American officials leaked out details of the extraordinary intelligence coup two days after the White House announced a plot by Al-Qaeda in the Arabian Peninsula (AQAP) had been successfully thwarted.
The double agent managed to spend weeks with AQAP before handing over information that allowed the United States to launch a drone strike on Sunday that killed Fahd al-Quso, a senior figure who was wanted for the 2000 bombing of the USS Cole in Yemen, the New York Times and other media reported, citing unnamed US officials.
A senior US official told the Times that a bomb for the would-be attack was sewn into "custom fit" underwear that would have been difficult to detect even in a careful pat-down at an airport.
Unlike the device used in the failed December 2009 plot by AQAP to blow up an airliner en route to Detroit, this explosive could have been detonated in two ways, in case one failed, the unnamed official was quoted as saying.
The main charge was a high-grade military explosive that "undoubtedly would have brought down an aircraft," the official said.
ABC News had reported earlier that the latest plot by AQAP was thwarted by a spy who infiltrated the group and took the explosive to Saudi Arabia.
The CIA and other government agencies declined to comment on the reports when contacted by AFP.
FBI experts on Tuesday were analyzing the seized explosive that officials said was an updated version of the "underwear bomb" used in a failed attack on Christmas Day 2009.
Although officials touted the disrupted plot as a success, they acknowledged AQAP remained determined to strike and its master bombmaker, Ibrahim Hassan Taleh Al-Asiri, was apparently hard at work seeking to circumvent airport security.
Saudi intelligence likely had the lead role in disrupting the conspiracy, possibly providing the double agent, former US officials said.
The Los Angeles Times reported that the double agent was "an informant" overseen by Saudi Arabia's intelligence service, which devotes great efforts to monitoring Al-Qaeda's branch in Yemen.
The leak on the double agent was upsetting to the chairman of the US House Homeland Security Committee, lawmaker Peter King of New York.
"It's really, to me, unfortunate that this has gotten out, because this could really interfere with operations overseas," King told CNN late Tuesday. "My understanding is a major investigation is going to be launched because of this."
Former intelligence officials noted that Saudi Arabia was credited with uncovering an AQAP plot in 2010 to blow up cargo planes headed to the United States and that the Saudis are known to keep a close eye on Al-Qaeda's branch in Yemen.
"The Saudis have the best insight to Al-Qaeda in the Arabian Peninsula. Presumably the Saudis working with the US disrupted this plot perhaps inside Yemen, seized the device, turned that over to the US," Fran Townsend, a former counterterrorism official under ex-president George W. Bush, told CNN.
Bruce Riedel, a former CIA officer, agreed.
"My guess is we probably had someone on the inside, my guess is someone working for the Saudis, who tipped us off that the bomb was somewhere and the bomber was somewhere and they were able to wrap the two of them up," Riedel told AFP.
"From previous AQAP experiences, it's usually the Saudis who give us the critical intelligence," said Riedel, a fellow at the Brookings Institution.
US officials are concerned about AQAP's expansion in Yemen, as the affiliate has exploited recent turmoil despite the threat of strikes from American drones overhead.
The fresh plot to blow up a passenger plane bore a "striking similarity" to the Christmas Day attempt of 2009, said Riedel, who worked on the government's case against the Nigerian man blamed for the botched attack.
Umar Farouk Abdulmutallab admitted to the conspiracy and was sentenced to four consecutive life sentences in February.
Both cases likely involved the same bomb master, Asiri, and the same approach in which the attacker was allowed to decide the timing and precise location of his suicide strike, he said.