Al Qaeda's most diabolical bomb maker, who has targeted the American homeland at least four times, has trained other terrorists who are now being hunted down, the top US aviation security official has said.

Transportation Security Administrator John Pistole told ABC News during a discussion at a counterterrorism conference that accused Saudi terrorist Ibrahim al-Asiri had shared his expertise at building almost undetectable bombs with a number of Al Qaeda operatives.

"There is intel that he has unfortunately trained others," Pistole said at the annual Aspen Security Forum.