A mole recruited by British intelligence is the deep-cover operative who penetrated al Qaeda's most recent bomb plot, intelligence sources told ABC News.
The infiltrator, who entered Yemen on a British passport, had been in place for several months, and intelligence agencies had wanted to keep the operation running, where he would have gathered information about several sets of terror plans in various stages of development.
"They had in some cases gone from aspiration to intent, and in other cases gone beyond that," said a senior intelligence official, citing as an example of a fully developed plot Umar Farouk Abdulmutallab's attempt to take down Northwest flight 253 with an underwear bomb.
The American role in the operation only took on operational urgency in the past two or so months, the officials said. As the scope of the Yemen bomb plot spread through the U.S. security apparatus, the effect at major U.S. airports was an increased volume of passengers put through full body screeners, additional teams of behavioral profilers placed on duty, and an increased number of shifts for bomb-sniffing dogs.
The operation was pulled up short in the past week when leaks developed and put the infiltrator in jeopardy.
"This was gold dust," the senior intelligence official said. "Such assets are few and far between."
Officials told ABC News that British intelligence recruited the mole from the U.K.'s Middle Eastern population to pass himself off as a suicide bomber.
With a British passport, he would have been able to fly to the U.S. without first obtaining a visa from the American embassy, perfect bait for the undercover sting, given the desire of al Qaeda bombmaker Ibrahim al-Asiri to hit the U.S.
"He must convince al Qaeda that he is a genuine recruit," said Saudi security analyst Dr. Mustafa Alani, "and he genuinely believes in the suicide mission and he genuinely believes in the punishment of the United States."
Authorities say the British spy was sent to Yemen ostensibly to enroll in one of the country's Arab language schools, the same recruitment path followed by real terrorists, including Abdulmutallab.
Both British spy agencies, the domestic MI5 and the overseas MI6, were involved, and authorities say they used Saudi intelligence officials to help facilitate the operation. The CIA was brought in once there was clear indication of a plot.
Authorities say they wanted to keep the undercover operation going but were forced to cut it short because of news reports in the U.S. that revealed the operation.
Sources involved in the intelligence operation said they had not yet gleaned all the information they could about the Yemen-based cell, its bombmaker, plotters, martyrs, potential devices, targets and launch points when the mole was pulled out.
But the information successfully extracted included the underwear bomb that the infiltrator brought out of Yemen by car, as well as operational detail on other improvised devices, including bombs designed to fit inside camera bodies and external hard drives. The drives would have exploded when plugged into the laptop.
ABC News was also told the bombmaker intended to use bombs that were surgically implanted inside humans -- and housepets.