WASHINGTON – Two mail bombs intercepted in Britain and Dubai contained enough packed explosives to cause “significant” damage to the planes carrying them, German officials said Monday, a grim warning as investigators tried to trace bomb parts and scanned for more hidden explosives possibly sent from Yemen.

Authorities believe Yemeni-based terrorists sent two mail bombs addressed to Jewish synagogues in Chicago last week, but the devices appear to 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.

Yemeni security officials said Monday that a leading al-Qaida militant in Yemen who surrendered to Saudi Arabia last month provided information that helped in thwarting the mail bomb plot.

But he had been captured in Yemen and turned over to Saudi Arabia before the packages were shipped and likely would not have known the tracking numbers. Officials have said the tip that came in just before the plot unraveled was specific enough that it identified the tracking numbers.

The mail bombs intercepted last week contained 10.58 ounces (300 grams) and 15.11 ounces (400 grams) of the explosive PETN, a German security official said Monday. The official, who briefed reporters in Berlin on condition of anonymity in line with department guidelines, said that if the bombs had gone off the explosive effect would have been “significant.” The detonation mechanism was very sophisticated, said the official, who declined to elaborate.

By contrast, the bomb that failed to explode last Christmas Day on a Detroit-bound airliner used 80 grams of PETN.

One of the packages transited through a UPS hub at Germany’s Cologne airport before being intercepted at its next stop in England. The device had already left Germany when an alert from Saudi authorities was received in the early hours of Friday. Another package was intercepted in Dubai.

A second German official said that authorities in Berlin don’t yet know whether the bombs were supposed to be, or could have been, activated in the air or at their destination in the United States. Both officials said that Germany received its information on the bombs from authorities in Dubai and Britain.

“Depending on where it is placed on the plane and if it breached the exterior wall, it could bring the plane down,” said Leo West, former FBI explosives expert, who worked many major bomb cases including the 1998 U.S. embassy bombings in Africa. “It would be a sizable explosion. In a plane it doesn’t really take a whole lot at a high altitude.”

Bombs experts say the key is the altitude of the plane when the bomb goes off. The chances of the plane surviving a blast increase at lower altitude. At a higher altitude, the plane is pressurized and a little hole can easily become a big hole, bomber experts say.

An official United Arab Emirates security source said that 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.

Yemeni officials also said that Jabir al-Fayfi, a Saudi militant who had joined al-Qaida in Yemen, informed Saudi officials about the plan. The officials spoke on condition of anonymity because they were not authorized to talk to the press. Several tribal leaders with knowledge of the situation, who similarly spoke on condition of anonymity, also confirmed al-Fayfi’s role.

U.S. officials have said an alert from Saudi Arabia led to the interception Friday of two explosive devices hidden in packages addressed to Chicago-area synagogues, on planes transiting in Britain and Dubai. Al-Qaida in the Arabian Peninsula, the terror group’s affiliate in Yemen, is suspected in the attempted bombing.

The Saudi newspaper Al-Watan on Monday cited Saudi security officials saying that the kingdom gave U.S. investigators the tracking numbers of the packages.

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.

In the 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 from Yemen amid the current terrorist threat.

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.

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.

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.”