FORSSA, Finland — War in Syria and Iraq seems distant from the incongruously named Villa Eden Care, a glum former hotel now housing some 300 refugees on the edge of this tidy, snow-swept town of 18,000.

But the largest atrocity attributed to Islamic State fighters washed right up in Forssa this month. The local and national police swept in and detained two refugees, 23-year-old twin brothers from Iraq, suspected of shooting 11 people during the massacre of as many as 1,700 unarmed Iraqi Army recruits near Tikrit in June 2014.

As hundreds of thousands of refugees have marched their way onto Europe’s agenda this year, so, too, have fears that past, present and future jihadists are among them.

Even as security services confront evidence that some of the participants in the Paris attacks of Nov. 13 might have entered Europe via the migrant trail, and as they struggle to assess the threat from thousands of European citizens who have traveled to fight in Syria and then returned home, they are starting to encounter another issue: holding Europeans and non-Europeans to account for killings and other atrocities carried out on the battlefields of the Middle East.