This week Britain's House of Commons was told that a terrorist attack in Europe by jihadist fighters returning from Syria is "almost inevitable but may not happen for some time".
The warning came from a leading terrorism expert and author on al-Qaeda, Dr Thomas Hegghammer from the Norwegian Defence Research Establishment.
He told a House of Commons seminar that at least 1,200 volunteers had now departed from various European countries to join extremist groups fighting in Syria.
For some time now counter-terrorism officials here have been on the lookout for what they call "blowback" - returning fighters getting involved in militancy back home, if they survive the battlefield.