LONDON — British authorities say they’ve arrested a man at a London airport over suspicions that he was training for terrorism overseas.

The unnamed 21-year-old was arrested at the British capital’s Gatwick Airport on Thursday after arriving on a flight from Istanbul, Turkey. He was later released on bail and has yet to be charged.

No further details about the man’s background or travel were released, but British authorities have become increasingly concerned over the movement of jihadist fanatics to Syria to participate in the war against President Bashar Assad. Many travel via Turkey.

Hundreds of British Muslims are believed to have gone to Syria, and Britain’s security services fear that some of the men may return to commit acts of terror on their home soil.