A 21-year-old woman has been detained at Heathrow Airport on suspicion of preparing acts of terrorism, police said.
The woman, from north London, was arrested as she left a flight from Istanbul, Turkey, at just after 9pm on Thursday.
Officers said the arrest was Syria-related and the woman was being held at a south London police station.
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The Metropolitan Police said searches two addresses in the north of the capital had been searched.