The double agent in the foiled al-Qaida bomb plot had a British passport, making the U.K.'s intelligence agency key to the international sting operation.
Two officials briefed on the investigation said the double agent had a British passport. The officials requested anonymity to discuss the operational details. One official said the British intelligence agency, MI6, gave the double agent the passport as part of the ruse.
Al-Qaida wants terror recruits that have a U.S. or British passport because they are more likely to be able to travel to and from the West without raising suspicion.
British Prime Minister David Cameron's office on Friday declined to comment in any detail on the reports of the country's role in thwarting the new bomb plot uncovered in Yemen.
Last month, al-Qaida's Yemen branch entrusted a new, sophisticated underwear bomb designed to take down and airplane with a would-be suicide bomber. But the bomber was actually a double agent, working with the CIA, Saudi intelligence agencies and the MI6. The double agent turned the bomb over to the U.S. government.
The operation shows the close cooperation among the U.S., Britain, and Saudi Arabia, whose intelligence service played a major role in infiltrating the organization, and helping communicate with the agent.
The British intelligence role was first reported by NBC News.
The explosive has been described as an upgrade over the 2009 Christmas bomb that nearly brought down an airliner over Detroit. This new device contained lead azide, a chemical known as a reliable detonator. After the Christmas attack failed, al-Qaida used lead azide as the detonator in the 2010 plot against cargo planes.
Security procedures at U.S. airports were unchanged despite the plot, a reflection of both the U.S. confidence in its security systems and a recognition that the government can't realistically expect travelers to endure much more. Increased costs and delays to airlines and shipping companies from new security measures could have a global economic impact too.
Security officials said they believe airport security systems put in place in the United States in recent years could have detected the new device or one like it. But the attempt served as a stark reminder that security overseas is quite different.
While airline checks in the United States mean passing through an onerous, sometimes embarrassing series of pat-downs and body scans, procedures overseas can be a mixed bag. The U.S. cannot force other countries to permanently adopt the expensive and intrusive measures that have become common in American airports over the past decade.