets. The FBI did not confirm that, and both Zedek and Chadash Rabbi Larry Edwards said they had not spoken to law enforcement.
Or Chadash members took the news calmly, Edwards said. The synagogue has about 100 members, and serves Jews in the gay community and their families.
The White House said Brennan called Yemen's president and made clear that the U.S. is ready to help the Yemeni government in the fight against al-Qaida. The U.S. already assists Yemen with airstrikes and other counterterrorism support.
Yemen's al-Qaida branch is the most active of the terrorist group's affiliates and has increasingly become the face of its recruitment efforts in the West. The country is home to radical U.S.-born cleric Anwar al-Awlaki. He has been linked in the Christmas attack and has inspired other terrorists with his violent message.
Also hiding in Yemen is Samir Khan, an American who declared himself a traitor and helps produce al-Qaida propaganda.
The Homeland Security Department said it was stepping up airline security, but White House spokesman Robert Gibbs said Americans did not need to change their travel plans.