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
More than 4,600 academics have signed a letter calling for an inquiry into the death of PhD student Giulio Regeni in Cairo last month.
A Norfolk patient has become the first woman in the world to have a so-called 'dead kidney' transplant thanks to new technology.
As the number of whales dying on our beaches rises, we've spoken to an Evolutionary Biologist about why this tragic event keeps happening.