PARIS (AP) — The suspect in the recent killings at the Jewish Museum in Brussels provides a textbook example of a longstanding fear in the West — the threat posed by radicalized citizens returning from the battlefields of Syria.

Since Syria became a magnet for Westerners, European nations have been slowly fortifying themselves with measures to detect potential jihadi fighters and counter any plots. But French-born Mehdi Nemmouche — the first Western citizen returning from Syria to be implicated in a major attack — was caught by chance, in a spot check for drugs by a customs official, making a mockery of efforts to counter the threat.