Would-be jihadists, largely shut out of legal means of travel, turn to migrant-smuggling groups to try to slip out of the U.K.

police, faced with stopping what they say is an unprecedented flow of British extremists to and from the Middle East, last year intercepted unusual contraband in a truck bound for a crossing point into France: a Muslim extremist seeking passage to Syria to join Daesh ‘ISIS‘.

Gabriel Rasmus, a 29-year-old Muslim convert, and two alleged accomplices were hunkered down outside Dover in a shipment of blue barrels, expecting they would join other extremists leaving Britain through organized crime’s migrant-smuggling routes, U.K. police say.

But their presence wasn’t a secret to police: The trio had been infiltrated by an undercover operative in a sting, one of several successful efforts by authorities in the U.K. and elsewhere to thwart would-be jihadists.

British authorities have confronted the problem of local extremists heading to conflict zones in the Middle East since 2012, when jihadists gained ground in Syria and called on Muslims elsewhere to join them.

That has led to near-daily arrests, officials say. On Monday, counterterrorism police in London said they had detained a 20-year-old woman and a 16-year-old girl on suspicion of planning to travel to Syria to join extremists.

Most of the 850-plus known to have left the U.K. for Syria took planes, buses, trains and passenger ferries, as others did elsewhere in Europe

Read More: The Wall Street Journal