Now, they're trying to tackle their apparent infiltration problem by offering a cash reward to anyone who can handover "an agent collaborating with the Crusaders."
Paper fliers, attributed to the ISIS command in the sprawling northern Syrian city of Aleppo, have been seen on the ground and widely disseminated on social media platforms used by the group. They offer $5,000 to anyone who can capture an alleged informant, "or for information leading to such spies."
Reports in Iraqi media last week said airstrikes on Haweija, near the northern Iraqi city of Kirkuk, left ISIS' de-facto governor of the city, Aboul Hassan, dead. Also killed last week, according to reports in local media and a reliable activist group with extensive contacts in the region, were two other senior ISIS commanders near Mosul.
One of them, Radwan Taleb al-Hamdouni, was allegedly killed when a missile struck his vehicle west of the province. If true, it would suggest he was singled out as a target -- and that type of attack is only possible with solid intelligence.
Since the beginning of the U.S.-led international military operation against ISIS, gathering good intelligence on the group has been a primary objective, and a considerable challenge, for Western agencies.
ISIS is believed to have a strong centralized command and control, with leader Abu Bakr al-Baghdadi at the top and a cadre of top lieutenants distributed across the group's territory answering directly to him.
But far beneath that level, the group has so actively -- and successfully -- welcomed recruits from around the world, that in spite of efforts to screen those newcomers it has created a possible route in for infiltrators working with intelligence agencies.
One of the first indicators that the U.S. government was getting reliable information on the movements of ISIS leaders came in early November, in the form of an airstrike on an ISIS vehicle convoy.
Rumors that Baghdadi himself might have been killed in the strike were eventually put to rest when he issued a new audio message, but other senior members of the group were killed in the attack.
"We hit a military convoy that we knew ISIS leadership was part of," CBS News contributor and former deputy CIA Director Mike Morrell told "CBS This Morning." He said regardless of whether Baghdadi was even in the convoy, "what's really important here is that we found a leadership target and we went after it... that takes very good intelligence."
Morrell said keeping the pressure on ISIS' leaders was essential to defeating the group.
Such targeted strikes "force the leadership of a terrorist group to think about their own security rather than conduct their operations. The other reason is that, as you remove leaders, over time you get weaker leaders, and that is one of the key ways to degrade a terrorist organization."
In addition to the offer of a $5,000 reward for spies, 산청출장안마 ISIS' online propaganda has included an increasing number of videos and photos in recent days showing the execution of people accused of spying or working for foreign agents.
It all serves as the latest example of how an adaptive, media savvy organization is trying to adjust its tactics to deal with its greatest perceived threat.