(AP) — 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 U.S. against al-Qaida’s spinoff group in Yemen and at least twice have disrupted plots to explode sophisticated bombs aboard airlines.

Details emerging about the latest unraveled plot revealed that a Saudi double agent fooled the terror group, known as al-Qaida 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-Qaida’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 oil-rich kingdom about a plot to blow up cargo planes inside the U.S., 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 one-time 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. Questions remain whether two Saudi citizens who had at least indirect links with two of the hijackers were reporting to Saudi government officials. U.S. law enforcement officials accused the Saudi government of failing to help adequately in investigations of the al-Qaida attack on the USS Cole in Yemen in 2000 and Hezbollah’s bombing of the Khobar Towers housing complex, which killed 19 U.S. servicemen in 1996.

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

Al-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. Al-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 Mueller said Wednesday that the FBI is examining the new al-Qaida bomb and urged Congress to renew wide-ranging surveillance authority to thwart similar terrorism plots.

The FBI is attempting to replicate bomb, trying to determine how destructive the bomb would have been and how easy it would be for AQAP to build another. The device is al-Qaida’s 2.0 version of the underwear bomb that very nearly brought down a Detroit-bound airliner on Christmas Day in 2009. This one was also nonmetallic, but is less bulky than the previous version, now shaped to fit nearly invisibly inside underwear to escape detection by security pat downs, two officials said.

Its trigger mechanism was also improved, replacing the flawed trigger design that failed to ignite the explosives in the previous attack.

The bomb bears the hallmarks of al-Qaida’s master bomb maker 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 al-Awlaki, but evidence emerged he was still around ahead of 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.”

Al-Asiri’s role also makes this particular mission personal for Saudi Arabia.

Saudi-born al-Asiri also turned his own brother into a suicide bomber in 2009, targeting Saudi Arabia’s top counterterrorism official, Prince Mohammed bin Nayef. The brother exploded a cavity bomb, killing himself, and injuring the prince.

Nayef’s forces are thought to have played a key role in sending the double agent that nabbed al-Asiri’s latest handiwork.

A tip from Saudi intelligence services led authorities in Dubai and Britain to uncover the al-Asiri-made the U.S.-bound parcel bombs sent from Yemen in 2010. Yemeni authorities have said they believe the tip came from an AQAP operative, Jabir al-Fayfi. Held at Guantanamo Bay, Cuba, before going through a Saudi militant rehab program, al-Fayfi may have been working as a double agent planted by Riyadh, Yemeni officials said.

White House counterterrorism chief John Brennan publicly thanked the Saudis for their role in stopping the cargo plane plot. Brennan’s personal ties have helped forge a closer Saudi relationship. A fluent Arabic speaker, he was once the CIA’s chief of station in Saudi Arabia.

Saudi Arabia’s intelligence services were also galvanised into their current more aggressive action after suicide bombers killed some 35 people at housing compounds for Westerners in Riyadh, in 2003, according to Philip Mudd, a former senior FBI and CIA official.

“The Saudis got serious and came up with a list of targets, and took them all down,” Mudd said. “It got too hot for al-Qaida in Saudi Arabia. They got squeezed out.”

Saudi militants fled southward, to the relative safety of Yemen. Exiled from their homeland, and some now disillusioned with al-Qaida, they are a ripe target to be turned, by Saudi intelligence, Mudd said.

The U.S. intelligence relationship with the Saudis waxes and wanes, said former House Intelligence Committee Chairman Pete Heokstra, R-Mich.

Before the Arab Spring left the Saudi royal family fearing for its rule, “the relationship was fraught with distrust, a lack of cooperation and coordination,” Hoekstra said.

Now, neither nation can afford to have al-Qaida in the neighbourhood, he said.