In the past two months, ISIS has taken over large swaths of land in Iraq and Syria, and recent reports indicate the Sunni militant group has pushed as far west as Jordan and Lebanon. But members of the hardcore radical group already may be in Western nations, too. Several of them are known to be holding Western passports, and some analysts have said that ISIS, also known as the Islamic State, could have already infiltrated the U.S.

When news broke that Abdirahman Muhumed, the second known American killed fighting with ISIS, worked at the airport in Minneapolis and reportedly had access to the tarmac, some suggested that meant he could have carried out an attack in the U.S. similar to what we saw in 2001. 

Yet others have shrugged off the idea, and still some who have studied terrorism for decades say all of that is speculation: No one really knows the magnitude of the group’s threat to the U.S. But given the Sept. 11, 2001, attacks, the work of an al Qaeda cell inside the U.S., it should be a worry, they said.

And according to John Horgan, director of the University of Massachusetts Lowell’s Center for Terrorism and Security Studies, the ISIS phenomenon is similar to what happened when al Qaeda formed under Osama bin Laden: lots of questions and speculation, but few concrete answers.

“When something dramatic happens, we scramble for answers to make ourselves feel better,” Horgan said. “We do not do a good job of understanding complexity.”

But the U.S. and others have invested heavily in understanding precisely this sort of thing. 

“Sept. 11, 2001, triggered a wave of interest in research on terrorism. The federal government has stimulated research by infusing millions of dollars into the field,” Marc Sageman, one of the leading researchers on terrorism, wrote in the Chronicle of Higher Education last year. But 12 years later, “overall, the same stale arguments about 'How can this happen?' are debated over and over again -- with very little new insight.”

"There have been few new significant insights about how or why some young people turn to indiscriminate political violence," Sageman wrote. "The disappointing state of the field, largely consisting of wild speculations without foundation, requires some explanation."

Since ISIS began its offensive in Iraq in June, Middle East experts and senior politicians have speculated about how the group functions, estimating everything from the number of fighters, the amount of money it possesses, and what kind of weaponry it has seized.

But that approach, Horgan said, ignores the more important questions, like how someone who has become radicalized becomes a terrorist.

“There is a gap there,” he said. “Everyone is looking at how these people are recruited and how they are radicalized. But radicalization is not a useful predictor of who becomes a terrorist.”

State Department spokeswoman Jen Psaki told a press briefing the U.S. did not "have information that [ISIS] was planning a 9/11-style attack."

Horgan said the media, experts and senior U.S. officials are looking at the ISIS issue all wrong.

"What we should really be looking at is this idea of disillusionment," he said. "If we can figure out what makes ISIS fighters walk away, if we can separate the reality from the fantasy of ISIS, then we might be able to stop this."

For now, the U.S. is fighting ISIS by conducting targeted airstrikes in Iraq and by forming an international coalition that will address the issue militarily and diplomatically. President Obama is expected to identify a specific strategy against ISIS in a speech Wednesday.