Yemen Terrorists Behind Latest Warning and New Concerns Over Syria

The Al Qaeda affiliate in Yemen that launched the Christmas Day bombing attempt in 2009 and the explosives-laden cargo plot in 2010 is behind Wednesday's warning to airlines around the world that terrorists could be developing new ways to smuggle "shoe bombs" and other deadly devices onto planes, sources told ABC News.

But what may be concerning U.S. officials even more is a covert plan by the group, Al Qaeda in the Arabian Peninsula, to shepherd American and European fighters to war-ravaged Syria, where they would receive paramilitary training and learn how to handle explosives, ABC News has learned.

The group - commonly called AQAP - has launched a systematic campaign online and elsewhere to recruit Westerners from countries that have visa-waiver agreements with the United States, making it easier for them to travel back to their home countries, sources said.

U.S. officials have recently sounded the alarm over the threat posed by foreign fighters heading to Syria, but they have never tagged AQAP as a source for some of their concern.

Complicating what he called the "apocalyptic disaster" in Syria are the more than 7,500 foreign fighters there from 50 countries around the world, Director of National Intelligence James Clapper told a Senate committee last week. And, he warned, among the foreign fighters is a group of Al Qaeda veterans from Afghanistan and Pakistan "who have aspirations for external attack in Europe, if not the [U.S.] homeland itself."

Speaking in Washington two weeks ago, the new head of homeland security said his agency is "very focused" on the issue, and it's the type of threat he worries about most.

"[E]xtremists are actively trying to recruit Westerners, indoctrinate them, and see them return to their home countries with an extremist mission," Department of Homeland Security Secretary Jeh Johnson said. "We must remain vigilant in detecting and countering all these threats."

In fact, the U.S. government and European partners are trying to identify and track foreign recruits in Syria, Clapper told lawmakers.

The U.S. government is also trying to work with the government of Turkey, Syria's neighbor, to shore up the country's borders, sources said. In his Senate testimony, Clapper said "part of the problem" with foreign fighters in Syria is that they transit through intermediary countries with sometimes lax rules.

The new intelligence that prompted Wednesday's notice to airlines indicates no specific threat, but federal authorities have a responsibility to communicate any such information to airlines as a precaution so they can take any measures they see fit, sources said.

It was at least the second such warning in as many weeks.

Two weeks ago, DHS issued a similar warning over explosives that could be hidden in toothpaste or cosmetic tubes, but those concerns were tied to potential attacks around the Winter Olympics in Sochi. At the time, the Transportation Security Administration announced it was temporarily banning travelers from bringing any amount of liquid, gel or aerosol in their carry-on luggage aboard flights between the United States and Russia.

Just three months after the attacks on Sept. 11, 2001, Richard Reid tried to detonate a homemade bomb hidden in his shoe aboard American Airlines Flight 63, which was carrying 14 crew members and 183 other passengers from Paris to Miami.

In 2006, British authorities uncovered a plot to use liquid explosives to blow up transatlantic flights headed to the United States and Canada, which was foiled by U.K. authorities.

That prompted U.S. and U.K. authorities to ban most liquids in carry-on luggage, but it also prompted terrorist groups such as AQAP to look for new ways to bring explosives onto planes.

Three years later, on Christmas Day 2009, Nigerian national Umar Farouk Abdulmutallab boarded a plane bound for Detroit and unsuccessfully tried to detonate explosives packed into his underwear - a bomb designed and built by AQAP.

The following year, AQAP hid explosives in printer cartridges that were shipped via cargo planes bound for the United States, but an overseas tip thwarted the plot. Most recently, AQAP developed another "underwear bomb," but the man they thought would detonate it was working with U.S. authorities.

Mike Levine, Jack Date and Jack Cloherty contributed to this report.