(AP) - Yemeni authorities are checking dozens morepackages in a search for the terrorists who tried to mail bombs toChicago-area synagogues in a brazen plot that heightened fears of anew al-Qaida terror attack.Click "NEXT" for President Obama's comments on the packages.

Authorities on three continents thwarted the attacks when theyseized explosives on cargo planes in the United Arab Emirates andEngland on Friday. The plot sent tremors throughout the U.S., whereafter a frenzied day searching planes and parcel trucks for otherexplosives, officials temporarily banned all new cargo from Yemen.

Several U.S. officials said they were increasingly confidentthat al-Qaida's Yemen branch, the group behind the failed Detroitairliner bombing last Christmas, was responsible.

President Barack Obama called the coordinated attacks a"credible terrorist threat." The bombs were discovered just daysbefore the U.S. national elections.

A Yemeni security official said investigators there wereexamining 24 other suspect packages in the capital, San'a. He spokeon condition on anonymity because he was not authorized to releaseinformation and refused to provide more details.

Authorities were questioning cargo workers at the airport aswell as employees of the local shipping companies contracted towork with FedEx and UPS, the official said.

In Dubai, where one of the two bombs was found in a FedExshipment from Yemen, police said it contained a powerful explosiveand bore the hallmarks of al-Qaida.

The white powder explosives were discovered in the ink cartridgeof a computer printer, said a police statement carried by theofficial state news agency WAM. The device was rigged to anelectric circuit, and a mobile phone chip was hidden inside theprinter, the statement said.

The police said the bomb was prepared in a "professionalmanner."

Yemen promised to investigate the plot. The U.S. has FBI,military and intelligence officers stationed in the country toconduct an inquiry. There are only a handful of internationalshipping locations in the impoverished Arab nation, but U.S.officials worried that record keeping would be sparse andinvestigators would have to rely more on intelligence sources toidentify the would-be bombers.

The other package was found at an airport in central England.Preliminary tests indicated both packages contained PETN, apowerful industrial explosive and the same chemical used in theattempted Christmas attack, U.S. officials said.

In San'a, there was no visible security presence Saturday at theUPS and FedEx offices, which are located on the same street.

An employee at the UPS office said they had been instructed notto receive any packages for delivery for the time being. He refusedto be identified by name because he said he had been instructed byauthorities not to talk to reporters.

No explosives were found on an Emirates Airlines passenger jetthat was escorted down the coast to New York by U.S. fighter jets.

"The forensic analysis is under way," Obama's counterterrorchief John Brennan said. "Clearly from the initial observation,the initial analysis that was done, the materials that were foundin 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 saidanyone associated with the group was a subject of concern.

That would include the radical U.S.-born Muslim cleric Anwaral-Awlaki, who now is in hiding in Yemen. He has been linked in theChristmas attack and has inspired other terrorists with hismessages advocating violence. Also hiding in Yemen is Samir Khan,an American who declared himself a traitor and helps produceal-Qaida propaganda.

The terrorist efforts "underscore the necessity of remainingvigilant against terrorism," Obama said.

The Homeland Security Department said it was stepping up airlinesecurity, but White House spokesman Robert Gibbs said Americans didnot need to change their travel plans. Obama was not canceling hisweekend campaigning plans that included stops in Philadelphia andChicago.

After a day of searches in Philadelphia, Newark, New Jersey, andNew York City, no explosives were found inside the United States,though the investigation was continuing on at least one suspiciouspackage late Friday night.

Intelligence officials were onto the suspected plot for days,officials said. The packages in England and Dubai were discoveredafter Saudi Arabian intelligence picked up information related toYemen and passed it on to the U.S., two officials said.

Most of the officials spoke on condition of anonymity, citingthe ongoing investigation.

U.S. intelligence officials warned last month that terroristshoped to mail chemical and biological materials as part of anattack on the United States and other Western countries. The alertcame in a Sept. 23 bulletin from the Homeland Security Departmentobtained by The Associated Press.

Since the failed Christmas bombing, Yemen has been a focus forU.S. counterterrorism officials. Before that attack, the U.S.regarded al-Qaida's branch there as primarily a threat in theregion, 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 andrecruiting. Authorities believe about 300 al-Qaida members operatein Yemen.

The Yemeni government has stepped up counterterrorismoperations, with help from the U.S. military and intelligenceofficials.FBI: No explosives on plane with cargo from Yemen at JFK