Counter terrorism officers stopped a man at Stansted Airport on Sunday night.
He was arrested on Monday on suspicion of terrorism offences and has been taken to a London police station where he remains in custody.
The 21-year-old had returned to the UK from Istanbul, Turkey, via Stuttgart.
Detectives says the arrest was not in response to any immediate threat or risk. Detectives have carried out searches of three addresses in connection with this investigation.
More top news
We may have needed our winter coats for much longer this year but April in the Anglia region was only the coldest and wettest since 2012.
The British astronaut spoke with five-year-old leukaemia patient Maddison Webb about being separated from their families.
There'll be some fair-weather cloud around today but, essentially, it will be a warm and sunny day. Clear tonight with a touch of frost.