LONDON -- The Justice Department's secret seizure of phone records from the Associated Press was prompted by a leak that put considerable strain on the relationship between American and British intelligence agencies.
The leak was the basis of an AP story in May 2012 about a CIA operation in Yemen that foiled an al Qaeda plot to detonate a bomb on an airplane headed for the United States.
There was anger in the British government over the leak and subsequent news reports that disclosed U.K. spies had been heavily involved in the operation.
The alleged details of the operation, which were never officially confirmed, were straight out of a John Le Carre novel. According to reports, a U.K. passport holder of Yemeni descent was recruited by British security officials and sent to Yemen to infiltrate an al Qaeda group.
The details of alleged U.K. involvement were attributed by many American media outlets to U.S. security sources. According to London's Times newspaper, the level of detail made public had left British officials "slack-jawed."
Deputy Attorney General James Cole, who approved getting the AP's phone records to track down the person that leaked classified information, said it was a last-resort effort after having conducted hundreds of interviews. NBC's Pete Williams reports
"I understand there is an investigation under way, being led by the Americans. It is clearly a matter for the U.S. authorities,", the official spokesperson for Britain's prime minister said at the time. "Clearly, we think that sensitive information should be protected."
Sir Malcolm Rifkind, chairman of the parliamentary Intelligence and Security Committee, went even further and said leaks about operations could be "extremely harmful."
"It can prevent the effective involvement of intelligence officers or agencies in operations that are designed to save lives either in this country or other countries," he added. "Whether a leak arises in the U.S., the U.K. or elsewhere it is equally serious."
In the wake of the leak, it was claimed that the double agent had managed to smuggle out a bomb that would have been used to blow up an airliner. The bomb was described as even more sophisticated than the underwear bomb that attempted to bring down an jet landing in Detroit on Christmas Day in 2009.
The British double agent was also said to have provided vital information about al Qaeda in the Arabian Peninsula and about its master bomb maker Ibrahim al-Asiri. Around the same time as the leak, a drone strike in Yemen killed a senior al Qaeda leader, Fahd al Quso, who had been involved in the USS Cole bombing. However, it not been confirmed that this killing was connected to the undercover operation.
US Attorney General Eric Holder tells reporters he recused himself from the investigation into leaks which led to a subpoena for AP phone records, a leak Holder said "put the American people at risk."
The leaked news potentially did more than put the operation it at risk. It also threatened the life of the double agent and his family and had an impact on the prospects for similar operations in the future. After all, why would similar recruits co-operate with the British knowing that information about what they did would go public?
"The revelations about the British agent in al Qaeda remind us that Beltway leaking is a major security threat," said Nigel Inkster, a former assistant chief of the British intelligence agency MI6.
Raffaelllo Pantucci, senior research fellow at London-based think tank RUSI, added: "It, of course, undermines the trust between the agencies. It’s a big problem."
The Saudis also substantially assisted in the operation, according to experts. Could their connections have been compromised? In 2010, Saudi intelligence had helped foil an attack out of Yemen involving bombs disguised as printer cartridges smuggled onto airplane cargo.
Did British disquiet help spur the U.S. investigation into the leak? British government sources would not say whether a complaint was lodged.
"It is a long standing policy of successive governments not to comment on intelligence matters," an official with the U.K.'s Foreign Office said Wednesday.
NBC News' Michele Neubert contributed to this report.