WASHINGTON - A decade after hijackers mostly from Saudi Arabia attacked the United States with passenger jets, the Saudis have emerged as the principal ally of the United States against al-Qaeda's spin-off group in Yemen and at least twice have disrupted plots to explode bombs aboard airlines.

Details emerging about the latest unraveled plot revealed that a Saudi double agent fooled the terror group al-Qaeda in the Arabian Peninsula, passing himself off as an eager would-be suicide bomber. Instead, he secretly turned over the group's most up-to-date underwear bomb to Saudi Arabia, which gave it to the CIA. Before he was whisked to safety, the spy provided intelligence that helped the CIA kill al-Qaeda's senior operations leader, Fahd al-Quso, who died in a drone strike last weekend.

The role of Saudi Arabia disrupting the plot follows warnings in 2010 from the kingdom about a plot to blow up cargo planes inside the United States, either on runways or over American cities. That plot involved a frantic chase across five countries of two packages containing bombs powerful enough to down an airplane. Twice, a bomb was aboard a passenger plane. Once, authorities were just minutes too late to stop a cargo jet with a bomb from departing for its next destination. Ultimately, no one died and the packages never exploded.

It hasn't always been this way.

Saudi Arabia, the onetime home of Osama bin Laden, failed to spot and stop the 15 Saudi-born hijackers of the 19 who carried out the September 2001 terror attacks.

But a series of al-Qaeda strikes against Saudi targets in 2003 and, more recently, fears al-Qaeda could try to trigger Arab Spring-style revolts in the kingdom, have energized the Saudi government in its war against al-Qaeda's spin-off in Yemen, which is composed mostly of ex-Saudi extremists. Saudi Arabia and the United States - with help from Yemen's government - have joined forces to penetrate the terror group at the highest levels. Drone strikes have killed the U.S.-born Anwar al-Awlaki last summer and Quso, his successor, more recently.

Quso personally briefed the Saudi double agent, giving him open-ended instructions to pick a U.S.-bound plane on a day of his choosing. Quso was hit in part due to information gleaned from the double agent, according to two former officials, speaking on condition of anonymity to preserve their ability to discuss details of current intelligence matters with current officials.

FBI Director Robert S. Mueller said Wednesday that the FBI is examining the new al-Qaeda bomb and urged Congress to renew wide-ranging surveillance authority to thwart similar plots.

The FBI is attempting to replicate the bomb, trying to determine how destructive the bomb would have been, and how easy it would be for al-Qaeda to build another.

The bomb bears the hallmarks of al-Qaeda's master bombmaker, Ibrahim Hassan al-Asiri, or one of his protégés, multiple officials say. U.S. officials had hoped the bomber was killed in the strike last year on Awlaki, but evidence he was still around emerged in the run-up to the one-year anniversary of the Navy SEAL raid that killed bin Laden.

"So Asiri is likely still out there, and he can still build these," or teach others to build them, said House intelligence committee member Adam Schiff (D., Calif.). "So it's not as though we can rest any easier."