As ISIS militants flee their falling strongholds, a Syrian human rights group is trying to track them and make sure they don't infiltrate other parts of the world.

"We want to get justice," says Mohammad Kheder, 31, CEO of Sound and Picture, which works with the Free Syrian Army (FSA) to identify ISIS fighters.

But human rights observers are concerned about whether suspected militants get due process.

If we want to fight against the radical ideology of ISIS, we have to fight it all over their territories. - Mohammad Kheder , Sound and Picture

Kheder, who was a math teacher, lived under ISIS for a year after the militant group took control of his hometown al-Bukamal, near the Iraqi border, in 2014. Then he moved his family to Turkey, where they spent a few months, before being granted asylum in Germany in 2016.

Sitting at his home now, with a Syrian flag on the wall behind him, Kheder reminisces over what his city and many others looked like before ISIS took over.

"They were very beautiful cities, very colourful cities. After that, ISIS came and stole those cities. They made it black," Kheder said via Skype. "We just want to give the justice to those people who suffered under ISIS control."

Kheder founded Sound and Picture in January 2015 to document human rights abuses by ISIS and other extremist groups — information it provides to media and NGOs free of charge.

Mohammad Kheder, CEO of Sound and Picture. (Raphael Knipping)

Kheder's brother Aghiad was among the organization's first 24 members, who began operating on the ground in Raqqa and Deir ez-Zor, and in the countryside of Hasakah, Aleppo and Homs, covering almost half of the country in the east. The group has grown to a network of 75 and extended its coverage to ISIS-controlled territories in Iraq.

"We believe that if we want to fight against the radical ideology of ISIS, we have to fight it all over their territories," says Kheder.

Using WhatsApp to ID militants

Between June and October 2017, when the U.S.-backed forces drove ISIS out of Raqqa, many of the militants headed for Aleppo and across the border to Turkey.

When the Turkish-backed FSA established roadside checkpoints in the area, guards set up a group chat with Sound and Picture using the WhatsApp messaging service.

"They needed to know who is [an] ISIS member and who is just civilian," Kheder said.

Every man who arrives at one of the checkpoints has his name and photo taken, which guards send to the WhatsApp group.

A checkpoint near al-Bab, Syria, controlled by the Free Syrian Army group Ahrar al-Sharqiya. The FSA flag can be seen in the background. (Isaa al-Khedhr)

"We have to be 100 per cent sure before saying anything," says Kheder.

Kheder understands the implications of his organization's work and says great care is taken in making identifications, requiring input from multiple Sound and Picture members on the ground.

"We don't take the information from one source," he explains.

The group has built a database by collecting as much information as possible about each suspected ISIS member. And the intelligence gathering isn't limited to men. About 30 per cent of Sound and Picture's members are female and trained to collect information on women.

Sound and Picture says it has identified hundreds of ISIS members trying to flee through FSA checkpoints — most of them are locals, but there are also nationals from Egypt, Morocco, Saudi Arabia, Tunisia and France.

In June 2017 when a Montreal woman contacted Sound and Picture about her daughter, Kheder recognized the daughter's husband — a German ISIS fighter known by the nom de guerre Abu Salaheddin.

"He was in al-Bukamal, and since I am originally from al-Bukamal, we could identify the neighbourhood he was in," explains Kheder.

Abu Abd al-Rahman, a commander of the FSA group Ahrar al-Sharqiya, confirmed to CBC News, via WhatsApp, that his group arrests suspected ISIS fighters identified by "trusted sources."

ISIS fighters detained

Sound and Picture's involvement ends with the identification. Any suspected ISIS members are taken into custody — and that's where things get murky. The FSA is the only group Sound and Picture works with, but it isn't the only rebel group on the ground.

Areas in Northern Syria are controlled by various FSA, Syrian Democratic Forces and Turkish-backed rebel groups, who are often in conflict with one another. No one group has jurisdiction.

Because of the patchwork of jurisdictional control, it's unclear what happens to the detainees.

A pile of cement blocks marks this checkpoint, also near al-Bab. (Isaa al-Khedhr)

Al-Rahman says his group has handed over ISIS fighters to courts in areas of the Euphrates Shield, a reference to an operation launched by Turkey in August 2016, aimed at driving ISIS out of the frontier town of Jarablus on the Euphrates River.

Foreign ISIS fighters who make it into Turkey and are caught are likely to be extradited back to their home countries by Turkish authorities.

Turkey's foreign affairs ministry declined to comment on whether or not it works with Sound and Picture and the FSA to identify ISIS members. A spokesperson from the Directorate General for Information told CBC News only that the country's "non-entry list" now has more than 56,300 people on it.

The Turkish government says it has arrested more than 10,000 ISIS "affiliates" and deported around 5,800 foreign terrorist fighters.

An investigation published in December 2017 by the U.K. Telegraph found that Kuwait and Qatar have reportedly offered to pay to have their citizens returned, while Ukraine, Chechnya, Turkmenistan and Uzbekistan have turned down Turkey's extradition requests.

The situation is more complicated when ISIS fighters are arrested by Kurdish officials, who are not universally recognized as an autonomous administration. Similarly, many foreign governments would not deal directly with FSA's ragtag collection of rebel groups.

Every time I see this video I feel proud of myself <br>I was the first person to write statements against <a href="https://twitter.com/hashtag/ISIS?src=hash&ref_src=twsrc%5Etfw">#ISIS</a> in <a href="https://twitter.com/hashtag/Syria?src=hash&ref_src=twsrc%5Etfw">#Syria</a><a href="https://twitter.com/hashtag/Raqqa?src=hash&ref_src=twsrc%5Etfw">#Raqqa</a> <a href="https://twitter.com/hashtag/DierEzor?src=hash&ref_src=twsrc%5Etfw">#DierEzor</a> <a href="https://twitter.com/hashtag/ISL?src=hash&ref_src=twsrc%5Etfw">#ISL</a> <a href="https://t.co/35yMYARMMN">pic.twitter.com/35yMYARMMN</a> —@aghiad_alkheder

'Huge' need for justice, accountability

Nadim Houry, director of the terrorism and counterterrorism program at Human Rights Watch (HRW), has visited a detention centre in a Kurdish-run part of Syria. While he declined to speak specifically about Sound and Picture, he says it's fine to use civilian tips to identify ISIS members, but he's concerned about due process.

"The question is, will there be a local process to check this information, a chance for the person to defend themselves, cross-check it, make sure the information is accurate?" says Houry.

There's a danger that in the absence of real justice mechanisms, we end up with various revenge mechanisms. - Nadim Houry , Human Rights Watch

"There is a huge need for justice and accountability in these areas," he says. "There have been a lot of abuses committed by all sorts of different parties. And there's a danger that in the absence of real justice mechanisms, we end up with various revenge mechanisms."

Kheder is also cognizant about the pitfalls of this process.

"Even ISIS members have to be dealt with in a civilized way," he says. "If we deal with former ISIS members in the same way that ISIS members dealt with civilians, then we are the same."

Kheder says detainees in this part of the world aren't treated the way they are in Western countries, and he too worries about human rights abuses in detention centres.

"It's not to find the best group, it's to find the least worst," he says. "We're trying to find the group who have less violations, as much as we can. It's impossible to find a group who have no violations."

Sound and Picture is featured in the CBC documentary The Way Out.