AL-Qaeda's leading bomb-maker escaped an operation by at least three Western intelligence agencies in which an undercover agent volunteered to be a suicide bomber using a device in his underpants.
The efforts of the CIA, MI6 and Saudi intelligence were aimed at stopping one of the most ruthless and inventive bomb-makers al-Qaeda has developed.
The agent, who risked his life to go undercover, is said to have been recruited by Saudi intelligence. MI6 offered support and advice to the Saudis and the CIA.
His target is thought to have been Ibrahim Hassan al-Asiri, the Saudi-born bomb-maker for al-Qaeda in the Arabian Peninsular.
US officials are understood to fear that Asiri may have trained protegé bomb-makers who are now "fanning out" to develop their own plots that could survive his capture or death. "We're always on the look-out for fresh threats against the US," said one security source.
Robert Mueller, the director of the FBI, told a Congressional hearing yesterday that the plot demonstrated that it was essential to renew far-reaching electronic surveillance powers that US officials have had at their disposal since 2008 but which are due to expire at the end of the year.
Their re-authorisation is "essential" to American efforts to counter the terrorism threat, he said.
The CIA helped run the operation and two weeks ago, the agent walked away from al-Qaeda with the device he was supposed to use in an attack on US-bound aircraft.
He travelled to the United Arab Emirates and then to Saudi Arabia with the device before handing it to the Saudis.
He was able to give information which led to a CIA drone strike on Sunday which killed Fahd al-Quso, AQAP's director of external operations and the man who handled the failed underpants bomber Umar Farouq Abdulmutallab. Asiri was not there and remains at large.
"Western security officials now have a clearer idea of how bomb-making has developed within AQAP and the terrorist organisation will be worried that they could be infiltrated again, limiting their access to new recruits," one source said.