The al-Qaeda operative tasked with carrying a high-tech new "underpants" bomb onto a US-bound jet was in fact an agent working for the CIA and Arab intelligence agencies, it emerged last night.
The agent penetrated al-Qaeda's deadly affiliate Yemen and tracked the bomb's movement for weeks before smuggling out the device out of the country and into the hands of American authorities.
The mole, a Saudi, is believed to have escaped safely out of Yemen and into Saudi Arabia with the device two weeks ago, leaving the US clear to kill several of the other plotters with a drone strike.
The sting operation demonstrates the extent of the US's penetration of the terrorist network but also the vital role played by Saudi intelligence, which has previously infiltrated al-Qaeda in the Arabian Peninsula (AQAP) inside Yemen.
“We know the route this thing took in terms of its movement,” a US official told the Washington Post.
The agent's identity is a closely-held secret and both he and his family are understood to have been moved to safe houses out of fear of reliation from al-Qaeda's operatives.
While the revelation that the bomber was controlled by the US and Saudi Arabia means there was no immediate danger to western aircraft, the advanced technology used in his device caused alarm among intelligence agencies.
It is believed to have been built by al-Qaeda's master bomb maker and explosives experts say it is more sophisticated than the bomb which almost brought down a transatlantic airliner to Detroit on Christmas Day in 2009.
There are fears that the bomb-maker, Ibrahim Hassan al-Asiri, has built other devices, including ones that can be surgically implanted, and may have passed on his skills to others.
John Brennan, the White House counter-terrorism adviser, told ABC News that he could not rule out that other bombers are still at large.
Flights to the US from Britain have also been carrying scores of extra air marshals amid fears of co-ordinated attacks on aircraft using the same bombs.
American counter-terrorism officials sent the gun-carrying undercover officers to Europe last week as they insisted on the extra cover
Every flight from Gatwick was carrying armed officers, according to one report.
The Department for Transport refused to comment last night. Around 1,000 US air marshals operate in teams of two or three and undergo intense firearms training to allow them to open fire on a crowded aircraft.
Hillary Clinton, the US secretary of state, said the latest plot indicated that terrorists "keep trying to devise more and more perverse and terrible ways to kill innocent people".
The Saudis have previously cracked a plot by al-Asiri to blow up airliners, including one travelling through Leicestershire, using devices disguised as desk-top printers.
The latest underpants device was handed over two weeks ago and President Barack Obama was informed as the CIA then targeted those who gave it to the agent.
On Sunday, a senior planner with AQAP called Fahd al-Quso was hit by a missile fired from an unmanned drone as he stepped out of his vehicle in Rafd, a mountain valley in the southern Yemeni province of Shabwa.
Quso, 37, who had a $5 million bounty on his head, was indicted in the US for his role in the bombing of the USS Cole in Yemen in 2000, which killed 17 American sailors. He was also one of the most senior al-Qaeda leaders to meet Umar Farouk Abdulmutallab, the Detroit underpants bomber, in Yemen before he left the country to launch his attack.
Al-Asiri is one of the most wanted terrorist leaders in the world. The son of a former officer in the Saudi armed forces, he has been described by US intelligence officials as the most "ruthless and fanatical of all al-Qaeda's followers".
He recruited his brother, Abdullah, to join al-Qaeda, and in one of his most chilling attacks, sent him back to Saudi Arabia for a meeting with Muhammad bin Nayef, the security minister.
Abdullah blew himself up using a prototype underpants bomb, but left the minister with only minor injuries.