Aziz, one of the activists profiled in “City of Ghosts,” wanted to put a face to a crisis. (Amazon Studios/A&E IndieFilms/IFC Films)

For his 2015 Oscar-nominated documentary “Cartel Land,” director Matthew Heineman put himself in harm’s way to film the drug trade along the U.S.-Mexico border. For his follow-up, “City of Ghosts,” he wasn’t in imminent danger — but his subjects were.

“City of Ghosts,” opening locally Friday, is about four men who are among the founders of Raqqa is Being Slaughtered Silently (RBSS), a network of citizen journalists risking their lives to expose the horrors of life in the Syrian city of Raqqa, basically the capital of ISIS.

“With ‘Cartel Land,’ I was in visceral, immediate danger,” Heineman says. “This was a much different experience, going with them from safe house to safe house, where the fear and the danger was omnipresent.”

Aziz, one of the film’s four subjects, says Heineman did what he could to address the danger.

“When we first met Matt, we were concerned about security things. Matt and his team, they were careful about that,” Aziz says. (The film withholds all of the subjects’ last names for obvious reasons.) “From the first day, there was a trust between us. We agreed he would film everything, [but] before the movie [was released] we would check it for any problems. And [there was] nothing.”

Because Heineman couldn’t travel to Syria to shoot his own footage, he had to rely on outside sources (mostly RBSS members) to get the images he needed. “It was a new experience not shooting the entire film,” he says. “It was definitely a fascinating exercise trying to weave in the footage shot in Syria, as well as the footage from ISIS, with what footage we shot.”

Yes, he said “footage from ISIS.” Heineman included some of the Islamic State group’s high-quality recruitment videos, which celebrate life under their regime, to show exactly what RBSS is up against.

“It’s shocking the production value that the ISIS videos have,” Heineman says. “I was intellectually fascinated about this war of ideas: There’s ISIS on the one hand, with their slick videos, and then there’s RBSS trying to dispel the myth that ISIS was creating this peaceful paradise for Muslims, when it’s the exact opposite.”

Aziz and the three other subjects all managed to get out of Syria alive, but that doesn’t mean they are safe. They all now live in Europe, but they remain in daily danger because of their work. Aziz travels frequently as the spokesperson for RBSS, and he came to D.C. with Heineman in June for a screening of “City of Ghosts” at the AFI Docs film festival. ISIS has put a bounty on all of them and, in some cases, their families — the father of one subject was killed in Syria and his murder broadcast due to his son’s involvement with RBSS.

For Aziz and the other subjects, the film is about putting faces to a crisis, even when it would be safer to stay hidden.

“Every one of us has his own personal story that should be told,” Aziz says. “That is the reason to show our faces, to reach out to thousands, millions of people around the world.”