Three men have been arrested on suspicion of possessing articles and documents with intent to use them for terrorist purposes overseas.
The men, from Birmingham, were detained at Heathrow Airport on Thursday night after arriving on a flight from Oman.
They were arrested under Section 57 of the Terrorism Act 2000, West Midlands Police said.
Two of the men, aged 33 and 39, are from the Small Heath area of the city and the third, aged 33, from Alum Rock.
The arrests, made by officers from the West Midlands counter terrorism unit, were pre-planned and intelligence-led.
West Midlands Police said they were not made in response to any immediate threat to public safety.