The "imminent" threat against the West that pushed the United States to strike the Khorasan Group in Syria Monday is linked to the same terrorist efforts that ABC News first disclosed this summer, just before the U.S. government announced heightened security measures for air travelers overseas, sources said today.

The little-known Khorasan Group was "nearing the execution phase for an attack in Europe or the homeland," and airstrikes overnight "removed their capability to act," senior law enforcement and intelligence officials told ABC News today.

But prior to the strikes, ABC News reported earlier this year that U.S. officials learned that a particularly extreme "subset" of terrorist groups in Syria was working alongside operatives from al Qaeda's prolific offshoot in Yemen to produce "creative" new designs for bombs packed into electronic devices like cell phones or laptops, sources said. The officials did not identify the group at the time.

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Specifically, associates of the al Qaeda affiliate in Syria - the al Nusrah Front - and radicals from other groups were teaming up with elements of the Yemen-based group Al Qaeda in the Arabian Peninsula (AQAP) to potentially down a U.S.- or Europe-bound plane, with help from one of the thousands of Americans and other foreign fighters carrying U.S. and European passports who have joined extremist groups in the region.

The group was made up of "seasoned Al Qaeda veterans" who had found a "safe haven" in Syria where they were able to "construct and test improvised explosive devices," one senior intelligence official said today. The joint effort with AQAP, which built such innovative devices as the "underwear bomb" that ultimately failed to detonate in a plane over Detroit on Christmas Day 2009, made the threat out of Syria "more frightening than anything" else the Obama administration had seen, Attorney General Eric Holder told ABC News in July.

The threat prompted airports overseas to increase security measures that month. At the time, the Department of Homeland Security announced that if some overseas passengers flying to the United States want to bring cell phones and other electronic devices onboard with them, they would have to show that the devices can turn on.

Though ABC News had previously reported on the "subset" of particularly dangerous terrorists working in Syria, the Associated Press first publicly put a name, Khorasan Group, to it last week.

On "Good Morning America" today Pentagon Press Secretary Rear Admiral John Kirby said the Khorasan Group was "very dangerous" and there was "active plotting going on for an attack on the U.S. homeland."

As part of the fight against the Islamic State of Iraq and Syria (ISIS) and al Qaeda-affiliated groups in Syria, FBI Director James Comey has said the government is spending "a tremendous amount of time and effort trying to identify" anyone who's gone to Syria, but "the challenge" is not missing anyone.

More than 12,000 foreign fighters, including more than 100 Americans, have now joined tens of thousands of other fighters operating in Syria and neighboring Iraq, where ISIS is now wreaking havoc and recruiting more Westerners to fight.