Last Updated 1:14 p.m. ET
After intercepting two mail bombs addressed to Chicago-area synagogues, investigators have located and cleared 10-20 other packages shipped from Yemen that were the focus of yesterday's search, a U.S. official told CBS News
. CBS News Homeland Security correspondent Bob Orr
also reports that the so-called printer bombs found in two packages that were en route to the United States contained significant amounts of the explosive PETN from 5 to 8 times the amount of explosive as the "underwear bomber."
Authorities on three continents thwarted the attacks when they seized explosives on cargo planes in the United Arab Emirates and England on Friday.
The U.S. Postal Service has temporarily suspended all inbound mail originating in Yemen effective today.
A U.S. official has told CBS News
that the cargo planes stopped in the U.K. and in Dubai were searched in response to specific warnings that they were carrying suspicious packages originating in Yemen and addressed to synagogues or Jewish centers in Chicago.
Other planes were searched as a precaution, the official said, because they carried packages originating in Yemen.
Orr reports that the specific tip which led to the discovery of the potential bombs overseas came from Saudi Arabia.
Appearing on "The Early Show on Saturday Morning,"
Homeland Security Secretary Janet Napolitano credited good intelligence gathering for discovering the two packages in Dubai and England before they made their way to the U.S..
"We have a good partnership with the Saudis, we have systems in place, so the minute anything is raised, we are able to work internationally with our private sector partners, identify packages and get them right into forensic examination," she said. Yemen: The Next Front Line Against al QaedaSecurity Gaps Plague Cargo Shipping
Napolitano said that this particular threat has the hallmarks of an al Qaeda plot. "We will trace it to its ultimate source and will be relentless doing so," she told anchor Chris Wragge
She also said while the discovery disrupted the plot, the extent of it is unknown. "We're not sitting back - we are being very, very vigilant. We don't know that this was the entire universe of the plot. That's why we've taken many of the measures we've taken over the past 48 hours," including heightened cargo screening and additional security at airports
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 Qaeda'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.
This morning the head of security at the Egyptian state-run Egypt Air, Essam Gamal, said authorities have begun manually inspecting all packages being sent abroad or brought to the country aboard cargo planes, especially those coming from Yemen.
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.
Orr reports that sources revealed the devices tested positive for PETN, a powerful industrial explosive - the same explosive compound used in last year's attempted Christmas Day attack in Detroit.
The white powder explosives were discovered in the ink cartridge of a computer printer, said a Dubai 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 Sana'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," President 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 Mr. Obama didn't specifically accuse Yemen's al Qaeda branch, Brennan called it the most active al Qaeda 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 Qaeda 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 Qaeda's branch there as primarily a threat in the region, not to the United States.
The Yemen branch, known as al Qaeda in the Arabian Peninsula, has since become a leading source of terrorist propaganda and recruiting. Authorities believe about 300 al Qaeda members operate in Yemen.
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