Security is tightening this morning at some overseas airports with direct flights to the U.S. because of concerns about al Qaeda's efforts to create an undetectable bomb, as well as the threat of Western militants trained in the Middle East.
Former CIA deputy director Mike Morell, now a CBS News analyst, said on "CBS This Morning" that the biggest threat would be those Western militants -- currently fighting in Syria -- joining forces with al Qaeda in Yemen, which has the unique capability to produce nonmetallic bombs.
"There a large number of foreign fighters who have gone to Syria -- 6,000 to 7,000 of them -- some of them might be thinking of coming home and conducting terrorist attacks," Morrell explained.
There are about 100 from the United States, he said, and thousands from Western Europe. Most foreign fighters end up dying on the battlefield, but those who want to return would be difficult to track. "It is very difficult to monitor them," Morell said, because "it is so easy to slip into Syria from the Turkish border."
Meanwhile, al Qaeda in Yemen is capable of producing bombs with a couple of ounces of liquid explosives and a nonmetallic initiation device.
"You worry about someone getting on an airliner with some sort of explosive device who can travel to the United States," Morrell said.