Sweden faces a rising threat from extremist groups because of radicalisation and financing by overseas terrorist groups, a report has said.
The report's publication came as police in Sweden arrested two women on Monday, on suspicion of links to ISIS, after they flew back from Syria.
The 115-page report, called Boundless Extremism: A Study of Transnational Connections to Swedish Radical Environments, gives a warning that extremist groups in Sweden may be “strengthened” through their links with foreign countries because of shared tactical knowledge, ideologies and funding.
The study, which was written by the Swedish National Defence College on behalf of the Centre against Violent Extremism, examined 19 terrorism cases and identified examples of terrorists communicating with foreign extremists and of them attending training camps abroad.
It cites the case of Rakhmat Akilov, who killed five people and injured 15 others in a terror attack in Drottninggatan in 2017.
Akilov was radicalised before the attack, the report said.
"Both before, during and after the act, Akilov was in contact with people outside Sweden," it said.
"These transnational contacts were made over the telephone, social media and communication applications such as Facebook, Zello, WhatsApp and Telegram.
"There were a total of 209 chat communications - individual or group chats - registered in Akilov's mobile phone and 16 were judged to have a direct bearing on the attack.
"The case of Akilov is a clear example of the existence of both operational and ideological transnational links, and where both forms of connections seem to have been significant to the course of events."
The study also mentioned links between the Nordic resistance movement and German extremist groups.
“The connections risk having a capacity-enhancing effect on the environments in all respects,” it said.
“This in the form of everything from increased tactical and practical knowledge, ideological inspiration, and financing."
The report highlighted the role of transnational links within the three Swedish extremist circles.
"The significance of these connections is likely to continue to increase," said the report.
"It should be emphasised that the number of transnational connections is significantly more than those involved in the study. This is because it is virtually impossible to get one comprehensive overview of all existing connections and their significance, based on available open information."
It recommended that the authorities review their ability to access encrypted networks and improve transparency in financing groups receive from abroad.
“Transparency in certain types of organisational forms, such as foundations and associations, is very limited,” said Magnus Ranstorp, one of the report’s authors.
“It is necessary to consider that significant donations from foreign actors should be declared publicly.”
The report called for more research into the links.
"As has been emphasised several times, the Swedish extremist milieus do not work together in a national vacuum - on the contrary, the transnational connections are crucial to their ability and development, which is likely to continue to increase in importance in the future," it said.
"An increased academic focus on this dimension is crucial for designing precise countermeasures against the violent extremist milieus, which in the long run can contribute to a safer environment and society."
This week Swedish prosecutors announced he was under investigation for "war crimes" committed in Syria.
He has also been implicated in the March 22, 2016, attacks in Brussels, and identified by Belgian investigators as one of the alleged executioners of a Jordanian pilot murdered by ISIS in early 2015 in Syria.
Originally from Malmo in southern Sweden, he is accused of joining ISIS in Syria in 2014 before returning to Europe by taking advantage of open routes for migrants.
About 300 Swedes or Swedish residents, a quarter of them women, joined extremist organisations in Syria and Iraq, mainly between 2013 and 2014, the country's intelligence service said.
Half of them have since returned home.
Due to a lack of Swedish legislation at the time to prosecute "returnees" for associating with a terrorist organisation, charges have been rare.
On Monday, police in Stockholm arrested two women after they arrived at the airport from camps in Syria.
A statement from the Prosecution Authority said multiple investigations were under way against men and women returning from areas that had been controlled by ISIS.
“The international crimes that are relevant for people for people returning from ISIS-controlled areas are war crimes, genocide and crimes against humanity,” public prosecutor Reena Devgun said.
“Sweden has an international commitment to investigate and prosecute these crimes.”