By Phil Stewart
ASPEN, Colo. (Reuters) - The United States believes the Saudi man suspected of designing underwear bombs for al Qaeda's Yemeni affiliate has trained a small number of people on his advanced bomb-making techniques, a senior U.S. official said on Friday.
The remarks by John Pistole, who heads the U.S. Transportation Security Administration, were some of the most detailed public comments to date about Ibrahim Hassan al-Asiri and the thwarted May 2012 plot by al Qaeda in the Arabian Peninsula, or AQAP, to blow up a plane with an underwear bomb.
"There is intelligence that he has unfortunately trained others and there's a lot of effort to identify those folks," Pistole told the Aspen Security Forum in Colorado.
Asked by Reuters afterward about the nature of that intelligence, Pistole said Asiri was believed to have trained a small number of people. He added that the intelligence was "credible."
Believed to be in his early 30s, Asiri, who survived a U.S. drone missile attack in 2011, has drawn scrutiny for his skill at fashioning hard-to-detect bombs and hiding them in clothing or equipment.
He became an urgent priority for Western counterterrorism officials after his suspected role in planning strikes on the United States in 2009 and 2010, plots that included the failed bombing of a Detroit-bound airliner on Christmas Day in 2009.
The Detroit plot was discovered only when the explosive sewn into the bomber's underwear misfired as the airliner flew over U.S. territory.
Pistole said Asiri's 2012 attempt utilized a more sophisticated device using a new type of explosive the United States had not seen previously.
Asiri also used a double-initiation system for igniting the bomb and enclosed the device in caulk to prevent leakage of any explosive vapors that could be detected by airport equipment or bomb-sniffing dogs, Pistole said.
"He gave (the presumed attacker) instructions to get on the plane ... and fly to the U.S. and blow himself up over the U.S.," he said.
"Fortunately, that terrorist was a double agent," Pistole said, referring to the U.S.-British-Saudi undercover counterterrorism operation.
A Riyadh-born former chemistry student who once plotted to bomb oil facilities in Saudi Arabia, Asiri served nine months in jail in Saudi Arabia for attempting to join a militant group in Iraq to fight U.S. troops there.
He later moved to Yemen and joined AQAP, and is suspected of providing the bomb that killed his younger brother in a failed bid to assassinate Saudi counterterrorism chief Prince Mohammed bin Nayef in 2009.
Pistole described Asiri as "the bombmaker who has made all of these" devices and acknowledged that U.S. officials harbored fears he and AQAP might try to strike again.
"That is our greatest threat," he said.
(Reporting by Phil Stewart; Editing by Peter Cooney)