The man chosen by al Qaeda's affiliate in Yemen to blow up a US-bound airliner was a double agent who infiltrated the group and volunteered to carry out the suicide attack, US and Yemeni officials say.The agent, who was apparently working with the knowledge of both Saudi intelligence and the CIA, escaped Yemen with the sophisticated bomb and delivered it to foreign intelligence authorities, according to reports.
He is now said to be safe in an unidentified country where he is being debriefed.
A senior US official told the New York Times that a bomb for the attack was sewn into "custom fit" underwear that would have been difficult to detect even in a careful pat-down at an airport.
The FBI is analysing the explosive.
"Initial exploitation indicates that the device is very similar to IEDs that have been used previously by al Qaeda in the Arabian Peninsula (AQAP) in attempted terrorist attacks, including against aircraft and for targeted assassinations," it said in a statement.
"The device never presented a threat to public safety, and the US Government is working closely with international partners to address associated concerns with the device."
Officials said it appeared to be an upgrade of the bomb that failed to detonate on board an airplane over Detroit on Christmas 2009, a plot which also originated in Yemen.
This new bomb contained no metal and used a chemical - lead azide - that was to be a detonator in the plot to attack cargo planes which nearly succeeded in 2010, officials said.
The New York Times said it had been designed by "the group's top explosives experts" to be undetectable by airport screening measures, particularly metal detectors.
A Department of Homeland Security official said that because the device was similar to the one in the failed 2009 attempt, security steps taken since "would have been able to prevent this device from bringing down an airplane".
Experts suggested airport body scanners, which use light doses of radiation to scan through a passenger's clothes, may have been able to detect an "anomaly" such as the device, which could then be further examined in a hands-on, pat down search.
However, the scanners have not been deployed in all airports across the US and are in very limited use elsewhere.
AQAP's master bomb-maker has previously been identified as Ibrahim Hassan al Asiri, a Saudi fugitive.
"I'm convinced that Asiri is behind this. He is an evil genius when it comes to bomb-making," House of Representatives homeland security committee chairman Peter King said on Fox News.