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

Authorities believe Yemeni-based terrorists sent two mail bombs addressed to Jewish synagogues in Chicago 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 warn there may be more in the system.

The UAE security official told The Associated Press Monday the emirates are sharing the bomb part serial numbers with the United States, Yemen and other countries involved in the probe in an effort to track the bombs’ origins.

The person spoke on condition of anonymity because the investigation is ongoing.

Meanwhile, U.S. counterterrorism officials warned local law enforcement and emergency personnel to be on the watch 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.

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 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. One device almost slipped through Britain and the other seized in Dubai 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.

Authorities in the UAE discovered the powerful explosive PETN in a shipment that arrived at Dubai airport on a Qatar Airways passenger flight last week. The same explosive is the hallmark of a series of recent terror attempts by al-Qaida in the Arab Peninsula, the militant faction suspected in the mail bomb plot.

U.S. intelligence officials believe the suspected bombmaker is a 28-year-old Saudi named Ibrahim Hassan al-Asiri, thought to be in Yemen. Al-Asiri has been previously tied to a failed bombing attempt last Christmas Day on a Detroit-bound plane and an earlier failed attempt to kill a Saudi counterterrorism official. Both attempts used PETN and a detonator hidden in militants’ underwear.

The UAE security source said Monday that a green mobile phone circuit board, but no SIM card, was connected to the HP printer head, which in turn was linked to the explosives. The security source said investigators are examining the circuit board to determine whether it had been used before in the hope it might provide clues to who sent the bomb.

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

A Yemeni security official said that security authorities are still hunting for a young woman who is believed to have dropped off the mail bombs at a shipping point in the capital, San’a. An anti-terrorism official told The Associated Press that the young woman acted as a “middleman” with the group.

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.

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.

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Schreck reported from Dubai. Associated Press writers Matt Apuzzo, Kimberly Dozier and Adam Goldman in Washington, Melissa Eddy in Berlin and Gregory Katz in London also contributed to this report.