Every time there's a terror attack, open-borders sympathizers cross their fingers and pray to the Magic Pixie that it wasn't one of the young men who came in through...open borders. To them, the only terror attacks that could possibly "count" must be committed only by people who entered Germany in the summer of 2015. And even then, they are "regrettable one-off cases".
But why are we supposed to be reassured by the fact that the perpetrators of IS-inspired terror attacks had been in Europe before 2015, or were even born and raised in a European country? What this means is that there is a large group of alienated, disaffected young males who are susceptible to radicalization even though they enjoyed every advantage (and all the disadvantages) of living in a prosperous European nation since birth.
A very small -- but very steady -- number of these fellow citizens can be convinced by foreign propaganda to murder dozens of their fellow Frenchmen or Germans at random, and then either blow themselves up or run into a hail of police bullets shrieking "Allahu Akbar!" Since they know the customs of the country they're living in, they can conceal their activities much more effectively.
And as we've seen, the authorities often (1) have no idea the risk they pose; or (2) know the risk, but do not have the legal tools to effectively counter it. See, e.g., the terrorist who slit the throat of an 84-year-old French priest while wearing a police ankle monitor. Or the mentally unstable man who was able to remain in Germany and commit a suicide bombing even though he had already been ordered deported to Bulgaria.
If you find this state of affairs reassuring, I don't think that word means what you think it means. And given that 83% of Germans (g) (the highest percentage in Europe) think immigration and integration are the most pressing challenges facing Germany today, they don't think so either.