The U.S. government is warning about the potential for retaliatory attacks from home-grown, lone-wolves who may be inspired to act in the name of al Qaeda and the Islamic State of Iraq and Syria.
The FBI and Department of Homeland Security issued a bulletin after the airstrikes were launched against extremists in Syria, telling law enforcement to "be vigilant." Officials have said there is no credible specific threat at the moment.
One of the groups targeted in Syria is an al Qaeda-offshoot called Khorasan.
Taking advantage of the lawlessness inside Syria, Khorason -- a terror cell of veteran al Qaeda operatives -- has been working on new hard-to-detect bombs that can be smuggled aboard airplanes, reports CBS News correspondent Bob Orr.
Lt. Gen. William Mayville, director of operations for the Joint Chiefs of Staff, said Tuesday the group was "nearing the execution phase of an attack either in Europe or the homeland."
Al Qaeda has experimented with non-metallic bombs that may be hidden in shoes, clothing, cell phones, laptops and tubes of toothpaste.
That intelligence prompted the TSA in July to tighten security for U.S.-bound flights from two dozen foreign airports.
Sources tell CBS News Khorasan, which takes orders from al Qaeda chief Ayman al Zawahiri, includes explosives experts who may have been trained by al Qaeda's top bomb-maker, Ibrahim al Asiri. Asiri, who is based in Yemen, is the architect of bombs hidden in underwear and computer printers.
Unlike the Islamic State of Iraq and Syria (ISIS), Khorasan is not battling for territory. Instead, it's plotting external attacks against the West and recruiting American and European radicals.
Western passport holders who have joined the jihad in Syria could be used to carry bombs onto planes.
Attorney General Eric Holder told Yahoo News that Khorasan "is a group that has been known to us for two years."
"I can say that the enhanced security measures that we took [in] the aviation sector some months ago [were] based on concerns we had about what the Khorasan Group was planning to do," Holder said.
The fear that American passport holders will take part in Khorasan's vision of attacking the homeland has led to a proposal for a new approach in tagging potential terrorists, Holder told Yahoo.
"It will be focused on people who have terrorist connections and come up with new ways in which information is shared between INTERPOL members that, frankly, don't exist now," he said. "We have red notices that we use for people who are charged with crimes. But we're gonna come up with a new kind of notice that deals with people who are suspected of engaging in terrorist activities."