WASHINGTON - The mail-bomb plot stretching from Yemen to Chicago may have been aimed at blowing up planes in mid-flight and was only narrowly averted, officials said yesterday, acknowledging that one device almost slipped through Britain and another seized in Dubai was unwittingly flown on two passenger jets.

Senior U.S. officials met yesterday to develop a U.S. response to the al Qaeda faction linked to the powerful explosives addressed to synagogues in Chicago that would have gone via Philadephia.

Investigators were still studying the two bombs they believed were designed by the top explosives expert working for al Qaeda in the Arab Peninsula, the Yemen-based militant faction thought to be behind the plot. Yemeni authorities yesterday released a woman engineering student arrested earlier, saying someone else had posed as her in signing the shipping documents.

Authorities admitted yesterday 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 there were other devices in the pipeline.

"We're trying to get a better handle on what else may be out there," deputy national security adviser John Brennan told NBC's "Meet the Press" yesterday.

"We're trying to understand better what we may be facing."

He told CNN's "State of the Union" that "it would be very imprudent . . . to presume that there are no others [packages] out there."

Brennan said authorities are "looking at the potential that they would have been detonated en route to those synagogues aboard the aircraft as well as at the destinations. But at this point we, I think, would agree with the British that it looks as though they were designed to be detonated in flight." He made those remarks on CBS' "Face the Nation."

British Prime Minister David Cameron had raised the possibility that the bombs were aimed at blowing up the planes carrying them, but Brennan and other officials had previously concentrated more on the threat to the American synagogues to which the bombs were addressed.

One of the explosive devices found inside a shipped printer cartridge in Dubai had flown on two airlines before it was seized.

After masterminding the attempt last December to blow up a U.S.-bound airliner with explosives hidden in a militant's underwear, the Yemen terror affiliate appears to have nearly pulled off its own audacious plot.

The U.S. has tried to kill or capture the group's leaders, but the American response to the thwarted attacks was still being developed yesterday. Brennan met with 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 free rein to the American military to take on the militants.

Details have emerged about the events leading up to the near-disaster.

U.S. officials said a call from Saudi intelligence with information about packages containing explosives led to searches in Dubai and in England.

The cargo plane landed in the dead of night at East Midlands Airport in central England on what seemed like a routine trans-Atlantic run. The plan was to stop at East Midlands - a relatively small airport that that handles both passengers and cargo - then continue to Philadelphia and Chicago in the United States.

British officials searched the plane to no avail and were set to clear it to continue its flight when officials in Dubai alerted their British counterparts that a suspicious computer-printer cartridge had been found to contain lethal PETN explosives. The Dubai officials told British police how to pinpoint the explosive, which was carefully placed to pass through an X-ray machine undetected.