Half of Sweden’s 300 Islamic State fighters have returned from the Middle East, but a survey has revealed several municipalities have little idea who among their residents are former fighters.

The survey found that out of the municipalities believed to be the homes of the 150 returnees, the local authorities could only identify 16 adults and ten children, Swedish public broadcaster SVT reports.

The cities and municipalities of Malmö, Borås Gothenburg, Stockholm, and Örebro saw the most fighters leave to fight with the terror group.

Estimates state that in Gothenburg and Västra Götaland, the number of returnees is somewhere in the dozens, but Gothenburg could only name ten adults and eight minors that they were aware of.

Stockholm, which identified one individual who turned out to live in another municipality, is thought to have 35 to 40.

The heavily migrant populated city of Malmö is thought to have at least 15 returnees, although the city did not answer the survey.

Report: Returning Islamic State Women and Children Pose Growing Terror Threat https://t.co/yaX1GghhRU — Breitbart London (@BreitbartLondon) July 24, 2018

Terrorist researcher Magnus Ranstorp said the survey showed there was a “deep culture of silence” in areas such as those in Stockholm where some locals are unwilling to speak to police and other authorities about returning fighters.

Ranstorp slammed the government saying the fighters presented a very real security risk and the government needed to do more to act and prosecute fighters if they engaged in terrorism overseas.

“They return, and their threshold of violence is totally different. This means that they can be extremely dangerous,” he said.

The terror expert has been vocal in the past about his criticism of the way the government has handled the issue of returning fighters.

In 2017, he came out against suggestions that the country should reintegrate returning fighters, saying “This is how ill-prepared we are to deal with returnees” in response to an interview with then-Swedish Minister for Culture and Democracy Alice Bah Kuhnke.

He also came out against a court ruling by the European Court of Justice (ECJ) earlier this year that prevented Sweden from deporting a migrant linked to terrorist activities.