Terrorists Team Up in Syria to Build Next Generation of Bombs Airlines on alert for explosives-laden toothpaste tubes, cosmetics and shoes.

June 24, 2014 -- An alliance has been building inside war-ravaged Syria, with al Qaeda-linked terrorists there now working alongside hardened operatives from the prolific al Qaeda affiliate in Yemen to develop a new generation of bombs that could be smuggled aboard commercial planes, ABC News has learned.

This potentially lethal partnership helps to explain why U.S. officials have so publicly expressed concern about thousands of Americans and other foreign fighters who joined terrorists in Syria, and it is at least part of what sparked a warning to airlines earlier this year to look out for explosives-laden toothpaste tubes, cosmetics and shoes.

The U.S. government had obtained intelligence that associates of an al Qaeda affiliate in Syria – the Al Nusrah Front – and extreme elements of other radical groups were being joined by operatives from al Qaeda in the Arabian Peninsula, the Yemen-based group behind the failed underwear bomb plot on Christmas Day 2009 and the plot a year later to take down cargo planes over the United States with explosives packed into printer cartridges.

And the groups are jointly working to produce new and “creative” designs for nonmetallic explosives, leading U.S. analysts to believe that the group of radicals, who have worked with Al Nusrah Front, might be looking to target a U.S.- or European-bound plane, sources told ABC News.

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The intelligence obtained by the U.S. government did not indicate a specific target or a specific timeline. But groups like Al Nusrah Front and al Qaeda in the Arabian Peninsula are now “leveraging each other,” as one source put it – with some linked to Al Nusrah Front leveraging their Yemen counterparts for their bomb-making expertise, and al Qaeda in the Arabian Peninsula leveraging Al Nusrah Front for its array of foreign fighters with U.S. and European passports.

Asked to comment, a Department of Homeland Security official issued a statement, saying, “DHS regularly monitors intelligence related to terrorist groups seeking to do us harm.”

“At this time, however, there is no specific or credible indication of an active plot against the homeland,” the statement said. “DHS continually monitors intelligence and regularly reevaluates our security apparatus, which includes a number of measure both seen and unseen, to fit an ever evolving threat environment.”

Speaking in Washington Monday, FBI Director James Comey reiterated his concern about radicals operating in Syria.

“We’re spending a tremendous amount of time and effort trying to identify those who go, so we can know who they are when they come back,” Comey told reporters at an unrelated news conference. “The challenge for us is if we don’t know they’ve gone. … American citizens travel back to the United States, hundreds of thousands on a regular basis every month. So it’s tougher to spot them that way.”

Asked about the deteriorating security situation in Iraq, Syria’s neighbor, Comey said the situation there changes the overall concern about foreign fighters “in degree, not in kind.”

“We’re still very concerned, as we’ve talked about before, about Syria as a breeding ground and a staging ground for terrorist groups,” Comey said. “To the extent that the activities of this [Iraqi terrorist] group expand that safe-haven – that launching ground – it’s obviously a [big] concern here.”

Nevertheless, it’s unclear whether al Qaeda in the Arabian Peninsula would ever share its bomb-making expertise with the group now wreaking havoc in Iraq, the Islamic State of Iraq and Syria, which has its sights on attacks against the West. Al Qaeda denounced and cut ties with the Iraq-based group earlier this year.