WASHINGTON - The U.S. and allied governments tightened their scrutiny of air cargo and shipped packages Monday, 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.

Authorities believe Yemeni-based terrorists sent two mail bombs addressed to Jewish synagogues last week, but the devices may have been aimed at blowing up planes in flight. While officials caught two bombs in the United Arab Emirates and the United Kingdom, U.S. officials say there may be more in the system.

Major cargo firms have already suspended shipments from Yemen and on Monday, Germany's aviation authority said the country has extended its ban on cargo aircraft from Yemen to include passenger flights amid the current terrorist threat.

One of the bombs that was mailed from Yemen and found by authorities was routed to London through the UPS hub in Cologne.

German aviation agency spokeswoman Cornelia Cramer said Monday that passenger flights from Yemen were being suspended until further notice. Germany stopped package deliveries from Yemen over the weekend.

The mail bomb plot 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 al-Qaida in the Arabian Peninsula, the Yemen-based faction thought to be behind the plot. 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 acknowledged 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.

Deputy national security adviser John Brennan, appearing on a round of television news shows Sunday, said that "it would be very imprudent ... to presume that there are no others (packages) out there."

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 said.

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.

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.

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.