Despite challenges, intelligence agencies know "quite a bit" about the current organization and its leadership, said a senior U.S. intelligence official, expressing a widely held view within the CIA and other agencies.
One hurdle is that much of the intelligence network the U.S. built up during eight years of fighting in Iraq has been dismantled, including a network of CIA and Pentagon sources and an NSA system that U.S. officials said made available the details of every Iraqi insurgent email, text message and phone-location signals in real time. Some monitoring is still possible, Zahner said.
The spy agencies appear to have been surprised by the sudden move by the ISIL to seize Mosul and other cities. The Senate Intelligence Committee is reviewing data from the past six months to determine what various agencies knew and said about the possibility of a major offensive, according to a committee staffer who was not authorized to be quoted.
Still, there was some warning. One top official, Lt. Gen Mike Flynn of the Defense Intelligence Agency, predicted to Congress in February that the ISIL "probably will attempt to take territory in Iraq and Syria to exhibit its strength in 2014, as demonstrated recently in Ramadi and Fallujah, and the group's ability to concurrently maintain multiple safe havens in Syria."
Fueling that analysis, he said, was the fact that some Sunni tribes and nationalist groups were working with the ISIL to oppose a Baghdad government they viewed as oppressive.
Behind the scenes, intelligence analysts warned about the increasing difficulties Iraq's security forces faced in combating the ISIL, and the political strains that were contributing to Iraq's declining stability, a senior intelligence official said. They reported on the ISIL's efforts to spark uprisings in areas with substantial Sunni populations and how the Iraqi military's failure to counter ISIL gains in Mosul allowed the group to deepen its influence there, the official said.
Some observers have urged an air campaign that they believe could effectively dislodge the ISIL from the towns it has seized.
"It would be challenging but certainly doable," said David Deptula, who retired in 2010 as the top Air Force general for intelligence and who planned the bombing campaign in the first Gulf War.