British police have arrested six people on suspicion of terrorism after Turkish authorities stopped a group crossing the Syrian border to join the Islamic State of Iraq and al-sham (ISIS), police said. The continuous influx of foreign fighters of British nationality has raised questions over the capability of U.K. officials to spot and prevent potential fighters that are looking to join ISIS.

Four people aged between 22 and 47 were arrested at Manchester airport in north west England early Wednesday, following on from the arrest on Tuesday of a 21-year-old man at Birmingham Airport in central England and a 31-year-old man in Rochdale, near Manchester.

All six remain in police custody for questioning, according to a statement from Greater Manchester Police. The investigation was launched after nine people - five adults and four children - were stopped by Turkish authorities trying to cross into Syria last month.

Turkey deported the nine Britons, including the son of a local councillor from Rochdale, on Tuesday, two weeks after arresting them for attempting to cross into Syria to join ISIS. The four children have been placed in the care of a guardian.

Turkish and other security services estimate some 600 Britons, including Mohammed Emwazi, named "Jihadi John" in the British media, who has appeared in several ISIS beheading videos have joined ISIS. About half of the 600 are thought to have returned home. Britain had already become the target of accusations following its failure to spot three British girls who left their homes to join ISIS through Turkey. The British, who initially dodged responsibility and placed the blame on Turkey, were forced to accept their failure in spotting the teens in time. They later apologized to the families of the teens for the mistake. However, apparently unable to learn its lesson, Britain continues failing to prevent its citizens from joining ISIS and causes Turkey difficulties, as it purportedly fights alone against the problem.

Turkish officials, who have requested more support from the West at every opportunity, are counting on more solid steps to stem the flow of foreign fighters. Foreign Minister Mevlüt Çavuşoğlu had previously said that one of the main problems is that individuals who are deported from Turkey keep returning, and highlighted the refusal of some countries to take the issue seriously.

As part of Turkey's efforts, the police force has tightened security at transfer points such as airports, terminals, bus stations and rental car companies via a special risk analysis system, and a team tasked with tracking suspicious foreigners who travel to Turkey for the first time and those who are minors and who do not have a hotel reservation.

Those deported were of different origins varying in age from 15 to 35. The suspected foreigners claimed that the purpose of their visit was work, tourism or visiting friends or relatives, but security forces who had suspicions about the possibility of them joining ISIS sent them back to the countries where they had traveled from.

Related: Turkey deports nine Syria-bound Britons, questions UK's intel capability