“It is a global expansionist, global jihadist organization” he said. “It is swollen with foreign fighters and suicide bombers” who will go “wherever the organization tells them to go. And that could very easily be capitals in the region, it could be capitals in Europe and, God forbid, it could be here.”

Still, there are factors that may limit the group’s ability to conduct a major al-Qaeda-style attack, the intelligence officials said. These include lacking the kind of technical know-how that al-Qaeda in the Arabian Peninsula’s top bomb designer has shown in building devices to be smuggled aboard aircraft, the officials said.

IS also may eventually face financial pressures because the areas it controls in Syria and Iraq are not economically self-sufficient, the officials said. Another vulnerability may be the militants’ main transit route through Turkey, if the Turkish government can be persuaded to crack down on would-be fighters traveling to Syria and Iraq, the officials said.