Potential terrorists “may be about to travel abroad, have just returned or be showing signs of becoming radicalized,” Mark Rowley, head of counter terrorism at the Metropolitan Police Service, said in a statement today.

The first six months of this year has seen a total of 69 terror-related arrests, with the biggest growth in offenses connected to Syria occurring in London, he said.

The execution of U.S. journalist James Foley by someone with a British accent has led to a bout of soul-searching in the U.K. as to how and why the country has become an exporter of extremists to the Middle East. U.K. Foreign Secretary Philip Hammond said that the Islamic State jihadis could strike on U.K. soil, in a Sunday Times article Aug. 24.