Following the usual patterns of behavior - Someone born to Muslim parents but never practiced the religion, then suddenly had an awakening, but learned from the wrong people.
"A report by the International Centre for the Study of Radicalisation (ICSR) found that more than half of known European jihadis had criminal histories, making them easy fodder for Isis recruiters offering 'redemption' in the name of jihad."
"Salman Abedi’s journey from cannabis-smoking university dropout to Isis suicide bomber is being gradually pieced together by investigators as Manchester reels from the worst terror attack to hit the UK in over a decade.
The 22-year-old was remembered as a “fun guy” who drank, took drugs and possibly had links with local gangs before appearing increasingly religious as his radicalisation deepened.
The same transformation has been documented in many of Europe’s deadliest terrorists, including petty criminals and drug dealers in the Isis “super cell” behind the Paris and Brussels attacks."
"“All of a sudden he travelled to Libya and then most likely to Syria, became radicalised and decided to commit this attack,” Gerard Collomb told BFMTV."
I am curious why that didn't pop up as a red flag? I hope Europe finds a better way to track these movements.
Also relevant:
http://www.independent.co.uk/news/world/europe/isis-foreign-fighters-british-european-western-dying-radicalised-islam-not-strongest-factor-cultural-a7421711.html