U.S. Official: Yemen Cargo Bomb Threat Ongoing

Updated 10:51 a.m. ET

Powerful explosives addressed to Chicago synagogues may have been intended to destroy the planes they were sent on, security officials acknowledged as they tried to figure out how to respond to the al Qaeda-linked plot.

Disaster was narrowly averted, officials said Sunday. One device almost slipped through Britain and another seized in Dubai in the United Arab Emirates was unwittingly flown on two passenger jets.

Investigators were still piecing together the potency and construction of two bombs they believed were designed by the top explosives expert working for the Yemen-based faction thought to be behind the plot.

CBS News homeland security correspondent Bob Orr reports that U.S. officials now have no doubt the Yemen-based al Qaeda franchise was behind the cargo bomb plot.

Sources tell CBS News that about 15 other "suspect" packages sent from Yemen around the same time as the bombs have now been located and cleared. But, Orr reports, officials have voiced concern that there could still be other explosive packages out there, and either way, AQAP presents a long term threat to the U.S.

On Monday, the U.S. and allied governments said they were tightening scrutiny of air cargo and shipped packages, asking consumers and businesses for more vigilance as investigators scanned for more mail bombs possibly sent from Yemen.

U.S. counterterrorism officials warned local law enforcement and first responders to be on the lookout for mail with unusual characteristics that could mean dangerous substances are hidden inside.

The FBI and Homeland Security Department cautioned that foreign-origin packages without return addresses and excessive postage require a second look, according to an advisory sent to local officials around the country that was obtained Monday by The Associated Press.

John Pistole, the head of the Transportation Security Administration, told CBS' "The Early Show" on Monday that the TSA was working with its partners across the globe to try to identify where any other suspect packages might be.

"We're aware that were facing a determined enemy," Pistole told "Early Show" co-anchor Maggie Rodriguez, adding that TSA agents had been sent to Yemen to help screen U.S.-bound cargo - so when a ban imposed Saturday on all shipments from the Arab country to the U.S. is dropped, packages can be shipped more safely.

An official security source said that United Arab Emirates authorities are tracing the serial numbers of a mobile phone circuit board and computer printer used in the mail bomb sent from Yemen and found in Dubai.

The source told The Associated Press on Monday the UAE is sharing the numbers with other countries including the United States in an effort to track the origins of the bomb parts. The person spoke on condition of anonymity because the investigation is ongoing.

According to CBS News terrorism consultant Juan Zarate, al Qaeda's affiliate group in Yemen is "the most dangerous of al Qaeda's branches or allies."

Yemeni authorities hunted suspects linked to the group, but released a female computer engineering student arrested Saturday, saying someone else had posed as her in signing the shipping documents.

Authorities admitted how close the terrorists came to getting their bombs through, and a senior U.S. official said investigators were still trying to figure out if other devices remained at large.

"We're not presuming that we have, you know, found all (the) devices that are out there," White House counterterrorism adviser John Brennan told Face the Nation host Bob Schieffer on Sunday.

Authorities are also "looking at the potential that they would have been detonated en route to those synagogues aboard the aircraft as well as at the destinations," Brennan told Schieffer.

Watch: Brennan on "Face the Nation"
"Face the Nation" Full Transcript (PDF)
GOP's King: Admin Handled Yemen Plot Perfectly

After masterminding the attempt last December to blow up a U.S.-bound airliner with explosives hidden in a passenger's underwear, the Yemen terror group appears to have nearly pulled off an audacious plot capitalizing on weak points in the world's aviation security and cargo systems.

The U.S. has been trying to kill or capture its leaders, and the American response to the thwarted attacks was still being developed Sunday. Brennan headed a meeting of national security and intelligence officials at the White House to determine the U.S. response in concert with a Yemeni government that has been reluctant to give the Americans free rein.

About 50 elite U.S. military experts are in Yemen training its counterterrorism forces and Washington is giving $150 million in military assistance this year for helicopters, planes and other equipment.

Yemen: The Next Front Line Against al Qaeda

A Yemeni official said Sunday his government is aiming for a "surgical" response with the help of the U.S. against the plotters. The official spoke on condition of anonymity because of the sensitivity of the talks.

New details have emerged about events leading up to the near-disaster. U.S. officials said a call from Saudi intelligence about packages containing explosives led to a frantic search in Dubai and England.

German Interior Minister Thomas de Maiziere said German Federal Police were tipped off to a suspicious package Friday. The package was flown from Yemen to Cologne-Bonn airport, where UPS has its hub. From there it was transferred to a plane bound for Britain's East Midlands airport in central England.

After the cargo plane landed at East Midlands, an initial search came up empty. But after consulting with officials in Dubai, British police found the lethal explosive PETN, or pentaerythritol tetranitrate.

PETN Explosive a Favorite of Terrorists

What happened in Dubai was even more troubling: The bomb had traveled on two commercial passenger planes, a Qatar Airways spokesman said.

The package with the second bomb arrived in Qatar Airways' hub in Doha, Qatar, on one of the carrier's flights from the Yemeni capital San'a. It was then shipped on a separate Qatar Airways plane to Dubai, where it was discovered by authorities late Thursday or early Friday.

U.S. intelligence officials believe the suspected bombmaker is a 28-year-old Saudi named Ibrahim Hassan al-Asiri, thought to be in Yemen.