Yemeni authorities are checking dozens more packages in a search for the terrorists who tried to mail bombs to Chicago-area synagogues in a brazen plot that heightened fears of a new al-Qaida terror attack.
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 US, where after a frenzied day searching planes and parcel trucks for other explosives, officials temporarily banned all new cargo from Yemen.
Several US 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." The bombs were discovered just days before the US national elections.
A Yemeni security official said investigators there were examining 24 other suspect packages in the capital, San'a. He spoke on condition on anonymity because he was not authorized to release information and refused to provide more details.
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 a powerful 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 US 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 US 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 both packages contained PETN, a powerful industrial explosive and the same chemical used in the attempted Christmas attack, US officials said.
In San'a, 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.
No explosives were found on an Emirates Airlines passenger jet that was escorted down the coast to New York by
US 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 US-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 messages advocating violence.
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," Obama 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.
Obama was not canceling his weekend campaigning plans that included stops in Philadelphia and Chicago.
After a day of searches in Philadelphia, Newark, New Jersey, 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 US, two officials said.
Most of the officials spoke on condition of anonymity, citing the ongoing investigation.
US 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 US counterterrorism officials. Before that attack, the US 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 US military and intelligence officials.