Activists from the group Raqqa is Being Slaughtered Silently work at their office in Gaziantep, Turkey. (Alice Martins/for The Washington Post)

The man’s voice was brisk and low as he called in his report from the dark heart of the Islamic State’s self-proclaimed capital, the north-central Syrian city of Raqqa.

Islamic State police are out in force in the city’s central square, he said. They are stopping ­passersby at random and scrutinizing their mobile phones. Two people have been detained. Islamic State fighters have also set up extra checkpoints on roads approaching the city and seem in an unusually jumpy mood.

“Don’t call me back unless I call you,” said the man, who also seemed nervous, before he hung up.

In this Turkish city more than 200 miles away, the screen of a colleague’s cellphone, which had identified the caller as “Raqqa Reporter 3,” went blank.

Such calls, made at great peril by a network of undercover activists living under Islamic State control, are the lifeblood of a group called Raqqa is Being Slaughtered Silently, organized to expose Islamic State atrocities through postings on Facebook and Twitter.

Comprising about two dozen 20-something Syrians who honed their activism — and their subterfuge — during the uprising against President Bashar al-
Assad in 2011, Raqqa is Being Slaughtered Silently has become a leading source of news and information about life under the Islamic State.

The activists launched their campaign in April 2014, at a time when the world was still largely oblivious to the threat posed by the extremists rampaging through Syria, beheading opponents, crucifying critics and imposing other harsh punishments.

The word “silently” in the group’s name attests to the sense of abandonment felt by many Syrians who watched in horror as their revolution for democratic change was hijacked by brutal jihadists.

The network claims credit for changing that.

“Raqqa is not being slaughtered silently now,” said Abu Ibrahim al-Raqqawi, one of the group’s founders, speaking by Skype from an undisclosed location and using a pseudonym. “Because of this campaign, the whole world knows about Raqqa and the reality of ISIS.”

The Islamic State, also known as ISIS or ISIL, has played its own part in broadcasting its atrocities, along with the slick propaganda videos that portray life under its rule as idyllic.

The Raqqa Slaughtered network has filled a crucial gap, presenting an alternative narrative to the Islamic State’s from people who are living inside Raqqa, said Hassan Hassan, co-author of the book “ISIS: Inside the Army of Terror.”

In footage provided to The Post by the activist group Raqqa is Being Slaughtered Silently, residents of the Islamic State’s self-proclaimed capital city line up for food distribution from a charity group. (Raqqa is Being Slaughtered Silently)

“These activists put their lives on the line to put out vital information from the ground,” he said. “What we don’t want is just ISIS telling the story, and people in the outside world saying that there’s no proof people are not happy under their rule.”

That is one of the main goals, said Abu Mohammed, 26, a former law student and another of the founders, who works out of the group’s small office in ­Gaziantep.

The group’s 23,000 followers on Twitter include diplomats, journalists and Pentagon officials. More than 39,000 people have “liked” its Facebook page.

Abu Mohammed says the group’s Facebook followers include Syrians living under Islamic State rule who don’t dare click “like” in case they are detained and discovered.

They are the group’s target audience, the people who are chafing under Islamic State rule and might otherwise feel that they are voiceless, he said. The group also hopes its reports will deter aspiring foreign fighters who might otherwise be tempted to volunteer.

It has clearly become a source of irritation to the Islamic State, judging by persistent attempts to disrupt the network, said Abu Ibrahim.

Imams have railed against it at Friday prayers. Its Twitter account has been hacked. Facebook suspended its page on several occasions previously after complaints — suspected to have come surreptitiously from the Islamic State — that it was violating rules against posting atrocities.

The militants recently announced that they had installed closed-circuit cameras in Raqqa to catch “the like of Raqqa Being Slaughtered Silently,” according to the Twitter account of one Islamic State member.

Whether that is true, the threat prompted even greater caution by the group’s members, for whom secrecy is paramount. They operate a cell-like structure, with activists inside Syria mostly unaware of the identities of the others, in case they should be captured and tortured to reveal names. They use encrypted software to communicate, don’t divulge their affiliation even to family members and regularly move locations, according to one of the group’s members, who uses the pseudonym Taim Ramadan and until last month was reporting from Abu Kamal, a town on the Syrian-Iraqi border.

Dispatches are relayed to Gaziantep, where Abu Mohammed and a half-dozen or so other activists, all from Raqqa, are on standby round-the-clock to post the feeds on social media. They receive funding from an American nongovernmental organization, which they do not want to identify publicly because of safety concerns.

Mistakes have been made, with tragic results. One of the group’s founders, Moataz Billah, was captured by the Islamic State at a checkpoint within days of the network’s formation. The militants found videos and photographs on his cellphone that proved his activism, and a month later he was killed.

On another occasion last year, the group posted a video sent in by an ordinary citizen of a coalition airstrike on the border town of Tal Abiyad. The man’s voice could fleetingly be heard on the video, and the Islamic State used its location to track him down, identify him and imprison him. He has not been heard from since.

“Any mistake means death. If you are arrested, they will kill you,” said Abu Mohammed, who spent a week in an Islamic State prison in 2013 for taking photographs of the extremists, before the organization was as powerful and pervasive as it is now.

Verifying the accuracy of the reports is usually impossible, because access to information in Islamic State areas is so difficult. But the network has often been proved correct.

One of its scoops was a detailed account reported in July of the clandestine U.S. operation to free American hostages held at a secret location east of Raqqa, more than six weeks before the Obama administration acknowledged the raid, which ended in disappointment. After destroying antiaircraft weapons, the American commandos were unable to find the prisoners they had hoped to free — including journalist James Foley, who was later beheaded.

When the militants captured a Jordanian pilot late last year, the Raqqa network reported his death within days of his capture. After the militants released a video depicting his brutal, fiery death, more than a month later, Jordanian intelligence confirmed that Lt. Muath al-Kaseasbeh had indeed been killed about the time pegged by the Raqqa group.

Many of its postings are more mundane, focusing on the electricity cuts, bread lines and shortages of food and medicine that reveal life under the Islamic State to be less rosy than the militants portray. The group also reports details of coalition airstrikes, and it drew criticism in one instance from other activists for allegedly inflating the toll of an attack.

Abu Mohammed stressed that the members do not consider themselves journalists, but activists, dedicated to overthrowing the Assad regime as much as the Islamic State. Many of them spent time in government prisons for participating in the 2011 revolt, and they see the two goals as inseparable, he said.

“We are nonviolent activists. We can’t fight Daesh with weapons. We can only fight them with words,” he said, using an Arabic acronym for the Islamic State. “To defeat us, they would have to shut down the Internet. And they can’t do that because all of them use the Internet.”

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