A 25-year-old man from Derry has been arrested by detectives investigating alleged activity in Syria earlier this year.
The suspect was arrested in the Creggan area of the city and has been taken to the Serious Crime Suite at Antrim Police Station for questioning.
Earlier this year, the man's family expressed fears he had gone to Syria to fight in the country's civil war.
He originally told them he was going to Turkey on holiday.
It is believed the suspect returned to Derry a few days ago.
It is understood detectives from the PSNI's Serious Crime Branch are working to establish if the man was indeed in Syria over the last number of months and, if so, what he was doing there.