A 19-year old man has been arrested at Heathrow airport on suspicion of "preparing for acts of terrorism" in relation to Syria, police said.
The man is being held at a police station in London and an address in the north of the city is being searched, the Metropolitan police said in a statement.
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On Friday, police said it appeared the girl had shot herself in the face with the weapon.
The showers will be slow-moving, heavy and thundery at times, particularly in central and eastern areas.