Stopping the planned attack was "a team sport" that highlights the growing counterterrorism relationship with Saudi Arabia.
The U.S. intelligence community has won plaudits for helping derail a series of recent terror plots, including this week's disclosure of an al-Qaida affiliate's attempt to use a sophisticated underwear bomb to destroy a Western jetliner.
But the U.S. has a quiet ally who has been instrumental in stopping both the new plot and an earlier militant attempt to destroy U.S. cargo planes by secretly shipping explosives in conventional mail packages: Saudi Arabia, a country once derided as a financial supporter of Islamic militants around the globe.
The underwear bomb plot, for instance, was blocked by an undercover Saudi intelligence agent who infiltrated Yemen's branch of al-Qaida and volunteered for the planned suicide mission to ensure he could take possession of the bomb and that it was never used, according to information first reported by The Associated Press.
The CIA and Saudi embassies declined to comment on Saudi Arabia's role in obtaining the device before it could be detonated, but a senior U.S. official, speaking on background, said the safe recovery of the underwear bomb was "a team sport."
"It is another example of outstanding international counterterrorism cooperation," the official said.
Bruce Riedel, a former high-level CIA official, said neither terrorist plan would have been stopped without the Saudi assistance.
"In this plot and the 2010 parcel-post plot, it was literally the difference between success and failure," said Riedel, who is now a senior fellow at the Brookings Institution. "The Saudis get a lot of negative press because 15 of the 19 9/11 hijackers were from Saudi Arabia, but the track record of cooperation has been extraordinary."
The relationship between the CIA and the Saudi General Intelligence Service improved sharply in 2009 after al-Qaida in the Arabian Peninsula -- the group behind the newly disclosed terror plot -- mounted an audacious suicide attack designed to kill Prince Muhammad bin Nayef, a high-ranking Saudi counterterrorism official.
The strike failed, but the Saudis responded by ramping up their efforts to recruit informers and infiltrate the terror group with intelligence operatives despite the enormous risks to the agents themselves.
The push allowed Riyadh to help Washington derail a 2010 plot to ship explosives masked as printer cartridges aboard U.S.-bound FedEx and UPS cargo planes. In that case, Saudi Arabia gave the CIA the actual routing numbers of the two packages; without the information, the planes could have been destroyed over Chicago or another major U.S. city.
The warming security relationship marks a sea change from just a few years ago, when lawmakers passed a 2007 measure that barred any American aid to Saudi Arabia amid criticism that Riyadh wasn't doing enough to block its citizens from funneling money to militants in Iraq and other countries. Given the paltry amount of U.S. aid at stake, the legislation was primarily a symbolic gesture.
"By cutting off aid and closing the loophole, we send a clear message to the Saudi Arabian government that they must be a true ally in advancing peace in the Middle East," then-Rep. Anthony Weiner, a New York Democrat, said at the time.
Today, ties have become so close that the Obama administration didn't make much of an effort to convince Riyadh to halt its crackdown on the country's small number of pro-democracy activists during the Arab Spring.
The United States has given Saudi Arabia permission to acquire 84 next-generation F-15s and to upgrade another 70. The $29.4 billion deal is the largest single weapons sale in U.S. history. Riyadh is also negotiating with the Pentagon to buy up to a dozen next-generation Navy vessels equipped with the Aegis missile-defense system, which can be used to shoot down incoming ballistic missiles; such a purchase would cost at least $10 billion more.
Still, word of the Saudi role in stopping the new plot may have come at a heavy price. The double agent and his family have been placed into protective custody in Saudi Arabia, and Riyadh has expressed fears about the safety of its undercover agents.
Here at home, powerful lawmakers like House Homeland Security Committee Chairman Peter King, R-N.Y., have called for investigations into who leaked the information to AP. The disclosures, the lawmakers argue, could make the Saudis less likely to help in the future.
Director of National Intelligence James Clapper has ordered an internal review across the intelligence community to determine if leaks came from any of the 16 agencies he oversees.
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