While I get the gist of your message, from a technical standpoint it isn't that hard to build and acquire materials to make an IED. More so these days with the proliferation of how-tos on the internet.
A fact well known to security and counter-terrorism experts, who would be closely monitoring the sales and purchase patterns of any items that could be so abused.
You could buy tiny quantities of the stuff you need as to not raise flags, but that also massively increase the time you need to gather enough materials to make a bomb. The longer a cell remains active, the greater the chance of discovery by police.
Drugs and people smuggling does happen. All the time. And those are done by common criminals who are out to make a profit.
A terrorist organisation like ISIS, with its vast wealth and probably support from several sovereign states, could be far more sophisticated in their operations and attempts.
So I don't think it's very clear cut at all which is more risky for the terrorist - either spend months or even years in the target country/city slowly accumulating dual use items in order to extract enough materials to make a big bomb; or smuggle materials and people through in bulk days or even hours before launching an attack.
As to the bombers I can only speculate that they are probably not recent refugees (at least not all of the perps) but Jura's point is still valid. They may or may not have been recent refugees but most certainly they were products of one. Almost 100% of home grown European terrorists were either foreign fighters or disenfranchised young men/women born to refugee/asylum seekers.
No, that's entirely self contradictory! One cannot be a 'home grown' terrorist and a foreign refugee at the same time!
The fact that you have second, third or later generation immigrants turning to terrorism isn't damning on the immigrants ancestors who first came to European countries, but actually on the host countries that turned immigrants' children into terrorists!
I have never ever heard of even a single case where those second+ gen immigrants who later became terrorists were taught to be terrorists by their parents or grandparents, you know, the original immigrants!
This entire line of proto-ethnic discrimination is a massive part of the problem that made so many later generations of immigrants into terrorists in the first place.
It's the world's saddest chicken and egg question, which came first, discrimination/alienation or radicalisation?
But it's actually entirely the wrong question, especially in light of recent developments.
Europe has already failed spectacularly with its politics of fear and discrimination against migrants from certain parts of the world and who has a certain shared ethnic and religious background.
Instead of abandoning a clearly failed policy and trying something that has actually worked well in the past - embracing and actively integrating new arrivals like as done in the US, most (with the notable, noble exception of Germany) are actually taking all the bad parts of the old policies that helped spawn all those late gen home grown terrorists and making it exponentially worse in every way possible!
You have millions of innocents fleeing war, persecution and death, most with sometime child-like naivety about how wonderful they think the west is, risking death (with a heartbreaking number of them dying on the way), and then seeing all their hopes, dreams and sacrifices brutally stamped out by jackbooted masked boarder security guards who often use excessive brutal forced against desperate refugees.
One could hardly purposely create a better breeding ground for hatred and radiacliasation!
It's not the few mythical undercover terrorists masquerading as refugees that Europe should be truely concerned about. It's creating concentration camp style refugee towns on its very boarders where hopes and dreams of millions wither and die, leaving only dispare, anger and hatred in its place!
