In particular, several counter-terrorism officials expressed deep concern about ISIS asserting itself in Libya, where the U.S. is still reeling from the Sept. 11, 2012 attack on a diplomatic post in Benghazi that killed four Americans, including the U.S. Ambassador to Libya.
"I am becoming concerned with how quickly their new affiliates are getting organized," one counter-terrorism official closely watching ISIS developments told ABC News last week.
"They pretty much own Libya," a second official involved in counter-terrorism operations said. "We have zero collection there and zero authorities there."
The first official referred to aspirational ISIS-produced maps showing their black jihadi flag representing the caliphate -- which was declared last summer by ISIS leader Abu Bakr al-Baghdadi -- enveloping much of the Middle East, Europe and North Africa and obliterating traditional borders. Their broad ambition is being realized in certain areas, the official said.
"Some of that territory they're actually legitimately claiming now," the official said.
"They've been preparing this for a while. A lot of them fought with ISIS in Syria and they're coming back home to Libya," Aaron Zelin, an expert on ISIS at the Washington Institute for Near-East Policy, told ABC News.
The group has used a tactic of distracting the West with individual hostage dramas – like the recent one that ended in the death of a Japanese journalist, a Japanese adventurer and a Jordanian F-16 pilot – while carrying out a far more shadowy campaign of expansion, expert observers told ABC News.
Last month, while the world was riveted by ultimately doomed negotiations for the release of the Japanese citizens and the pilot, ISIS’s division in Libya perpetrated a deadly suicide attack against Tripoli's Corinthia Hotel, which killed a dozen people including decorated U.S. Marine Corps veteran David Berry, who was working as a contractor.
"Those guys are good, that's true," a senior intelligence official said, referring to their regional strategy intertwined with sophisticated public messaging campaigns.
For several months, ISIS besieging the strategically-insignificant town of Kobane in Syria garnered considerable global attention even as ISIS essentially overran much of Iraq's Anbar province west of Baghdad and made barely-noticed inroads within Libya, the Sinai and Yemen.
The U.S.-led coalition, working with Kurdish peshmerga ground spotters, killed at least 1,500 ISIS fighters with airstrikes in and around Kobane, a defense official told ABC News last week. ISIS ultimately lost the fight for Kobani and lost ground in some areas north of Baghdad near Baiji and in Diyala province. But they also have managed their own expectations and effected tactical withdrawals like disciplined military forces, defense officials have said.
Since ISIS announced in November it had accepted a pledge of loyalty from a group it set up in Libya, the affiliate there has released about a dozen communiques in the model of its parent organization announcing beheadings to enforce religious Shariah law, assassinations, the kidnappings of 21 Christian "crusaders" and the Corinthia Hotel attack. But to date the Libyan affiliate has not commanded the spotlight the way core-ISIS has in Iraq and Syria.
However, in the foreseeable future Libya has the "most potential" for ISIS to duplicate its success in Iraq, Zelin said.
"They're trying to build up their capability in Libya and then they'll probably go after insurgent factions there like they did in Syria," he added. From there, ISIS will try to make its disturbing map of an expanded “caliphate” a reality.
Zelin argued that as ISIS has expanded its reach outside its traditional areas of control, it has avoided the past mistakes of al Qaeda, which anointed franchises to assist in its goal of attacking the west.
That plan faltered with its onetime Iraqi affiliate headed by Abu Musab al-Zarqawi, who led the predecessor organization of ISIS and started a sectarian war -- which core-al Qaeda didn't desire -- between Iraq's Sunni and Shi'a Muslims. Instead, ISIS has opted to enlarge its self-proclaimed Muslim caliphate while putting attacking the west as a much lower strategic priority.
“It's all very ambitious. But the impossible hasn't stopped them in the past," Zelin said.