TSA chief warns of 'Underwear 2' bomb

The head of the Transportation Security Administration (TSA) described a second, highly dangerous al Qaeda underwear bomb in unusual detail Friday.

TSA chief John Pistole said authorities are on high alert because the "Underwear 2" bomb, designed to blow up an airliner over the United States, had never been seen before.

The remarks came during the third annual Aspen Security Forum in Aspen, Colo.

Pistole said he had readjusted TSA security systems to respond to the bomb, which was discovered after a double agent foiled the plot last year.

The "next-generation device" was "new and improved in many respects" over the original, but failed underwear bomb worn by Umar Farouk Abdulmutallab on Christmas Day, 2009, Pistole said.

The explosive was designed to mix volatile liquids with two syringes, effectively preventing the failure that plagued Abdulmutallab's device.

The bomb-maker had used household caulk to seal the device and prevent any vapors from alerting detectors or bomb-sniffing dogs.

Pistole said his previous security detection system would not have discovered the bomb. He ascribed the device to Ibrahim al-Asiri, a member of al Qaeda's Yemen branch and one of the world's most wanted terrorists.

"That is our greatest threat," Pistole, who was interviewed by ABC's Brian Ross, said.

"All the intel folks here know that is a clear and present danger …

"[Members of al Qaeda] want to show that we can spend billions and billions of dollars … and we still can't stop them."  

Pistole's comments were first reported by TIME magazine.

--This report was updated at 1:35 p.m.