The tip reflects how Saudi Arabia has worked aggressively for years to infiltrate al-Qaida in the Arabian Peninsula, which is operating in the unruly, impoverished nation on its southern doorstep.

The tip came from Jabir al-Fayfi, a Saudi who was held for years at the U.S. military prison at Guantanamo Bay, Cuba, and was released to Saudi Arabia in 2007. Soon after, he fled Saudi Arabia and joined the al-Qaida affiliate in Yemen, until he turned himself in to Saudi authorities in late September.

Yemeni security officials said they believe al-Fayfi may have been a double agent, planted by Saudi Arabia in Yemen among al-Qaida in the Arabian Peninsula militants to uncover their plots. The officials said that after his return to the kingdom, he told authorities that al-Qaida was planning to send bomb-laden packages.

The officials spoke on condition of anonymity because they were not authorized to talk to the media. Tribal leaders in Yemen aware of the situation, and similarly speaking on condition of anonymity, confirmed al-Fayfi’s role. Saudi officials did not respond to calls for comment.

Saudi Arabia has been recruiting informants in the terrorist network and also has been paying Yemeni tribal chiefs — and even gives cash to figures in the Yemeni military — to gain their loyalty.

President Barack Obama thanked Saudi King Abdullah, a top U.S. ally, in a Saturday telephone call for the “critical role” by Saudi counterterrorism authorities in uncovering the plot. After the Saudi alert, two bombs hidden in packages mailed from Yemen and addressed to synagogues in Chicago were discovered Friday on planes transiting through Dubai and Britain.

Ibrahim Hassan al-Asiri, considered a key figure in al-Qaida in the Arabian Peninsula, is the chief suspect behind assembling the sophisticated mail bombs, according to U.S. intelligence officials.

German officials said Monday the mail bombs contained 10.58 ounces (300 grams) and 15.11 ounces (400 grams) of the explosive PETN — enough to cause “significant” damage to the planes. By contrast, the explosives that failed to work last Christmas on a Detroit-bound airliner used 80 grams of PETN secreted in the underwear of a Nigerian passenger. Al-Qaida in the Arabian Peninsula claimed responsibility for that.

The warning from Germany came as investigators tried to trace bomb parts and look for any more explosives possibly sent from Yemen.

The Yemeni National civil aviation committee decided late Sunday to tighten security in Yemeni airports, according to the state Saba news agency. The committee, headed by the minister of transport, said cargo leaving the airports will be thoroughly inspected and shipping agents will have to get licenses in line with international standards. The committee also approved a new airport security force.

While al-Fayfi may have provided broad outlines about the plot, it appears Saudi Arabia had other sources.

U.S. officials have said the tip was specific enough that it identified the tracking numbers of the packages. The Saudi newspaper Al-Watan on Monday cited Saudi security officials as saying the kingdom gave U.S. investigators the tracking numbers, which al-Fayfi likely would not have known since he surrendered well before the packages were mailed.

Al-Fayfi’s surrender may have revealed other plots as well. In mid-October, a couple of weeks after his surrender, Saudi Arabia warned European authorities of a threat from al-Qaida in the Arabian Peninsula, saying the group’s operatives were active on the continent, particularly France.

Al-Fayfi, who is in his mid-30s and is known by the nom de guerre of Abu Jaafar al-Ansari, was captured by U.S. forces in Afghanistan after the 2001 toppling of the Taliban there. According to documents from Guantanamo, he spent time at Osama bin Laden’s hideaway at Tora Bora in eastern Afghanistan in November-December 2001, during a U.S. air assault on the al-Qaida mountain stronghold.

Al-Fayfi was held at Guantanamo until early 2007, when he was released to Saudi Arabia.

There, he was put through the kingdom’s rehabilitation program for militants. But soon after leaving the program, he fled to Yemen and joined al-Qaida, according to the Saudi Interior Ministry. In September, he contacted Saudi authorities, saying he wanted to turn himself in. A private jet was sent to the capital of San’a to bring him to Riyadh, Saudi security officials told the Saudi-owned daily Al-Hayat.

Al-Qaida in the Arabian Peninsula is made up of several hundred militants and appears to be aggressively seeking to recruit American and European radicals who could provide a way for the group to carry out attacks in their homelands. Yemen provides a potentially easy entry point for foreign radicals to link up with al-Qaida, with a number of popular Islamic religious and Arabic-language schools that attract students from around the world.

Most of the militants, however, are Yemenis and Saudis — including many Saudis who belonged to al-Qaida’s branch in the kingdom until it was crushed by a heavy crackdown in the mid-2000s.

Since then, Saudi intelligence has aggressively been pursuing them, even as the militants have vowed both to kill Saudi officials and to topple the Yemeni government of President Ali Abdullah Saleh. The Saudis deeply distrust the ability of Saleh’s regime to crack down on militants, seeing Yemen’s security forces as incompetent.

The frustration with the Yemenis climaxed last year when al-Qaida in the Arabian Peninsula came close to killing Prince Muhammad bin Nayef, a member of the royal family who runs the Saudi counterterrorism program.

Al-Asiri’s brother, Abdullah, posing as a reformed jihadist, detonated a bomb hidden inside a body cavity, killing himself but only slightly wounding the prince.

Forensic analysis indicates that Ibrahim Hassan al-Asiri built the bomb carried by his brother, as well as the explosives carried by the Nigerian on the Detroit-bound flight.

The attack on the prince “was the thing that infuriated the Saudis and made them step up their intelligence operations in Yemen and almost completely sidestep the Yemenis,” said a Yemeni security official familiar with the kingdom’s activity in his country.

“They recruited hundreds of informers and began to spend even more lavishly on their allies,” said the official, who agreed to share the information in exchange for anonymity because he was not authorized to speak to the media.

For years, Saudi Arabia has also been known to be giving cash rewards to tribal chiefs, senior military officers and politicians.

In large areas of mountainous Yemen, where infrastructure is often poor to nonexistent, tribes hold far more power than the central government and are better aware of militants’ comings and going. Some tribes, disenchanted with San’a, have provided shelter to al-Qaida fighters.

President Saleh, meanwhile, is hampered by trying to balance his policies toward the tribes, making it difficult for him to crack down on those harboring militants for fear of a backlash. With oil revenues declining, he does not have the cash to tempt them to surrender the militants in exchange for better services or jobs for their followers. His security forces lack discipline and are poorly armed, save for those reporting directly to him or close family members, and are mostly deployed in San’a.

The Saudis are fully aware of all this.

“It is a case of the Saudis distrusting the Yemenis on the war against terror,” said Mohammed al-Sabry, a Yemeni analyst. “What was once a lack of coordination between the two nations is now a serious problem between them.”

Jamal Khashoggi, a prominent Saudi expert on al-Qaida, said the Saudis “managed to do a superb job in Yemen. … You have to have someone inside in order to get the job done.”

He said Saudi Arabia has been working to infiltrate al-Qaida elsewhere, especially in Iraq and Afghanistan, “but we have done a better job in Yemen.”

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Associated Press writers Salah Nasrawi and Maggie Michael in Cairo contributed to this report.