British intelligence played a central role in the undercover investigation to foil an underwear bomb plot involving al Qaeda's Yemeni offshoot, counterterrorism sources told Reuters.
The undercover informant in the plot linked to al Qaeda in the Arabian Peninsula, or AQAP, was a British citizen, possibly of Saudi origin, the sources said on condition of anonymity. The informant was working in cooperation with Britain's two principal spy agencies.
The Obama administration had been under heavy pressure not to disclose the role of British authorities in the investigation.
U.S. officials revealed publicly on Monday that AQAP tried to arm a suicide bomber with a non-metallic device that was an upgraded version of the "underwear bomb" that was carried on to a Detroit-bound plane on Christmas Day 2009.
U.S. officials said earlier this week that the latest plot was foiled by the CIA and allied foreign intelligence services, without identifying the allies.
British intelligence played a key role in the operation that foiled the plot to send a suicide bomber on to an airplane, and it was a cooperative effort between Britain's domestic and foreign intelligence services known as MI5 and MI6, officials told Reuters on condition of anonymity.
A spokesman for Britain's Foreign Office declined to comment, saying that in such cases it never confirmed or denied the involvement of British intelligence.
Several U.S. officials said the operation was severely disrupted when leaks of some of the details began to surface, and they believed the operation could have continued at least another week or two if the leaks to the media had not occurred.
The foiled plot ended with the explosive device being delivered to the FBI, which is examining it at its lab at Quantico, Virginia.
U.S. officials say the device bears the hallmarks of fugitive Saudi militant Ibrahim Hassan al-Asiri, suspected of being a bomb-maker working with AQAP.
"He is very dangerous, he is smart, he is vicious. He's the type of person that would put his own brother's life at risk for the cause," Representative C.A. "Dutch" Ruppersberger, the senior Democrat on the House intelligence committee, said.
He would not comment on British involvement in the investigation, saying only, "It was a team effort with different countries."