After intercepting two mail bombs addressed to Chicago-area synagogues, investigators are searching for two dozen more suspect packages that terrorists in Yemen attempted to smuggle onto aircraft in a brazen al-Qaida terror plot.
Authorities on three continents thwarted the attacks when they seized explosives on cargo planes in the United Arab Emirates and England on Friday. The plot sent tremors throughout the U.S., where after a frenzied day searching planes and parcel trucks for other explosives, officials temporarily banned all new cargo from Yemen.
Several U.S. officials said they were increasingly confident that al-Qaida's Yemen branch, the group behind the failed Detroit airliner bombing last Christmas, was responsible.
President Barack Obama called the coordinated attacks a "credible terrorist threat."
A Yemeni security official said the investigation involved about 26 suspected packages. Some had left Yemen and others were still in the country, he said, speaking on condition of anonymity because he was not authorized to release information.
Authorities were questioning cargo workers at the airport as well as employees of the local shipping companies contracted to work with FedEx and UPS, the official said.
In Dubai, where one of the two bombs was found in a FedEx shipment from Yemen, police said it contained PETN, a powerful industrial explosive, and bore the hallmarks of al-Qaida.
The white powder explosives were discovered in the ink cartridge of a computer printer, said a police statement carried by the official state news agency WAM. The device was rigged to an electric circuit, and a mobile phone chip was hidden inside the printer, the statement said.
The police said the bomb was prepared in a "professional manner."
Yemen promised to investigate the plot. The U.S. has FBI, military and intelligence officers stationed in the country to conduct an inquiry. There are only a handful of international shipping locations in the impoverished Arab nation, but U.S. officials worried that record keeping would be sparse and investigators would have to rely more on intelligence sources to identify the would-be bombers.
The other package was found at an airport in central England. Preliminary tests indicated the packages contained PETN, the same chemical as in the attempted Christmas attack, U.S. officials said.
In San'a, the capital of Yemen, there was no visible security presence Saturday at the UPS and FedEx offices, which are located on the same street.
An employee at the UPS office said they had been instructed not to receive any packages for delivery for the time being. He refused to be identified by name because he said he had been instructed by authorities not to talk to reporters.
Rep. Jane Harman, a California Democrat who serves on the Homeland Security Committee and was briefed by John Pistole, the administrator of the Transportation Security Administration, told The New York Times that both packages contained explosive-filled printer cartridges. One, she said, had a timer and the other was rigged to use a cell phone as a detonator.
No explosives were found on an Emirates Airlines passenger jet that was escorted down the coast to New York by American fighter jets.
"The forensic analysis is under way," Obama's counterterror chief John Brennan said. "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."
While Obama didn't specifically accuse Yemen's al-Qaida branch, Brennan called it the most active al-Qaida franchise and said anyone associated with the group was a subject of concern.
That would include the radical U.S.-born Muslim cleric Anwar al-Awlaki, who now is in hiding in Yemen. 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 terrorist efforts "underscore the necessity of remaining vigilant against terrorism," the president said.
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.
After a day of searches in Philadelphia, Newark, N.J., and New York City, no explosives were found inside the United States, though the investigation was continuing on at least one suspicious package late Friday night.
Intelligence officials were onto the suspected plot for days, officials said. The packages in England and Dubai were discovered after Saudi Arabian intelligence picked up information related to Yemen and passed it on to the U.S., two officials said.
Most of the officials spoke on condition of anonymity, citing the ongoing investigation.
U.S. intelligence officials warned last month that terrorists hoped to mail chemical and biological materials as part of an attack on the United States and other Western countries. The alert came in a Sept. 23 bulletin from the Homeland Security Department obtained by The Associated Press.
Since the failed Christmas bombing, Yemen has been a focus for U.S. counterterrorism officials. Before that attack, the U.S. regarded al-Qaida's branch there as primarily a threat in the region, not to the United States.
The Yemen branch, known as al-Qaida in the Arabian Peninsula, has since become a leading source of terrorist propaganda and recruiting. Authorities believe about 300 al-Qaida members operate in Yemen.
The Yemeni government has stepped up counterterrorism operations, with help from the U.S. military and intelligence officials.