ISIS: How countries are dealing with returning ISIS fighters

Recently, I've been reading articles about how countries are dealing with citizens who are returning (wish to return) after fighting (being a part) of ISIS.

Here's a look at three countries:

Sweden:

From The Local:

None of the 150 Swedish citizens who have returned home after joining the terror group Islamic State have been convicted of crimes committed while abroad. While some European countries are investigating all returning foreign fighters, Sweden is not . . . In Sweden, however, suspicion of a specific crime is required in order to initiate a preliminary investigation. If there is no concrete information to work off, the police's ability to access any digital evidence, for example in returning foreign fighters’ phones, decreases drastically.

It would appear that Sweden is prevented from investigating ISIS fighters due to existing law. Sweden wishes to make changes to that law.