The terror threat emanating from al Qaeda in the Arabian Peninsula in Yemen has focused attention on the group’s cunning leader, Nasir al-Wuhayshi. And for good reason. Wuhayshi, who was personally groomed by Osama bin Laden, is said to be a highly effective organizer and charismatic leader in the mold of his now-deceased mentor. Recently, Wuhayshi was elevated to the No. 2 position in al Qaeda’s core organization, even while running AQAP. But despite Wuhayshi’s growing stature among terror masterminds, no AQAP operative worries the United States more than Ibrahim al-Asiri, the group’s diabolically clever bombmaker, according to two U.S. counterterrorism officials.

Al-Asiri, a drop-out chemistry student from Saudi Arabia who was radicalized after the U.S. invasion of Iraq in 2003, has proved uniquely adept at devising bombs that can elude even the most sophisticated forms of detection. He rose to prominence in 2009 after a suicide bomber nearly succeeded in killing Saudi prince Muhammed Bin Nayef. Al-Asiri had devised the bomb used in the assassination attempt. The attacker was his own brother. Early reports indicated that the bomb was implanted in the younger al-Asiri’s rectum, but it turned out that it had been sewn into his underwear. Al-Asiri absorbed most of the blast, which killed him instantly. Bin Nayef managed to escape with only minor injuries. Still, the fact that an al Qaeda operative was able to penetrate Saudi security and come some close to killing one of the kingdom’s top counterterrorism officials spooked American officials. A few days later, John Brennan traveled to Saudi Arabia to learn about the attack and the innovative bomb that had been used.

Only four months later, AQAP struck again—this time with a new version of the same kind of bomb. A Nigerian AQAP recruit came perilously close to blowing up a commercial airliner over Detroit. The device was traced back to al-Asiri. In 2010, AQAP managed to place bombs in two U.S. cargo planes. The explosive material was ingeniously placed in printer ink cartridges where the ink powder normally goes. It was, once again, the devilish handiwork of al-Asiri. A Saudi tipster helped foil the plot.

For U.S. intelligence officials, there was nothing reassuring about the fact that al-Asiri had yet to successfully pull off any major attacks. They attributed it to pure luck. “We felt like we were cheating fate,” says one U.S. intelligence official. “The question was: would we get him before he got us?” Al-Asiri would soon move to the top of the Obama administration’s kill or capture list.

Few things chilled U.S. officials more than the belief that al-Asiri has succeeded in developing a new kind of bomb that can be surgically implanted inside the human body. The military refers to such a weapon as a “surgically implanted improvised explosive device,” or SIIED. In 2011, U.S. intelligence learned that al-Asiri was working closely with AQAP-affiliated doctors who had tested the bomb on dogs and other animals. Like his previous bombs, it contained no metal and so could pass through conventional detectors.

Last year, U.S. intelligence officials circulated a secret report that laid out in vivid detail how al-Asiri and his doctors had developed the surgical technique. A source familiar with the report described it as 15 to 20 pages, single spaced, and replete with pictures and schematics. “It was almost like something you’d see in Scientific American,” the source said.

President Obama has received regular updates on al-Asiri’s progress and has been involved in numerous discussions about killing or otherwise neutralizing the bombmaker, according to a senior administration official who declined to be named discussing sensitive national security matters.

The CIA may have gotten close to taking out al-Asiri in the spring of 2012. Prince Bin Nayef hatched a plan to retaliate against his would-be killer. According to author and Yemen expert Gregory Johnsen, bin Nayef turned to British intelligence and the CIA to help mount a sting operation using a Saudi national who’d been recruited by the British as bait. The undercover agent successfully penetrated the AQAP, a major intelligence coup, though he was never able to meet directly with al-Asiri. Another AQAP operative gave the mole a new, more sophisticated, version of the underwear bomb along with instructions to get on a plane from a safe airport. Instead, the agent was able to give the bomb to his Saudi handler in Yemen who spirited it out of the country on his private plane.

A few days later, an Associated Press story reported that Western intelligence agencies had foiled an AQAP plot. The article indicated that the CIA had obtained the explosive device, thereby revealing that a spy had likely penetrated AQAP.

Some U.S. officials have suggested that the leak to the AP might have disrupted the best chance the U.S. had to take out al-Asiri, though it is unclear how the operation would have been pulled off once the mole had left the country on a supposed suicide mission.

The CIA’s quest to kill AQAP’s master bomber goes on. Last year a CIA drone strike took out Ahmed Said Saad, a Syrian medical doctor who had been working with al-Asiri on his experiments. On more than one occasion over the past few months, U.S. counterterrorism operators thought they had al-Asiri in their crosshairs. But a senior U.S. official says he has eluded American drones and remains on the loose.