US officials have said the would-be suicide bomber in a foiled plot by al-Qaeda was planted in the group by an allied intelligence agency or turned into an informant early in the conspiracy.
The CIA and its allies tracked the plot for several weeks and then managed to get the informant to deliver the bomb outside Yemen, possibly to Saudi Arabia or the United Arab Emirates.
US intelligence officials wanted to keep a lid on details of how the plot was uncovered to make sure the informant and his family was safe.
Sources said the informant was believed to be connected to a foreign intelligence agency allied with the CIA.
The New York Times reported the would-be suicide bomber was an intelligence agent for Saudi Arabia who volunteered for the mission to detonate the bomb aboard a US-bound airliner.
Ten days ago, officials seized an improved version of the "underwear bomb" that was used in a failed airline bombing attempt over Detroit on Christmas Day 2009.
The plot to introduce the bomb aboard an aircraft was the work of Yemen-based al-Qaeda in the Arabian Peninsula, or AQAP, considered the group's most dangerous offshoot.
The latest device appeared to be similar to the work of fugitive Saudi militant Ibrahim Hassan al-Asiri, who US sources believe is a bomb-maker working with AQAP.
There is no immediate sign the Obama administration is ordering changes in aviation security procedures. Officials said the plot never came close to fruition and no aircraft was in danger.
Security steps taken since the failed 2009 attempt, in which a similar device was carried onto a plane by Nigerian militant, Umar Farouk Abdulmutallab, "would have been able to prevent this device from bringing down an airplane," a US official said.
Other US officials said airport metal detectors probably would have trouble spotting a device that had no metal parts.
But airport body scanners, which use light doses of radiation to scan through a passenger's clothes, ought to be able to detect "anomalies" which could then be further examined in a hands-on, pat down search, they said.
