ASPEN: US security officials are on the lookout for a new type of explosive, after analysis of an upgraded underwear bomb intercepted by a CIA operation in Yemen.

Transportation Security Administrator John Pistole told an audience at the Aspen Security Forum that the device smuggled out by a double-agent in an operation earlier this year was an upgrade from the underwear bomb carried by Nigerian Umar Farouk Abdulmutallab, to try to bring down a Detroit-bound airliner on Christmas 2009.

''We found in the Underwear Plot, Part 2...that a different type of explosive had been used than the previous one,'' Pistole said, ''so we have gone back and recalibrated all the equipment and we have been working with our canine to detect this different type of explosive.''

The CIA intercepted the device earlier this year, thwarting an ambitious plot by al Qaeda's affiliate in Yemen to destroy a US-bound airliner around the one-year anniversary of the killing of Osama bin Laden.

The new model also had a more sophisticated trigger mechanism, an apparent attempt to fix the defective trigger that burned the bomber but failed to ignite the bomb in the Christmas attack.