The two package bombs intercepted by authorities in Britain and Dubai last week appear to have been built to detonate "in flight" and to bring down the planes carrying them, President Obama's top counterterrorism adviser said.

"At this point we, I think, would agree with the British that it looks as though they were designed to be detonated in flight," said the adviser, John Brennan, speaking Sunday on the CBS program "Face the Nation."

The assessment, combined with the revelation that one of the packages traveled on passenger flights in the Middle East, underlined just how narrowly authorities had averted a potential catastrophe. It also raised puzzling questions about why the packages, which contained bombs skillfully packed inside modified printer cartridges, were addressed to two synagogues in Chicago, a potential warning flag given that the packages originated in Yemen.

Germany on Monday said it would halt passenger flights from German airports to Yemen because of the terrorist threat, wire services reported. Germany suspended package deliveries from Yemen over the weekend. The bomb intercepted in London had been mailed from Yemen and routed through the UPS hub in Cologne, the Associated Press said.

On other Sunday morning talk shows, Brennan was more circumspect about the ultimate targets of the attack. He said on the ABC program "This Week with Christiane Amanpour" that authorities have "to look very carefully at whether or not they were going to be detonated on the aircraft or they were intended for the destination, and that's where they were going to be detonated."

British officials have been more categorical. Prime Minister David Cameron said Saturday that "we believe the device was designed to go off on the aeroplane."

A U.S. counterterrorism official said that forensics work is still in its early stages on the packages and that FBI experts are involved. The preliminary conclusion that the devices were designed to detonate aboard aircraft, and not at the addresses in the Chicago area, is based in part on the fact that the parcels were not rigged to explode upon opening.

The devices employed cellphone technology, but it remains unclear why they were built that way. Among the questions authorities are asking: How and when, during a transatlantic passage, would the cellphone components have been in range to receive a signal?

"There are a whole lot of theories being kicked around about whether [they were set] on a timer, whether somebody was going to call, or another triggering mechanism would set them off," the counterterrorism official said.

With early evidence suggesting that the plot was directed by al-Qaeda's affiliate in Yemen, Brennan said U.S. officials cannot presume that there are no more package bombs circulating, even though some U.S. law enforcement officials think they have identified and cleared all packages that left Yemen at the same time as the two devices already intercepted.

In Yemen, police on Sunday released a woman they had arrested the previous day on suspicion of mailing the two bombs. Officials there said someone might have assumed the identity of the woman, 22, a computer engineering student at Sanaa University.

A shipping agent could not identify the woman, and Yemeni officials said that although there are no suspects in custody, they are pursuing a number of leads.