Intelligence estimates so far have counted at least 100 Americans who have joined or attempted to join terrorist groups in Syria — but U.S. Assistant Attorney General John Carlin says that not all are affiliated with the Islamic State, or ISIS.

"Those who they believe have gone to that region, attempted to go to that region, or have gone to that region and come back, it is not the number that they have linked to a designated foreign terrorist organization," Carlin said.

Among that group is Ahmad Abousamra, a Stoughton man who’s accused of being a key player in ISIS’s social media campaign and, last December, was added to the FBI’s Most Wanted Terrorists list. Carlin says people like Abousamra who become radicalized in the U.S. come from many walks of life.

"I can’t say, when it comes to the cases that we’ve brought in the criminal system, that I can see a trend, which I guess says something there; because it’s really cut across the spectrum in terms of the people that have been attracted by it," he said.

That’s why, Carlin says, it’s important for communities to take a more active role in stopping many from reaching what he calls the concrete steps that lead to radicalization — and prosecution.

"In over 80 percent of these cases, there’s been someone in the community who’s seen the person go down the road of radicalization, and been in a position where they could’ve taken some steps to intervene — either by reporting it or trying to dissuade the person," he said.

Carlin was in Boston for a U.S. Attorneys conference and Thursday — in a rare sit down with media — discussed the many issues his office handles, including the response to terrorist threats like ISIS.

Carlin says he wouldn’t characterize Boston as having a graver terrorist threat than other places. But, in part due to its community policing apparatus, it’s been chosen to be part of a federal pilot program tasked with preventing people from joining extremist groups. It’s a partnership that will see the likes of law enforcement, public health, community and religious groups working together. U.S. Attorney Carmen Ortiz, who joined Carlin’s sit down with reporters, said Boston’s program is still in its early planning stages.

"We’re focusing on what are those signs," Ortiz said. "What are indicators that could alert individuals that would be observing a particular person into a behavior that would concern them? And what are the resources and what should the response be?"

Ortiz estimates the program’s plan won’t be put be in place until the end of the year.