Have hundreds of disaffected Americans traveled overseas to train as terrorists with the brutal Islamic State?

That’s what Rep. Mike Rogers (R) of Michigan, chairman of the House Intelligence Committee, said on Sunday during an appearance on Fox News. He said that IS foreign fighters – Canadian and British citizens, as well as Americans – now pose “a very serious threat” to US interests.

IS, also known as ISIS and ISIL, has focused on seizing slices of Syria and Iraq to create what it calls a new Islamic caliphate. But Representative Rogers said that an attack on a Jewish museum in Brussels in May was likely led or inspired by IS members and may show the group is eager to attack Westerners where they live.

“I’m very concerned, because we don’t know every single person that has an American passport that has gone and trained and learned how to fight,” said Rogers.

Clearly some Americans have indeed joined the IS ranks. A US citizen named Douglas McCain died fighting with IS troops in Syria last month. Other Americans have turned up among IS battlefield casualties. The question is the scale of this trend – and whether it poses a threat to the US homeland as well as US interests in the Middle East.

In July, Attorney General Eric Holder said “dozens” of Americans were present in IS units. In total there are some 7,000 foreign IS fighters, out of a total strength of 23,000 operating in the Syria-Iraq axis, according to US intelligence estimates.

This matches up with the estimates made by some knowledgeable groups outside government.

Last December an extensive survey of credible sources by the Center for the Study of Terrorist Radicalization at the Foundation for Defense of Democracies judged the most likely number of foreign fighters in IS to be about 8,500.

Of those, only a scattering were Americans or Canadians, according to the Center, which is based at Kings College in London.

However, Western Europeans made up almost 20 percent of the foreign fighter population in Syria at that time, according to the Center figures. The largest contingent, between 63 and 412 individuals, came from France. Second was Britain, with a range of 43 to 366. Third was Germany, with 34 to 240.

Back in June, Rogers framed the threat somewhat differently. In a Fox News appearance then he said that “thousands of Westerners and Americans” are fighting with IS in the Middle East.

Politifact.com rated this statement “mostly true.” While the “Americans” part of that statement might have been stretched, according to the Pulitzer-winning fact-checking web site, it’s undeniable that disaffected Westerners are flocking to a group they see as making progress towards its extremist goal of a radical Islamic-run state.

“Rogers may have amplified the numbers a bit, but he did not overstate the threat. Experts we spoke with, along with US and Western governments, have expressed legitimate concerns about this issue,” wrote Politifact.com in June.

Right now there is bipartisan dissatisfaction with President Obama’s approach to IS. Rogers opined that Obama’s foreign policy is in “free fall”. Sen. Dianne Feinstein (D) of California, chairman of the Senate Intelligence Committee, said Sunday on “Meet the Press” that Obama has been “too cautious” in confronting the group.

Obama travels to Wales for a NATO summit this week where he may push to persuade other nations to join the US in a more concerted effort to roll back IS gains.

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As to the IS threat to the United States itself, doom-laden worry about them crossing the US border is alarmist and unhelpful in developing a realistic strategy to defeat the group, according to Daniel Benjamin, a senior fellow in foreign policy at the Brookings Institution.

“Even if marauding [IS] operatives in Land Cruisers may be humiliating Iraq’s hollowed-out military, that doesn’t mean they have genuine terrorist skills ... we’ve seen no demonstrated ability to carry out the kind of complex international strike that kills dozens or hundreds, let along engulfs a US city in flames,” wrote Benjamin in mid-August.