FBI Director Robert S. Mueller III told a Senate committee Wednesday the bureau is investigating the source of leaks about a plot by al Qaeda terrorists to place a sophisticated explosive device aboard a U.S.-bound airliner.

In testimony before the Senate Judiciary Committee, Mr. Mueller - in his first public confirmation of the investigation - said leaks threaten ongoing law enforcement operations, put the lives of sources at risk, make it harder to recruit sources, and damage relationships with foreign law-enforcement partners.

“We have initiated an investigation into this leak” and “will investigate thoroughly,” Mr. Mueller said.

Earlier this month, the Associated Press and other news organizations reported the CIA had thwarted a plot by al Qaeda operatives in Yemen to destroy a U.S.-bound airliner using a bomb with a sophisticated new design. The attack was planned to coincide with the first anniversary of the killing of Osama bin Laden.

The plot, according to federal officials, was an upgrade of the underwear bomb that failed to detonate aboard a jetliner over Detroit on Christmas Day 2009. The new bomb also was designed to be used in a passenger’s underwear, but this time al Qaeda developed a more-refined detonation system, the officials said.

The new bomb did not contain metal, meaning it probably could have passed through an airport metal detector, and it was unclear whether new body scanners used in many airports would have detected it. The would-be suicide bomber, based in Yemen, actually was a double agent working with the CIA and Saudi intelligence agencies.

At the time, the FBI said only an improvised explosive device (IED) designed to carry out a terrorist attack had been seized abroad as the result of “close cooperation with our security and intelligence partners overseas.” The bureau said the FBI had possession of the IED and was conducting technical and forensics analysis on it.

“Initial exploitation indicates that the device is very similar to IEDs that have been used previously by al Qaeda in the Arabian Peninsula in attempted terrorist attacks, including against aircraft and for targeted assassinations,” the FBI said.

The White House acknowledged at the time that President Obama had learned of the plot in April and was assured the device posed no threat to the public.

The AP discovered the thwarted plot in early April, but agreed to White House and CIA requests not to publish it immediately because the sensitive intelligence operation was ongoing. Once officials said those concerns were allayed, the AP disclosed the plot despite requests from the administration to wait for an official announcement.

It remains unclear who built the new bomb, but because of its sophistication and its similarity to the Christmas bomb, authorities suspected it was the work of master bomb maker Ibrahim Hassan al-Asiri.

Al-Asiri constructed the first underwear bomb and two others that al Qaeda built into printer cartridges and shipped to the U.S. on cargo planes in 2010. Both of those bombs used a powerful industrial explosive and both were nearly successful.

Last week, Mr. Mueller urged the reauthorization of an act passed by Congress in 2008 - but slated to expire at the end of this year - that lets federal authorities conduct warrantless searches. He told the House Judiciary Committee the law allows the collection of vital information about international terrorists “while providing a robust protection for the civil liberties and privacy of Americans.”

Mr. Mueller said the FISA Amendments Act (FAA) gave law enforcement authorities wide-ranging surveillance authority to target terrorism plots as al Qaeda and its affiliates continue to scheme to attack U.S. sites. He said records seized from Osama bin Laden’s compound in Pakistan, as well as the recent conviction of an al Qaeda operative plotting to conduct coordinated suicide bombings in New York, confirmed that the group was committed to renewed attacks.

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